implement instances via one base URL per instance

Previously, instances were addressed in various ways depending on then
request (GET parameter, some instance_id in the POST body).  With this
commit, users of the merchant backend don't see the instance, they just
see another base URL.

47 files changed, 470 insertions, 640 deletions

diff --git a/src/backend/taler-merchant-httpd.c b/src/backend/taler-merchant-httpd.c
index 5269c4c4..0accbb85 100644
--- a/src/backend/taler-merchant-httpd.c
+++ b/src/backend/taler-merchant-httpd.c
@@ -301,6 +301,10 @@ url_handler (void *cls,
   struct TM_HandlerContext *hc;
   struct GNUNET_AsyncScopeId aid;
   const char *correlation_id = NULL;
+  char *instance_id;
+  char *effective_url;
+  int ret;
+
 
   hc = *con_cls;
 
@@ -338,23 +342,53 @@ url_handler (void *cls,
                 method,
                 url);
 
-  for (unsigned int i = 0; NULL != handlers[i].url; i++)
+  /* Find out the merchant backend instance for the request.
+   * If there is an instance, remove the instance specification
+   * from the beginning of the request URL. */
+  {
+    const char *instance_prefix = "/instances/";
+    if (0 == strncmp (url, instance_prefix, strlen (instance_prefix)))
+    {
+      // url starts with "/instance/"
+      instance_id = GNUNET_strdup (url + strlen (instance_prefix));
+      char *slash = strchr (instance_id, '/');
+      if (NULL == slash)
+      {
+        GNUNET_free (instance_id);
+        return TMH_MHD_handler_static_response (&h404,
+                                                connection,
+                                                con_cls,
+                                                upload_data,
+                                                upload_data_size,
+                                                NULL);
+      }
+      effective_url = GNUNET_strdup (slash);
+      *slash = '\0';
+    }
+    else
+    {
+      effective_url = GNUNET_strdup (url);
+      instance_id = NULL;
+    }
+  }
+
+
+  for (unsigned int i=0;NULL != handlers[i].url;i++)
   {
     struct TMH_RequestHandler *rh = &handlers[i];
 
-    if ( (0 == strcmp (url,
-                       rh->url)) &&
+    if ( (0 == strcasecmp (effective_url,
+                           rh->url)) &&
          ( (NULL == rh->method) ||
            (0 == strcasecmp (method,
                              rh->method)) ) )
     {
-      int ret;
-
       ret = rh->handler (rh,
-                         connection,
-                         con_cls,
-                         upload_data,
-                         upload_data_size);
+			 connection,
+			 con_cls,
+			 upload_data,
+			 upload_data_size,
+                         instance_id);
       hc = *con_cls;
       if (NULL != hc)
       {
@@ -363,14 +397,18 @@ url_handler (void *cls,
          * we get another callack for this request. */
         hc->async_scope_id = aid;
       }
+      GNUNET_free_non_null (instance_id);
       return ret;
     }
   }
-  return TMH_MHD_handler_static_response (&h404,
-                                          connection,
-                                          con_cls,
-                                          upload_data,
-                                          upload_data_size);
+  ret = TMH_MHD_handler_static_response (&h404,
+                                         connection,
+                                         con_cls,
+                                         upload_data,
+                                         upload_data_size,
+                                         instance_id);
+  GNUNET_free_non_null (instance_id);
+  return ret;
 }
 
 
@@ -1132,23 +1170,26 @@ instances_iterator_cb (void *cls,
 
 
 /**
- * Lookup a merchant instance by its name.
+ * Lookup a merchant instance by its instance ID.
  *
- * @param name name of the instance to resolve
+ * @param instance_id identifier of the instance to resolve
  * @return NULL if that instance is unknown to us
  */
 struct MerchantInstance *
-TMH_lookup_instance (const char *name)
+TMH_lookup_instance (const char *instance_id)
 {
   struct GNUNET_HashCode h_instance;
 
-  GNUNET_CRYPTO_hash (name,
-                      strlen (name),
+  if (NULL == instance_id)
+    instance_id = "default";
+
+  GNUNET_CRYPTO_hash (instance_id,
+                      strlen (instance_id),
                       &h_instance);
   GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
               "Looking for by-id key %s of '%s' in hashmap\n",
               GNUNET_h2s (&h_instance),
-              name);
+              instance_id);
   /* We're fine if that returns NULL, the calling routine knows how
      to handle that */
   return GNUNET_CONTAINER_multihashmap_get (by_id_map,
@@ -1157,35 +1198,6 @@ TMH_lookup_instance (const char *name)
 
 
 /**
- * Extract merchant instance from the given JSON; if not
- * 'instance' field was found, then "default" instance is
- * returned.
- *
- * @param json the JSON to inspect; it is not required to
- * comply with any particular format. It will only be checked
- * if the field "instance" is there.
- * @return a pointer to a `struct MerchantInstance`. This will be
- * the 'default' merchant if the frontend did not specify any
- * "instance" field. The user should not care to free the returned
- * value, as it is taken from a global array that will be freed
- * by the general shutdown routine. NULL if the frontend specified
- * a wrong instance
- */
-struct MerchantInstance *
-TMH_lookup_instance_json (struct json_t *json)
-{
-  struct json_t *instance;
-  const char *instance_str;
-
-  if (NULL == (instance = json_object_get (json, "instance")))
-    instance_str = "default";
-  else
-    instance_str = json_string_value (instance);
-  return TMH_lookup_instance (instance_str);
-}
-
-
-/**
  * Iterate over locations in config in order to populate
  * the location data.
  *
diff --git a/src/backend/taler-merchant-httpd.h b/src/backend/taler-merchant-httpd.h
index ee3d88fe..4e939b96 100644
--- a/src/backend/taler-merchant-httpd.h
+++ b/src/backend/taler-merchant-httpd.h
@@ -203,13 +203,16 @@ struct TMH_RequestHandler
    * @param[in,out] connection_cls the connection's closure (can be updated)
    * @param upload_data upload data
    * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+   * @param instance_id mechant backend instance ID, or NULL if no explicit
+   *        instance has been specified
    * @return MHD result code
    */
   int (*handler)(struct TMH_RequestHandler *rh,
                  struct MHD_Connection *connection,
                  void **connection_cls,
                  const char *upload_data,
-                 size_t *upload_data_size);
+                 size_t *upload_data_size,
+                 const char *instance_id);
 
   /**
    * Default response code.
@@ -351,21 +354,4 @@ struct MerchantInstance *
 TMH_lookup_instance (const char *name);
 
 
-/**
- * Extract merchant instance from the given JSON
- *
- * @param json the JSON to inspect; it is not required to
- * comply with any particular format. It will only be checked
- * if the field "instance" is there.
- * @return a pointer to a #struct MerchantInstance. This will be
- * the 'default' merchant if the frontend did not specif any
- * "instance" field. The user should not care to free the returned
- * value, as it is taken from a global array that will be freed
- * by the general shutdown routine. NULL if the frontend specified
- * a wrong instance
- */
-struct MerchantInstance *
-TMH_lookup_instance_json (struct json_t *json);
-
-
 #endif
diff --git a/src/backend/taler-merchant-httpd_check-payment.c b/src/backend/taler-merchant-httpd_check-payment.c
index 59ed60d1..2ba4cc4a 100644
--- a/src/backend/taler-merchant-httpd_check-payment.c
+++ b/src/backend/taler-merchant-httpd_check-payment.c
@@ -291,6 +291,8 @@ check_order_and_request_payment (struct MHD_Connection *connection,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -298,12 +300,12 @@ MH_handler_check_payment (struct TMH_RequestHandler *rh,
                           struct MHD_Connection *connection,
                           void **connection_cls,
                           const char *upload_data,
-                          size_t *upload_data_size)
+                          size_t *upload_data_size,
+                          const char *instance_id)
 {
   const char *order_id;
   const char *contract_url;
   const char *session_id;
-  const char *instance_str;
   const char *fulfillment_url;
   char *final_contract_url;
   char *h_contract_terms_str;
@@ -315,12 +317,7 @@ MH_handler_check_payment (struct TMH_RequestHandler *rh,
   int ret;
   int refunded;
 
-  instance_str = MHD_lookup_connection_value (connection,
-                                              MHD_GET_ARGUMENT_KIND,
-                                              "instance");
-  if (NULL == instance_str)
-    instance_str = "default";
-  mi = TMH_lookup_instance (instance_str);
+  mi = TMH_lookup_instance (instance_id);
   if (NULL == mi)
     return TMH_RESPONSE_reply_bad_request (connection,
                                            TALER_EC_CHECK_PAYMENT_INSTANCE_UNKNOWN,
@@ -343,7 +340,7 @@ MH_handler_check_payment (struct TMH_RequestHandler *rh,
   {
     final_contract_url = TALER_url_absolute_mhd (connection,
                                                  "/public/proposal",
-                                                 "instance", instance_str,
+                                                 "instance", instance_id,
                                                  "order_id", order_id,
                                                  NULL);
     GNUNET_assert (NULL != final_contract_url);
diff --git a/src/backend/taler-merchant-httpd_check-payment.h b/src/backend/taler-merchant-httpd_check-payment.h
index 14c2a67b..98fa3a11 100644
--- a/src/backend/taler-merchant-httpd_check-payment.h
+++ b/src/backend/taler-merchant-httpd_check-payment.h
@@ -34,6 +34,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -41,6 +43,7 @@ MH_handler_check_payment (struct TMH_RequestHandler *rh,
                           struct MHD_Connection *connection,
                           void **connection_cls,
                           const char *upload_data,
-                          size_t *upload_data_size);
+                          size_t *upload_data_size,
+                          const char *instance_id);
 
 #endif
diff --git a/src/backend/taler-merchant-httpd_config.c b/src/backend/taler-merchant-httpd_config.c
index 51d44729..3e7b7b39 100644
--- a/src/backend/taler-merchant-httpd_config.c
+++ b/src/backend/taler-merchant-httpd_config.c
@@ -39,6 +39,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -46,19 +48,13 @@ MH_handler_config (struct TMH_RequestHandler *rh,
                    struct MHD_Connection *connection,
                    void **connection_cls,
                    const char *upload_data,
-                   size_t *upload_data_size)
+                   size_t *upload_data_size,
+                   const char *instance_id)
 {
   int ret;
-  const char *instance_str;
   struct MerchantInstance *mi;
 
-  instance_str = MHD_lookup_connection_value (connection,
-                                              MHD_GET_ARGUMENT_KIND,
-                                              "instance");
-  if (NULL == instance_str)
-    instance_str = "default";
-
-  mi = TMH_lookup_instance (instance_str);
+  mi = TMH_lookup_instance (instance_id);
 
   if (NULL == mi)
     return TMH_RESPONSE_reply_bad_request (connection,
diff --git a/src/backend/taler-merchant-httpd_config.h b/src/backend/taler-merchant-httpd_config.h
index cff766eb..08594a5f 100644
--- a/src/backend/taler-merchant-httpd_config.h
+++ b/src/backend/taler-merchant-httpd_config.h
@@ -31,6 +31,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -38,6 +40,7 @@ MH_handler_config (struct TMH_RequestHandler *rh,
                    struct MHD_Connection *connection,
                    void **connection_cls,
                    const char *upload_data,
-                   size_t *upload_data_size);
+                   size_t *upload_data_size,
+                   const char *instance_id);
 
 #endif
diff --git a/src/backend/taler-merchant-httpd_history.c b/src/backend/taler-merchant-httpd_history.c
index 2d620890..36ca7290 100644
--- a/src/backend/taler-merchant-httpd_history.c
+++ b/src/backend/taler-merchant-httpd_history.c
@@ -122,6 +122,8 @@ pd_cb (void *cls,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -129,7 +131,8 @@ MH_handler_history (struct TMH_RequestHandler *rh,
                     struct MHD_Connection *connection,
                     void **connection_cls,
                     const char *upload_data,
-                    size_t *upload_data_size)
+                    size_t *upload_data_size,
+                    const char *instance_id)
 {
   #define LOG_INFO(...) GNUNET_log (GNUNET_ERROR_TYPE_INFO, __VA_ARGS__)
   const char *str;
@@ -172,10 +175,7 @@ MH_handler_history (struct TMH_RequestHandler *rh,
     }
   }
 
-  str = MHD_lookup_connection_value (connection,
-                                     MHD_GET_ARGUMENT_KIND,
-                                     "instance");
-  mi = TMH_lookup_instance (NULL != str ? str : "default");
+  mi = TMH_lookup_instance (instance_id);
 
   if (NULL == mi)
   {
diff --git a/src/backend/taler-merchant-httpd_history.h b/src/backend/taler-merchant-httpd_history.h
index a06e926f..5ea2ed23 100644
--- a/src/backend/taler-merchant-httpd_history.h
+++ b/src/backend/taler-merchant-httpd_history.h
@@ -34,6 +34,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -41,7 +43,8 @@ MH_handler_history (struct TMH_RequestHandler *rh,
                     struct MHD_Connection *connection,
                     void **connection_cls,
                     const char *upload_data,
-                    size_t *upload_data_size);
+                    size_t *upload_data_size,
+                    const char *instance_id);
 
 /* end of taler-merchant-httpd_history.c */
 #endif
diff --git a/src/backend/taler-merchant-httpd_mhd.c b/src/backend/taler-merchant-httpd_mhd.c
index e4e5329c..a17485cc 100644
--- a/src/backend/taler-merchant-httpd_mhd.c
+++ b/src/backend/taler-merchant-httpd_mhd.c
@@ -37,6 +37,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -44,7 +46,8 @@ TMH_MHD_handler_static_response (struct TMH_RequestHandler *rh,
                                  struct MHD_Connection *connection,
                                  void **connection_cls,
                                  const char *upload_data,
-                                 size_t *upload_data_size)
+                                 size_t *upload_data_size,
+                                 const char *instance_id)
 {
   struct MHD_Response *response;
   int ret;
@@ -81,6 +84,8 @@ TMH_MHD_handler_static_response (struct TMH_RequestHandler *rh,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -88,7 +93,8 @@ TMH_MHD_handler_agpl_redirect (struct TMH_RequestHandler *rh,
                                   struct MHD_Connection *connection,
                                   void **connection_cls,
                                   const char *upload_data,
-                                  size_t *upload_data_size)
+                                  size_t *upload_data_size,
+                                  const char *instance_id)
 {
   const char *agpl =
     "This server is licensed under the Affero GPL. You will now be redirected to the source code.";
@@ -136,6 +142,8 @@ TMH_MHD_handler_agpl_redirect (struct TMH_RequestHandler *rh,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -143,7 +151,8 @@ TMH_MHD_handler_send_json_pack_error (struct TMH_RequestHandler *rh,
                                          struct MHD_Connection *connection,
                                          void **connection_cls,
                                          const char *upload_data,
-                                         size_t *upload_data_size)
+                                         size_t *upload_data_size,
+                                         const char *instance_id)
 {
   return TMH_RESPONSE_reply_json_pack (connection,
                                        rh->response_code,
diff --git a/src/backend/taler-merchant-httpd_mhd.h b/src/backend/taler-merchant-httpd_mhd.h
index 5f0e69be..0ec9830b 100644
--- a/src/backend/taler-merchant-httpd_mhd.h
+++ b/src/backend/taler-merchant-httpd_mhd.h
@@ -37,6 +37,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -44,7 +46,8 @@ TMH_MHD_handler_static_response (struct TMH_RequestHandler *rh,
                                  struct MHD_Connection *connection,
                                  void **connection_cls,
                                  const char *upload_data,
-                                 size_t *upload_data_size);
+                                 size_t *upload_data_size,
+                                 const char *instance_id);
 
 
 /**
@@ -56,6 +59,8 @@ TMH_MHD_handler_static_response (struct TMH_RequestHandler *rh,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -63,7 +68,8 @@ TMH_MHD_handler_agpl_redirect (struct TMH_RequestHandler *rh,
                                struct MHD_Connection *connection,
                                void **connection_cls,
                                const char *upload_data,
-                               size_t *upload_data_size);
+                               size_t *upload_data_size,
+                               const char *instance_id);
 
 
 /**
@@ -98,6 +104,8 @@ TMH_MHD_helper_send_json_pack (struct TMH_RequestHandler *rh,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -105,7 +113,8 @@ TMH_MHD_handler_send_json_pack_error (struct TMH_RequestHandler *rh,
                                       struct MHD_Connection *connection,
                                       void **connection_cls,
                                       const char *upload_data,
-                                      size_t *upload_data_size);
+                                      size_t *upload_data_size,
+                                      const char *instance_id);
 
 
 #endif
diff --git a/src/backend/taler-merchant-httpd_pay.c b/src/backend/taler-merchant-httpd_pay.c
index d381c7ee..e72a8e25 100644
--- a/src/backend/taler-merchant-httpd_pay.c
+++ b/src/backend/taler-merchant-httpd_pay.c
@@ -1471,27 +1471,6 @@ parse_pay (struct MHD_Connection *connection,
     pc->mode = PC_MODE_PAY;
   else
     pc->mode = PC_MODE_ABORT_REFUND;
-  pc->mi = TMH_lookup_instance_json (merchant);
-  if (NULL == pc->mi)
-  {
-    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
-                "Unable to find the specified instance\n");
-    GNUNET_JSON_parse_free (spec);
-    if (MHD_NO ==
-	TMH_RESPONSE_reply_not_found (connection,
-                                  TALER_EC_PAY_INSTANCE_UNKNOWN,
-                                  "Unknown instance given"))
-    {
-      GNUNET_break (0);
-      return GNUNET_SYSERR;
-    }
-    return GNUNET_NO;
-  }
-
-  GNUNET_log (GNUNET_ERROR_TYPE_INFO,
-              "/pay: picked instance %s\n",
-              pc->mi->id);
-
   {
     const char *fulfillment_url;
     struct GNUNET_JSON_Specification espec[] = {
@@ -2136,6 +2115,8 @@ handler_pay_json (struct MHD_Connection *connection,
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a
  *       upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -2143,7 +2124,8 @@ MH_handler_pay (struct TMH_RequestHandler *rh,
                 struct MHD_Connection *connection,
                 void **connection_cls,
                 const char *upload_data,
-                size_t *upload_data_size)
+                size_t *upload_data_size,
+                const char *instance_id)
 {
   struct PayContext *pc;
   int res;
@@ -2160,6 +2142,25 @@ MH_handler_pay (struct TMH_RequestHandler *rh,
     pc->hc.cc = &pay_context_cleanup;
     pc->connection = connection;
     *connection_cls = pc;
+    pc->mi = TMH_lookup_instance (instance_id);
+    if (NULL == pc->mi)
+    {
+      GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
+                  "Unable to find the specified instance\n");
+      if (MHD_NO ==
+          TMH_RESPONSE_reply_not_found (connection,
+                                    TALER_EC_PAY_INSTANCE_UNKNOWN,
+                                    "Unknown instance given"))
+      {
+        GNUNET_break (0);
+        return GNUNET_SYSERR;
+      }
+      return GNUNET_NO;
+    }
+    GNUNET_log (GNUNET_ERROR_TYPE_INFO,
+                "/pay: picked instance %s\n",
+                pc->mi->id);
+
   }
   else
   {
diff --git a/src/backend/taler-merchant-httpd_pay.h b/src/backend/taler-merchant-httpd_pay.h
index d4f4958a..cffb13bb 100644
--- a/src/backend/taler-merchant-httpd_pay.h
+++ b/src/backend/taler-merchant-httpd_pay.h
@@ -40,6 +40,8 @@ MH_force_pc_resume (void);
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -47,6 +49,7 @@ MH_handler_pay (struct TMH_RequestHandler *rh,
                 struct MHD_Connection *connection,
                 void **connection_cls,
                 const char *upload_data,
-                size_t *upload_data_size);
+                size_t *upload_data_size,
+                const char *instance_id);
 
 #endif
diff --git a/src/backend/taler-merchant-httpd_proposal.c b/src/backend/taler-merchant-httpd_proposal.c
index a93d6f17..7f61d215 100644
--- a/src/backend/taler-merchant-httpd_proposal.c
+++ b/src/backend/taler-merchant-httpd_proposal.c
@@ -149,10 +149,10 @@ json_parse_cleanup (struct TM_HandlerContext *hc)
  */
 static int
 proposal_put (struct MHD_Connection *connection,
-              json_t *order)
+              json_t *order,
+              const struct MerchantInstance *mi)
 {
   int res;
-  struct MerchantInstance *mi;
   struct TALER_Amount total;
   const char *order_id;
   const char *summary;
@@ -182,7 +182,6 @@ proposal_put (struct MHD_Connection *connection,
     GNUNET_JSON_spec_end ()
   };
   enum GNUNET_DB_QueryStatus qs;
-  const char *instance;
   struct WireMethod *wm;
 
   /* Add order_id if it doesn't exist. */
@@ -309,111 +308,81 @@ proposal_put (struct MHD_Connection *connection,
                          json_array ());
   }
 
-  instance = json_string_value (json_object_get (order,
-                                                 "instance"));
-  if (NULL == instance)
-  {
-    TALER_LOG_DEBUG ("Giving 'default' instance\n");
-    instance = "default";
-  }
-
   /* Fill in merchant information if necessary */
+  if (NULL == json_object_get (order, "merchant"))
   {
-    /* The frontend either fully specifieds the "merchant" field,
-     * or just gives the backend the "instance" name and lets it
-     * fill out. */
-    struct MerchantInstance *my_mi = TMH_lookup_instance (instance);
-
-    if (NULL == my_mi)
+    const char *mj = NULL;
+    const char *ma = NULL;
+    json_t *locations;
+    char *label;
+    json_t *jmerchant;
+
+    jmerchant = json_object ();
+    json_object_set_new (jmerchant,
+                         "name",
+                         json_string (mi->name));
+    json_object_set_new (jmerchant,
+                         "instance",
+                         json_string (mi->id));
+    locations = json_object_get (order,
+                                 "locations");
+    if (NULL != locations)
     {
-      TALER_LOG_WARNING ("Does 'default' instance exist?\n");
-      return TMH_RESPONSE_reply_not_found
-        (connection,
-         TALER_EC_CONTRACT_INSTANCE_UNKNOWN,
-         "merchant instance (order:instance) not found");
-    }
-
-    /**
-     * Potential bug: if the outer 'instance' field is not
-     * given and the 'merchant' object is also missing, then
-     * is not possible to extract the instance!
-     */
-    if (NULL == json_object_get (order,
-                                 "merchant"))
-    {
-      const char *mj = NULL;
-      const char *ma = NULL;
-      json_t *locations;
-      char *label;
-      json_t *jmerchant;
-
-      jmerchant = json_object ();
-      json_object_set_new (jmerchant,
-                           "name",
-                           json_string (my_mi->name));
-      json_object_set_new (jmerchant,
-                           "instance",
-                           json_string (instance));
-      locations = json_object_get (order,
-                                   "locations");
-      if (NULL != locations)
+      json_t *loca;
+      json_t *locj;
+
+      /* Handle merchant address */
+      GNUNET_assert (0 < GNUNET_asprintf (&label,
+                                          "%s-address",
+                                          mi->id));
+      loca = json_object_get (default_locations,
+                              label);
+      if (NULL != loca)
+      {
+        loca = json_deep_copy (loca);
+        ma = STANDARD_LABEL_MERCHANT_ADDRESS;
+        json_object_set_new (locations,
+                             ma,
+                             loca);
+        json_object_set_new (jmerchant,
+                             "address",
+                             json_string (ma));
+      }
+      GNUNET_free (label);
+
+      /* Handle merchant jurisdiction */
+      GNUNET_assert (0 < GNUNET_asprintf (&label,
+                                          "%s-jurisdiction",
+                                          mi->id));
+      locj = json_object_get (default_locations,
+                              label);
+      if (NULL != locj)
       {
-        json_t *loca;
-        json_t *locj;
-
-        /* Handle merchant address */
-        GNUNET_assert (0 < GNUNET_asprintf (&label,
-                                            "%s-address",
-                                            my_mi->id));
-        loca = json_object_get (default_locations,
-                                label);
-        if (NULL != loca)
+        if ( (NULL != loca) &&
+             (1 == json_equal (locj,
+                               loca)) )
         {
-          loca = json_deep_copy (loca);
-          ma = STANDARD_LABEL_MERCHANT_ADDRESS;
-          json_object_set_new (locations,
-                               ma,
-                               loca);
-          json_object_set_new (jmerchant,
-                               "address",
-                               json_string (ma));
+          /* addresses equal, re-use */
+          mj = ma;
         }
-        GNUNET_free (label);
-
-        /* Handle merchant jurisdiction */
-        GNUNET_assert (0 < GNUNET_asprintf (&label,
-                                            "%s-jurisdiction",
-                                            my_mi->id));
-        locj = json_object_get (default_locations,
-                                label);
-        if (NULL != locj)
+        else
         {
-          if ( (NULL != loca) &&
-               (1 == json_equal (locj,
-                                 loca)) )
-          {
-            /* addresses equal, re-use */
-            mj = ma;
-          }
-          else
-          {
-            locj = json_deep_copy (locj);
-            mj = STANDARD_LABEL_MERCHANT_JURISDICTION;
-            json_object_set_new (locations,
-                                 mj,
-                                 locj);
-          }
-          json_object_set_new (merchant,
-                               "jurisdiction",
-                               json_string (mj));
+          locj = json_deep_copy (locj);
+          mj = STANDARD_LABEL_MERCHANT_JURISDICTION;
+          json_object_set_new (locations,
+                               mj,
+                               locj);
         }
-        GNUNET_free (label);
-      } /* have locations */
-      json_object_set_new (order,
-                           "merchant",
-                           jmerchant);
-    } /* needed to synthesize merchant info */
-  } /* scope of 'my_mi' */
+        json_object_set_new (merchant,
+                             "jurisdiction",
+                             json_string (mj));
+      }
+      GNUNET_free (label);
+    } /* have locations */
+    json_object_set_new (order,
+                         "merchant",
+                         jmerchant);
+  } /* needed to synthesize merchant info */
 
   /* extract fields we need to sign separately */
   res = TMH_PARSE_json_data (connection,
@@ -444,45 +413,6 @@ proposal_put (struct MHD_Connection *connection,
        "order:products");
   }
 
-  mi = TMH_lookup_instance_json (merchant);
-
-  if (NULL == mi)
-  {
-    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
-                "Not able to find the specified instance\n");
-    GNUNET_JSON_parse_free (spec);
-    return TMH_RESPONSE_reply_not_found
-      (connection,
-       TALER_EC_CONTRACT_INSTANCE_UNKNOWN,
-       "Unknown instance (order:merchant:instance) given");
-  }
-
-  /* The outer instance field, and the one included
-   * in the merchant object are different */
-  if (0 != strcmp (mi->id,
-                   instance))
-  {
-    TALER_LOG_ERROR
-      ("Inconsistent instance specified"
-       " by merchant ('%s' vs '%s')\n",
-       instance,
-       mi->id);
-
-    TALER_LOG_DEBUG ("Dump wrong order: %s\n",
-                     json_dumps (order,
-                                 JSON_INDENT (1)));
-
-    return TMH_RESPONSE_reply_not_found
-      (connection,
-       TALER_EC_CONTRACT_INSTANCE_INCONSISTENT,
-       "Inconsistent instance given");
-  }
-
-  /* Setting the instance on the order directly is just a shortcut,
-     the wallet shouldn't see that. */
-  json_object_del (order, "instance");
-  instance = NULL;
-
   /* add fields to the contract that the backend should provide */
   json_object_set (order,
                    "exchanges",
@@ -499,7 +429,7 @@ proposal_put (struct MHD_Connection *connection,
   {
     GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                 "No wire method available for"
-                " specified instance\n");
+                " instance '%s'\n", mi->id);
     GNUNET_JSON_parse_free (spec);
     return TMH_RESPONSE_reply_not_found
       (connection,
@@ -618,10 +548,12 @@ proposal_put (struct MHD_Connection *connection,
  *
  * @param connection the MHD connection to handle
  * @param[in,out] connection_cls the connection's closure
- * (can be updated)
+ *                (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in
- * @a upload_data
+ *                @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -629,10 +561,12 @@ MH_handler_proposal_put (struct TMH_RequestHandler *rh,
                          struct MHD_Connection *connection,
                          void **connection_cls,
                          const char *upload_data,
-                         size_t *upload_data_size)
+                         size_t *upload_data_size,
+                         const char *instance_id)
 {
   int res;
   struct TMH_JsonParseContext *ctx;
+  struct MerchantInstance *mi;
   json_t *root;
   json_t *order;
 
@@ -662,6 +596,12 @@ MH_handler_proposal_put (struct TMH_RequestHandler *rh,
        (NULL == root) )
     return MHD_YES;
 
+  mi = TMH_lookup_instance (instance_id);
+  if (NULL == mi)
+    return TMH_RESPONSE_reply_not_found (connection,
+                                         TALER_EC_CONTRACT_INSTANCE_UNKNOWN,
+                                         "instance");
+
   order = json_object_get (root,
                            "order");
   if (NULL == order)
@@ -672,8 +612,7 @@ MH_handler_proposal_put (struct TMH_RequestHandler *rh,
        "order");
   }
   else
-    res = proposal_put (connection,
-                        order);
+    res = proposal_put (connection, order, mi);
   json_decref (root);
   return res;
 }
@@ -691,6 +630,8 @@ MH_handler_proposal_put (struct TMH_RequestHandler *rh,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -698,10 +639,10 @@ MH_handler_proposal_lookup (struct TMH_RequestHandler *rh,
                             struct MHD_Connection *connection,
                             void **connection_cls,
                             const char *upload_data,
-                            size_t *upload_data_size)
+                            size_t *upload_data_size,
+                            const char *instance_id)
 {
   const char *order_id;
-  const char *instance;
   const char *nonce;
   int res;
   enum GNUNET_DB_QueryStatus qs;
@@ -710,14 +651,7 @@ MH_handler_proposal_lookup (struct TMH_RequestHandler *rh,
   struct GNUNET_CRYPTO_EddsaSignature merchant_sig;
   const char *stored_nonce;
 
-  instance = MHD_lookup_connection_value (connection,
-                                          MHD_GET_ARGUMENT_KIND,
-                                          "instance");
-  if (NULL == instance)
-    return TMH_RESPONSE_reply_arg_missing (connection,
-					   TALER_EC_PARAMETER_MISSING,
-                                           "instance");
-  mi = TMH_lookup_instance (instance);
+  mi = TMH_lookup_instance (instance_id);
   if (NULL == mi)
     return TMH_RESPONSE_reply_not_found (connection,
                                          TALER_EC_CONTRACT_INSTANCE_UNKNOWN,
diff --git a/src/backend/taler-merchant-httpd_proposal.h b/src/backend/taler-merchant-httpd_proposal.h
index fe5b319a..137c32a6 100644
--- a/src/backend/taler-merchant-httpd_proposal.h
+++ b/src/backend/taler-merchant-httpd_proposal.h
@@ -33,6 +33,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -40,7 +42,8 @@ MH_handler_proposal_put (struct TMH_RequestHandler *rh,
                          struct MHD_Connection *connection,
                          void **connection_cls,
                          const char *upload_data,
-                         size_t *upload_data_size);
+                         size_t *upload_data_size,
+                         const char *instance_id);
 
 
 /**
@@ -60,5 +63,6 @@ MH_handler_proposal_lookup (struct TMH_RequestHandler *rh,
                             struct MHD_Connection *connection,
                             void **connection_cls,
                             const char *upload_data,
-                            size_t *upload_data_size);
+                            size_t *upload_data_size,
+                            const char *instance_id);
 #endif
diff --git a/src/backend/taler-merchant-httpd_refund.c b/src/backend/taler-merchant-httpd_refund.c
index 1f80491c..a20149a1 100644
--- a/src/backend/taler-merchant-httpd_refund.c
+++ b/src/backend/taler-merchant-httpd_refund.c
@@ -155,6 +155,8 @@ json_parse_cleanup (struct TM_HandlerContext *hc)
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -162,7 +164,8 @@ MH_handler_refund_increase (struct TMH_RequestHandler *rh,
                             struct MHD_Connection *connection,
                             void **connection_cls,
                             const char *upload_data,
-                            size_t *upload_data_size)
+                            size_t *upload_data_size,
+                            const char *instance_id)
 {
   int res;
   struct TMH_JsonParseContext *ctx;
@@ -171,7 +174,6 @@ MH_handler_refund_increase (struct TMH_RequestHandler *rh,
   json_t *contract_terms;
   const char *order_id;
   const char *reason;
-  const char *merchant;
   struct MerchantInstance *mi;
   struct GNUNET_HashCode h_contract_terms;
   struct TALER_MerchantRefundConfirmationPS confirmation;
@@ -180,7 +182,6 @@ MH_handler_refund_increase (struct TMH_RequestHandler *rh,
     TALER_JSON_spec_amount ("refund", &refund),
     GNUNET_JSON_spec_string ("order_id", &order_id),
     GNUNET_JSON_spec_string ("reason", &reason),
-    GNUNET_JSON_spec_string ("instance", &merchant),
     GNUNET_JSON_spec_end ()
   };
   enum GNUNET_DB_QueryStatus qs;
@@ -226,11 +227,12 @@ MH_handler_refund_increase (struct TMH_RequestHandler *rh,
     return MHD_NO;
   }
 
-  mi = TMH_lookup_instance (merchant);
+  mi = TMH_lookup_instance (instance_id);
   if (NULL == mi)
   {
+    GNUNET_assert (NULL != instance_id);
     GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
-                "No instance found\n");
+                "Instance '%s' not found\n", instance_id);
     GNUNET_JSON_parse_free (spec);
     json_decref (root);
     return TMH_RESPONSE_reply_not_found (connection,
@@ -491,6 +493,8 @@ process_refunds_cb (void *cls,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -498,34 +502,23 @@ MH_handler_refund_lookup (struct TMH_RequestHandler *rh,
                           struct MHD_Connection *connection,
                           void **connection_cls,
                           const char *upload_data,
-                          size_t *upload_data_size)
+                          size_t *upload_data_size,
+                          const char *instance_id)
 {
   const char *order_id;
-  const char *instance;
   struct GNUNET_HashCode h_contract_terms;
   json_t *contract_terms;
   struct MerchantInstance *mi;
   enum GNUNET_DB_QueryStatus qs;
 
-  instance = MHD_lookup_connection_value (connection,
-                                          MHD_GET_ARGUMENT_KIND,
-                                          "instance");
-  if (NULL == instance)
-  {
-    GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
-                "Argument 'instance' not given\n");
-    return TMH_RESPONSE_reply_arg_missing (connection,
-					   TALER_EC_PARAMETER_MISSING,
-                                           "instance");
-  }
-
-  mi = TMH_lookup_instance (instance);
+  mi = TMH_lookup_instance (instance_id);
 
   if (NULL == mi)
   {
+    GNUNET_assert (NULL != instance_id);
     GNUNET_log (GNUNET_ERROR_TYPE_ERROR,
                 "Unknown instance given: %s\n",
-                instance);
+                instance_id);
     return TMH_RESPONSE_reply_not_found (connection,
                                          TALER_EC_REFUND_INSTANCE_UNKNOWN,
                                          "Unknown instance given");
diff --git a/src/backend/taler-merchant-httpd_refund.h b/src/backend/taler-merchant-httpd_refund.h
index 32083273..eadd24a9 100644
--- a/src/backend/taler-merchant-httpd_refund.h
+++ b/src/backend/taler-merchant-httpd_refund.h
@@ -34,6 +34,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -41,7 +43,8 @@ MH_handler_refund_increase (struct TMH_RequestHandler *rh,
                             struct MHD_Connection *connection,
                             void **connection_cls,
                             const char *upload_data,
-                            size_t *upload_data_size);
+                            size_t *upload_data_size,
+                            const char *instance_id);
 
 
 /**
@@ -52,6 +55,8 @@ MH_handler_refund_increase (struct TMH_RequestHandler *rh,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -59,7 +64,8 @@ MH_handler_refund_lookup (struct TMH_RequestHandler *rh,
                           struct MHD_Connection *connection,
                           void **connection_cls,
                           const char *upload_data,
-                          size_t *upload_data_size);
+                          size_t *upload_data_size,
+                          const char *instance_id);
 
 /**
  * Get the JSON representation of a refund.
diff --git a/src/backend/taler-merchant-httpd_tip-authorize.c b/src/backend/taler-merchant-httpd_tip-authorize.c
index 4c2451e8..1a79d772 100644
--- a/src/backend/taler-merchant-httpd_tip-authorize.c
+++ b/src/backend/taler-merchant-httpd_tip-authorize.c
@@ -45,11 +45,6 @@ struct TipAuthContext
   void *json_parse_context;
 
   /**
-   * Merchant instance to use.
-   */
-  const char *instance;
-
-  /**
    * Justification to use.
    */
   const char *justification;
@@ -112,6 +107,8 @@ cleanup_tac (struct TM_HandlerContext *hc)
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -119,7 +116,8 @@ MH_handler_tip_authorize (struct TMH_RequestHandler *rh,
                           struct MHD_Connection *connection,
                           void **connection_cls,
                           const char *upload_data,
-                          size_t *upload_data_size)
+                          size_t *upload_data_size,
+                          const char *instance_id)
 {
   struct TipAuthContext *tac;
   int res;
@@ -153,7 +151,6 @@ MH_handler_tip_authorize (struct TMH_RequestHandler *rh,
   {
     struct GNUNET_JSON_Specification spec[] = {
       TALER_JSON_spec_amount ("amount", &tac->amount),
-      GNUNET_JSON_spec_string ("instance", &tac->instance),
       GNUNET_JSON_spec_string ("justification", &tac->justification),
       GNUNET_JSON_spec_end()
     };
@@ -181,12 +178,13 @@ MH_handler_tip_authorize (struct TMH_RequestHandler *rh,
     tac->parsed_json = GNUNET_YES;
   }
 
-  mi = TMH_lookup_instance (tac->instance);
+  mi = TMH_lookup_instance (instance_id);
   if (NULL == mi)
   {
+    GNUNET_assert (NULL != instance_id);
     GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                 "Instance `%s' not configured\n",
-                tac->instance);
+                instance_id);
     return TMH_RESPONSE_reply_not_found (connection,
                                          TALER_EC_TIP_AUTHORIZE_INSTANCE_UNKNOWN,
                                          "unknown instance");
@@ -195,7 +193,7 @@ MH_handler_tip_authorize (struct TMH_RequestHandler *rh,
   {
     GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                 "Instance `%s' not configured for tipping\n",
-                tac->instance);
+                (NULL != instance_id) ? instance_id : "default");
     return TMH_RESPONSE_reply_not_found (connection,
                                          TALER_EC_TIP_AUTHORIZE_INSTANCE_DOES_NOT_TIP,
                                          "exchange for tipping not configured for the instance");
diff --git a/src/backend/taler-merchant-httpd_tip-authorize.h b/src/backend/taler-merchant-httpd_tip-authorize.h
index b7c3b9a1..025b9580 100644
--- a/src/backend/taler-merchant-httpd_tip-authorize.h
+++ b/src/backend/taler-merchant-httpd_tip-authorize.h
@@ -32,6 +32,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -39,6 +41,7 @@ MH_handler_tip_authorize (struct TMH_RequestHandler *rh,
                           struct MHD_Connection *connection,
                           void **connection_cls,
                           const char *upload_data,
-                          size_t *upload_data_size);
+                          size_t *upload_data_size,
+                          const char *instance_id);
 
 #endif
diff --git a/src/backend/taler-merchant-httpd_tip-pickup.c b/src/backend/taler-merchant-httpd_tip-pickup.c
index eba99e38..6232c4b8 100644
--- a/src/backend/taler-merchant-httpd_tip-pickup.c
+++ b/src/backend/taler-merchant-httpd_tip-pickup.c
@@ -494,6 +494,8 @@ parse_planchet (struct MHD_Connection *connection,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -501,7 +503,8 @@ MH_handler_tip_pickup (struct TMH_RequestHandler *rh,
                        struct MHD_Connection *connection,
                        void **connection_cls,
                        const char *upload_data,
-                       size_t *upload_data_size)
+                       size_t *upload_data_size,
+                       const char *instance_id)
 {
   int res;
   struct GNUNET_HashCode tip_id;
@@ -608,6 +611,8 @@ MH_handler_tip_pickup (struct TMH_RequestHandler *rh,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -615,10 +620,10 @@ MH_handler_tip_pickup_get (struct TMH_RequestHandler *rh,
                            struct MHD_Connection *connection,
                            void **connection_cls,
                            const char *upload_data,
-                           size_t *upload_data_size)
+                           size_t *upload_data_size,
+                           const char *instance_id)
 {
   struct MerchantInstance *mi;
-  const char *instance_str;
   const char *tip_id_str;
   char *exchange_url;
   json_t *extra;
@@ -630,12 +635,7 @@ MH_handler_tip_pickup_get (struct TMH_RequestHandler *rh,
   int ret;
   int qs;
 
-  instance_str = MHD_lookup_connection_value (connection,
-                                              MHD_GET_ARGUMENT_KIND,
-                                              "instance");
-  if (NULL == instance_str)
-    instance_str = "default";
-  mi = TMH_lookup_instance (instance_str);
+  mi = TMH_lookup_instance (instance_id);
   if (NULL == mi)
     return TMH_RESPONSE_reply_bad_request (connection,
                                            TALER_EC_TIP_INSTANCE_UNKNOWN,
diff --git a/src/backend/taler-merchant-httpd_tip-pickup.h b/src/backend/taler-merchant-httpd_tip-pickup.h
index 181725a7..f0e9733d 100644
--- a/src/backend/taler-merchant-httpd_tip-pickup.h
+++ b/src/backend/taler-merchant-httpd_tip-pickup.h
@@ -32,6 +32,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -39,7 +41,8 @@ MH_handler_tip_pickup (struct TMH_RequestHandler *rh,
                        struct MHD_Connection *connection,
                        void **connection_cls,
                        const char *upload_data,
-                       size_t *upload_data_size);
+                       size_t *upload_data_size,
+                       const char *instance_id);
 
 /**
  * Manages a GET /tip-pickup call.
@@ -49,6 +52,8 @@ MH_handler_tip_pickup (struct TMH_RequestHandler *rh,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -56,7 +61,8 @@ MH_handler_tip_pickup_get (struct TMH_RequestHandler *rh,
                            struct MHD_Connection *connection,
                            void **connection_cls,
                            const char *upload_data,
-                           size_t *upload_data_size);
+                           size_t *upload_data_size,
+                           const char *instance_id);
 
 
 #endif
diff --git a/src/backend/taler-merchant-httpd_tip-query.c b/src/backend/taler-merchant-httpd_tip-query.c
index 2b90b795..bd61a3e6 100644
--- a/src/backend/taler-merchant-httpd_tip-query.c
+++ b/src/backend/taler-merchant-httpd_tip-query.c
@@ -52,6 +52,12 @@ struct TipQueryContext
   const char *instance;
 
   /**
+   * GNUNET_YES if the tip query has already been processed
+   * and we can queue the response.
+   */
+  int processed;
+
+  /**
    * Context for checking the tipping reserve's status.
    */
   struct CheckTipReserve ctr;
@@ -126,6 +132,8 @@ generate_final_response (struct TipQueryContext *tqc)
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -133,7 +141,8 @@ MH_handler_tip_query (struct TMH_RequestHandler *rh,
                       struct MHD_Connection *connection,
                       void **connection_cls,
                       const char *upload_data,
-                      size_t *upload_data_size)
+                      size_t *upload_data_size,
+                      const char *instance_id)
 {
   struct TipQueryContext *tqc;
   struct MerchantInstance *mi;
@@ -172,28 +181,21 @@ MH_handler_tip_query (struct TMH_RequestHandler *rh,
     return res;
   }
 
-  if (NULL != tqc->instance)
+  if (GNUNET_YES == tqc->processed)
   {
     /* We've been here before, so TMH_check_tip_reserve() must have
        finished and left the result for us. Finish processing. */
     return generate_final_response (tqc);
   }
 
-  /* No error yet, so first time here, let's query the exchange */
-  tqc->instance = MHD_lookup_connection_value (connection,
-                                               MHD_GET_ARGUMENT_KIND,
-                                               "instance");
-  if (NULL == tqc->instance)
-    return TMH_RESPONSE_reply_arg_missing (connection,
-                                           TALER_EC_PARAMETER_MISSING,
-                                           "instance");
 
-  mi = TMH_lookup_instance (tqc->instance);
+  mi = TMH_lookup_instance (instance_id);
   if (NULL == mi)
   {
+    GNUNET_assert (NULL != instance_id);
     GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                 "Instance `%s' not configured\n",
-                tqc->instance);
+                instance_id);
     return TMH_RESPONSE_reply_not_found (connection,
                                          TALER_EC_TIP_AUTHORIZE_INSTANCE_UNKNOWN,
                                          "unknown instance");
@@ -202,7 +204,7 @@ MH_handler_tip_query (struct TMH_RequestHandler *rh,
   {
     GNUNET_log (GNUNET_ERROR_TYPE_WARNING,
                 "Instance `%s' not configured for tipping\n",
-                tqc->instance);
+                instance_id);
     return TMH_RESPONSE_reply_not_found (connection,
                                          TALER_EC_TIP_AUTHORIZE_INSTANCE_DOES_NOT_TIP,
                                          "exchange for tipping not configured for the instance");
@@ -236,6 +238,7 @@ MH_handler_tip_query (struct TMH_RequestHandler *rh,
     }
   }
 
+  tqc->processed = GNUNET_YES;
   TMH_check_tip_reserve (&tqc->ctr,
                          mi->tip_exchange);
   return MHD_YES;
diff --git a/src/backend/taler-merchant-httpd_tip-query.h b/src/backend/taler-merchant-httpd_tip-query.h
index f3a9ebff..10b1e1be 100644
--- a/src/backend/taler-merchant-httpd_tip-query.h
+++ b/src/backend/taler-merchant-httpd_tip-query.h
@@ -31,6 +31,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -38,6 +40,7 @@ MH_handler_tip_query (struct TMH_RequestHandler *rh,
                       struct MHD_Connection *connection,
                       void **connection_cls,
                       const char *upload_data,
-                      size_t *upload_data_size);
+                      size_t *upload_data_size,
+                      const char *instance_id);
 
 #endif
diff --git a/src/backend/taler-merchant-httpd_track-transaction.c b/src/backend/taler-merchant-httpd_track-transaction.c
index 6879612d..1376aa9a 100644
--- a/src/backend/taler-merchant-httpd_track-transaction.c
+++ b/src/backend/taler-merchant-httpd_track-transaction.c
@@ -1037,6 +1037,8 @@ find_exchange (struct TrackTransactionContext *tctx)
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -1044,13 +1046,12 @@ MH_handler_track_transaction (struct TMH_RequestHandler *rh,
                               struct MHD_Connection *connection,
                               void **connection_cls,
                               const char *upload_data,
-                              size_t *upload_data_size)
+                              size_t *upload_data_size,
+                              const char *instance_id)
 {
   struct TrackTransactionContext *tctx;
   const char *order_id;
-  const char *instance;
   enum GNUNET_DB_QueryStatus qs;
-  struct GNUNET_HashCode h_instance;
   struct json_t *contract_terms;
 
   if (NULL == *connection_cls)
@@ -1105,26 +1106,16 @@ MH_handler_track_transaction (struct TMH_RequestHandler *rh,
     return TMH_RESPONSE_reply_arg_missing (connection,
                                            TALER_EC_PARAMETER_MISSING,
                                            "order_id");
-  instance = MHD_lookup_connection_value (connection,
-                                          MHD_GET_ARGUMENT_KIND,
-                                          "instance");
-  if (NULL == instance)
-    instance = "default";
-
-  GNUNET_CRYPTO_hash (instance,
-                      strlen (instance),
-                      &h_instance);
-
-  tctx->mi = GNUNET_CONTAINER_multihashmap_get (by_id_map,
-                                                &h_instance);
-  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
-              "Tracking on behalf of instance '%s'\n",
-              instance);
 
+  tctx->mi = TMH_lookup_instance (instance_id);
   if (NULL == tctx->mi)
     return TMH_RESPONSE_reply_not_found (connection,
                                          TALER_EC_TRACK_TRANSACTION_INSTANCE_UNKNOWN,
                                          "unknown instance");
+  GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
+              "Tracking on behalf of instance '%s'\n",
+              instance_id);
+
 
   /* Map order id to contract terms; the objective is to get
      the contract term's hashcode so as to retrieve all the
diff --git a/src/backend/taler-merchant-httpd_track-transaction.h b/src/backend/taler-merchant-httpd_track-transaction.h
index 30896bf3..7efecaa9 100644
--- a/src/backend/taler-merchant-httpd_track-transaction.h
+++ b/src/backend/taler-merchant-httpd_track-transaction.h
@@ -32,6 +32,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -39,7 +41,8 @@ MH_handler_track_transaction (struct TMH_RequestHandler *rh,
                               struct MHD_Connection *connection,
                               void **connection_cls,
                               const char *upload_data,
-                              size_t *upload_data_size);
+                              size_t *upload_data_size,
+                              const char *instance_id);
 
 
 #endif
diff --git a/src/backend/taler-merchant-httpd_track-transfer.c b/src/backend/taler-merchant-httpd_track-transfer.c
index 54810037..f833c7e9 100644
--- a/src/backend/taler-merchant-httpd_track-transfer.c
+++ b/src/backend/taler-merchant-httpd_track-transfer.c
@@ -881,6 +881,8 @@ proof_cb (void *cls,
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -888,12 +890,12 @@ MH_handler_track_transfer (struct TMH_RequestHandler *rh,
                            struct MHD_Connection *connection,
                            void **connection_cls,
                            const char *upload_data,
-                           size_t *upload_data_size)
+                           size_t *upload_data_size,
+                           const char *instance_id)
 {
   struct TrackTransferContext *rctx;
   const char *str;
   const char *url;
-  const char *instance_str;
   const char *wire_method;
   int ret;
   enum GNUNET_DB_QueryStatus qs;
@@ -973,13 +975,7 @@ MH_handler_track_transfer (struct TMH_RequestHandler *rh,
   }
   rctx->wire_method = GNUNET_strdup (wire_method);
 
-  instance_str = MHD_lookup_connection_value (connection,
-                                              MHD_GET_ARGUMENT_KIND,
-                                              "instance");
-  if (NULL == instance_str)
-    instance_str = "default";
-
-  rctx->mi = TMH_lookup_instance (instance_str);
+  rctx->mi = TMH_lookup_instance (instance_id);
   if (NULL == rctx->mi)
     return TMH_RESPONSE_reply_not_found (connection,
                                          TALER_EC_TRACK_TRANSFER_INSTANCE_UNKNOWN,
diff --git a/src/backend/taler-merchant-httpd_track-transfer.h b/src/backend/taler-merchant-httpd_track-transfer.h
index 13a173c9..8da998dc 100644
--- a/src/backend/taler-merchant-httpd_track-transfer.h
+++ b/src/backend/taler-merchant-httpd_track-transfer.h
@@ -34,6 +34,8 @@
  * @param[in,out] connection_cls the connection's closure (can be updated)
  * @param upload_data upload data
  * @param[in,out] upload_data_size number of bytes (left) in @a upload_data
+ * @param instance_id merchant backend instance ID or NULL is no instance
+ *        has been explicitly specified
  * @return MHD result code
  */
 int
@@ -41,7 +43,8 @@ MH_handler_track_transfer (struct TMH_RequestHandler *rh,
                            struct MHD_Connection *connection,
                            void **connection_cls,
                            const char *upload_data,
-                           size_t *upload_data_size);
+                           size_t *upload_data_size,
+                           const char *instance_id);
 
 
 #endif
diff --git a/src/include/taler_merchant_service.h b/src/include/taler_merchant_service.h
index 82ac7160..f219be61 100644
--- a/src/include/taler_merchant_service.h
+++ b/src/include/taler_merchant_service.h
@@ -66,7 +66,6 @@ struct TALER_MERCHANT_RefundLookupOperation *
 TALER_MERCHANT_refund_lookup (struct GNUNET_CURL_Context *ctx,
                               const char *backend_url,
                               const char *order_id,
-                              const char *instance,
                               TALER_MERCHANT_RefundLookupCallback cb,
                               void *cb_cls);
 
@@ -108,7 +107,6 @@ typedef void
  * @param order_id id of the order whose refund is to be increased
  * @param refund amount to which increase the refund
  * @param reason human-readable reason justifying the refund
- * @param instance id of the merchant instance issuing the request
  * @param cb callback processing the response from /refund
  * @param cb_cls closure for cb
  */
@@ -118,7 +116,6 @@ TALER_MERCHANT_refund_increase (struct GNUNET_CURL_Context *ctx,
                                 const char *order_id,
                                 const struct TALER_Amount *refund,
                                 const char *reason,
-                                const char *instance,
                                 TALER_MERCHANT_RefundIncreaseCallback cb,
                                 void *cb_cls);
 
@@ -227,7 +224,6 @@ struct TALER_MERCHANT_ProposalLookupOperation *
 TALER_MERCHANT_proposal_lookup (struct GNUNET_CURL_Context *ctx,
                                 const char *backend_url,
                                 const char *order_id,
-                                const char *instance,
                                 const struct GNUNET_CRYPTO_EddsaPublicKey *nonce,
                                 TALER_MERCHANT_ProposalLookupOperationCallback plo_cb,
                                 void *plo_cb_cls);
@@ -331,7 +327,6 @@ struct TALER_MERCHANT_PayCoin
  *
  * @param ctx execution context
  * @param merchant_url base URL of the merchant
- * @param instance which merchant instance will receive this payment
  * @param h_wire hash of the merchant’s account details
  * @param h_contract hash of the contact of the merchant with the customer
  * @param transaction_id transaction id for the transaction between merchant and customer
@@ -352,7 +347,6 @@ struct TALER_MERCHANT_PayCoin
 struct TALER_MERCHANT_Pay *
 TALER_MERCHANT_pay_wallet (struct GNUNET_CURL_Context *ctx,
 			   const char *merchant_url,
-			   const char *instance,
                            const struct GNUNET_HashCode *h_contract,
                            const struct TALER_Amount *amount,
                            const struct TALER_Amount *max_fee,
@@ -424,7 +418,6 @@ typedef void
  *
  * @param ctx execution context
  * @param merchant_url base URL of the merchant
- * @param instance which merchant instance will receive this payment
  * @param h_wire hash of the merchant’s account details
  * @param h_contract hash of the contact of the merchant with the customer
  * @param transaction_id transaction id for the transaction between merchant and customer
@@ -445,7 +438,6 @@ typedef void
 struct TALER_MERCHANT_Pay *
 TALER_MERCHANT_pay_abort (struct GNUNET_CURL_Context *ctx,
 			  const char *merchant_url,
-			  const char *instance,
 			  const struct GNUNET_HashCode *h_contract,
 			  const struct TALER_Amount *amount,
 			  const struct TALER_Amount *max_fee,
@@ -524,7 +516,6 @@ struct TALER_MERCHANT_PaidCoin
  *
  * @param ctx execution context
  * @param merchant_url base URL of the merchant
- * @param instance which merchant instance will receive this payment
  * @param h_contract hash of the contact of the merchant with the customer
  * @param amount total value of the contract to be paid to the merchant
  * @param max_fee maximum fee covered by the merchant (according to the contract)
@@ -630,7 +621,6 @@ typedef void
  *
  * @param ctx execution context
  * @param backend_url base URL of the backend
- * @param instance which merchant instance is going to be tracked
  * @param wire_method wire method used for the wire transfer
  * @param wtid base32 string indicating a wtid
  * @param exchange base URL of the exchange in charge of returning the wanted information
@@ -641,7 +631,6 @@ typedef void
 struct TALER_MERCHANT_TrackTransferHandle *
 TALER_MERCHANT_track_transfer (struct GNUNET_CURL_Context *ctx,
                                const char *backend_url,
-                               const char *instance,
 			       const char *wire_method,
                                const struct TALER_WireTransferIdentifierRawP *wtid,
                                const char *exchange_url,
@@ -714,7 +703,6 @@ typedef void
  *
  * @param ctx execution context
  * @param backend_url base URL of the backend
- * @param instance which merchant instance is going to be tracked
  * @param transaction_id which transaction should we trace
  * @param track_transaction_cb the callback to call when a reply for this request is available
  * @param track_transaction_cb_cls closure for @a track_transaction_cb
@@ -723,7 +711,6 @@ typedef void
 struct TALER_MERCHANT_TrackTransactionHandle *
 TALER_MERCHANT_track_transaction (struct GNUNET_CURL_Context *ctx,
                                   const char *backend_url,
-                                  const char *instance,
                                   const char *order_id,
                                   TALER_MERCHANT_TrackTransactionCallback track_transaction_cb,
                                   void *track_transaction_cb_cls);
@@ -763,7 +750,6 @@ typedef void
  *
  * @param ctx execution context
  * @param backend_url base URL of the merchant backend
- * @param instance which merchant instance is performing this call
  * @param start return @a delta records starting from position @a start
  * @param delta return @a delta records starting from position @a start
  * @param date only transactions younger than/equals to date will be returned
@@ -774,7 +760,6 @@ typedef void
 struct TALER_MERCHANT_HistoryOperation *
 TALER_MERCHANT_history (struct GNUNET_CURL_Context *ctx,
                         const char *backend_url,
-                        const char *instance,
                         unsigned long long start,
                         long long delta,
                         struct GNUNET_TIME_Absolute date,
@@ -786,7 +771,6 @@ TALER_MERCHANT_history (struct GNUNET_CURL_Context *ctx,
  *
  * @param ctx execution context
  * @param backend_url base URL of the merchant backend
- * @param instance which merchant instance is performing this call
  * @param start return `delta` records starting from position `start`.
  * If given as zero, then no initial skip of `start` records is done.
  * @param delta return `delta` records starting from position `start`
@@ -798,7 +782,6 @@ TALER_MERCHANT_history (struct GNUNET_CURL_Context *ctx,
 struct TALER_MERCHANT_HistoryOperation *
 TALER_MERCHANT_history_default_start (struct GNUNET_CURL_Context *ctx,
                                       const char *backend_url,
-                                      const char *instance,
                                       long long delta,
                                       struct GNUNET_TIME_Absolute date,
                                       TALER_MERCHANT_HistoryOperationCallback history_cb,
@@ -849,7 +832,6 @@ typedef void
  * @param pickup_url frontend URL for where the tip can be picked up
  * @param next_url where the browser should proceed after picking up the tip
  * @param amount amount to be handed out as a tip
- * @param instance which backend instance should create the tip (identifies the reserve and exchange)
  * @param justification which justification should be stored (human-readable reason for the tip)
  * @param authorize_cb callback which will work the response gotten from the backend
  * @param authorize_cb_cls closure to pass to @a authorize_cb
@@ -861,7 +843,6 @@ TALER_MERCHANT_tip_authorize (struct GNUNET_CURL_Context *ctx,
                               const char *pickup_url,
                               const char *next_url,
                               const struct TALER_Amount *amount,
-                              const char *instance,
                               const char *justification,
                               TALER_MERCHANT_TipAuthorizeCallback authorize_cb,
                               void *authorize_cb_cls);
@@ -980,7 +961,6 @@ typedef void
  *
  * @param ctx execution context
  * @param backend_url base URL of the merchant backend
- * @param instance instance used for the transaction
  * @param order_id order id to identify the payment
  * @parem session_id sesion id for the payment (or NULL if the payment is not bound to a session) 
  * @param check_payment_cb callback which will work the response gotten from the backend
@@ -990,7 +970,6 @@ typedef void
 struct TALER_MERCHANT_CheckPaymentOperation *
 TALER_MERCHANT_check_payment (struct GNUNET_CURL_Context *ctx,
                               const char *backend_url,
-                              const char *instance,
                               const char *order_id,
                               const char *session_id,
                               TALER_MERCHANT_CheckPaymentCallback check_payment_cb,
@@ -1053,13 +1032,11 @@ TALER_MERCHANT_tip_query_cancel (struct TALER_MERCHANT_TipQueryOperation *tqo);
  *
  * @param ctx execution context
  * @param backend_url base URL of the merchant backend
- * @param instance instance to query
  * @return handle for this operation, NULL upon errors
  */
 struct TALER_MERCHANT_TipQueryOperation *
 TALER_MERCHANT_tip_query (struct GNUNET_CURL_Context *ctx,
                           const char *backend_url,
-                          const char *instance,
                           TALER_MERCHANT_TipQueryCallback query_cb,
                           void *query_cb_cls);
 
diff --git a/src/include/taler_merchant_testing_lib.h b/src/include/taler_merchant_testing_lib.h
index 8442bf24..b250af62 100644
--- a/src/include/taler_merchant_testing_lib.h
+++ b/src/include/taler_merchant_testing_lib.h
@@ -73,7 +73,6 @@ TALER_TESTING_run_merchant (const char *config_filename,
  *        the proposal request.
  * @param http_status expected HTTP status.
  * @param order the order to PUT to the merchant.
- * @param instance merchant instance performing the operation.
  *
  * @return the command
  */
@@ -81,8 +80,7 @@ struct TALER_TESTING_Command
 TALER_TESTING_cmd_proposal (const char *label,
                             const char *merchant_url,
                             unsigned int http_status,
-                            const char *order,
-                            const char *instance);
+                            const char *order);
 /**
  * Make a "proposal lookup" command.
  *
@@ -600,7 +598,6 @@ TALER_TESTING_get_trait_refund_entry
  *        the reserve from which the tip is going to be gotten.
  * @param http_status the HTTP response code which is expected
  *        for this operation.
- * @param instance which merchant instance is running this CMD.
  * @param justification human-readable justification for this
  *        tip authorization.
  * @param amount the amount to authorize for tipping.
@@ -612,7 +609,6 @@ TALER_TESTING_cmd_tip_authorize_with_ec
    const char *merchant_url,
    const char *exchange_url,
    unsigned int http_status,
-   const char *instance,
    const char *justification,
    const char *amount,
    enum TALER_ErrorCode ec);
@@ -642,7 +638,6 @@ TALER_TESTING_cmd_tip_authorize_fake
  *        the reserve from which the tip is going to be gotten.
  * @param http_status the HTTP response code which is expected
  *        for this operation.
- * @param instance which merchant instance is running this CMD.
  * @param justification human-readable justification for this
  *        tip authorization.
  * @param amount the amount to authorize for tipping.
@@ -652,7 +647,6 @@ TALER_TESTING_cmd_tip_authorize (const char *label,
                                  const char *merchant_url,
                                  const char *exchange_url,
                                  unsigned int http_status,
-                                 const char *instance,
                                  const char *justification,
                                  const char *amount);
 
@@ -664,13 +658,11 @@ TALER_TESTING_cmd_tip_authorize (const char *label,
  *        server the /tip-query request.
  * @param http_status expected HTTP response code for the
  *        /tip-query request.
- * @param instance the merchant instance running this CMD.
  */
 struct TALER_TESTING_Command
 TALER_TESTING_cmd_tip_query (const char *label,
                              const char *merchant_url,
-                             unsigned int http_status,
-                             const char *instance);
+                             unsigned int http_status);
 /**
  * Define a /tip-query CMD equipped with a expected amount.
  *
@@ -679,7 +671,6 @@ TALER_TESTING_cmd_tip_query (const char *label,
  *        server the /tip-query request.
  * @param http_status expected HTTP response code for the
  *        /tip-query request.
- * @param instance the merchant instance running this CMD.
  * @param expected_amount_picked_up expected amount already
  *        picked up.
  * @param expected_amount_authorized expected amount that was
@@ -691,7 +682,6 @@ TALER_TESTING_cmd_tip_query_with_amounts
   (const char *label,
    const char *merchant_url,
    unsigned int http_status,
-   const char *instance,
    const char *expected_amount_picked_up,
    const char *expected_amount_authorized,
    const char *expected_amount_available);
diff --git a/src/lib/merchant_api_check_payment.c b/src/lib/merchant_api_check_payment.c
index 8c057072..8848686d 100644
--- a/src/lib/merchant_api_check_payment.c
+++ b/src/lib/merchant_api_check_payment.c
@@ -174,7 +174,6 @@ handle_check_payment_finished (void *cls,
  *
  * @param ctx execution context
  * @param backend_url base URL of the merchant backend
- * @param instance instance used for the transaction
  * @param order_id order id to identify the payment
  * @parem session_id sesion id for the payment (or NULL if the payment is not bound to a session) 
  * @param check_payment_cb callback which will work the response gotten from the backend
@@ -184,7 +183,6 @@ handle_check_payment_finished (void *cls,
 struct TALER_MERCHANT_CheckPaymentOperation *
 TALER_MERCHANT_check_payment (struct GNUNET_CURL_Context *ctx,
                               const char *backend_url,
-                              const char *instance,
                               const char *order_id,
                               const char *session_id,
                               TALER_MERCHANT_CheckPaymentCallback check_payment_cb,
@@ -194,15 +192,13 @@ TALER_MERCHANT_check_payment (struct GNUNET_CURL_Context *ctx,
   CURL *eh;
 
   GNUNET_assert (NULL != backend_url);
-  GNUNET_assert (NULL != instance);
   GNUNET_assert (NULL != order_id);
 
   cpo = GNUNET_new (struct TALER_MERCHANT_CheckPaymentOperation);
   cpo->ctx = ctx;
   cpo->cb = check_payment_cb;
   cpo->cb_cls = check_payment_cb_cls;
-  cpo->url = TALER_url_join (backend_url, "/check-payment",
-                             "instance", instance,
+  cpo->url = TALER_url_join (backend_url, "check-payment",
                              "order_id", order_id,
                              "session_id", session_id,
                              NULL);
diff --git a/src/lib/merchant_api_history.c b/src/lib/merchant_api_history.c
index bab8d82a..d1cbfcfa 100644
--- a/src/lib/merchant_api_history.c
+++ b/src/lib/merchant_api_history.c
@@ -153,7 +153,6 @@ history_raw_cb (void *cls,
  *
  * @param ctx execution context
  * @param backend_url base URL of the merchant backend
- * @param instance which merchant instance is performing this call
  * @param start return `delta` records starting from position `start`.
  *        If given as zero, then no initial skip of `start` records is done.
  * @param use_default_start do NOT include the 'start' argument in URL.
@@ -166,7 +165,6 @@ history_raw_cb (void *cls,
 static struct TALER_MERCHANT_HistoryOperation *
 TALER_MERCHANT_history2 (struct GNUNET_CURL_Context *ctx,
                          const char *backend_url,
-                         const char *instance,
                          unsigned long long start,
                          int use_default_start,
                          long long delta,
@@ -184,21 +182,19 @@ TALER_MERCHANT_history2 (struct GNUNET_CURL_Context *ctx,
   ho->cb = history_cb;
   ho->cb_cls = history_cb_cls;
   seconds = date.abs_value_us / 1000LL / 1000LL;
-  base = TALER_url_join (backend_url, "/history", NULL);
+  base = TALER_url_join (backend_url, "history", NULL);
 
   if (GNUNET_YES == use_default_start)
     GNUNET_asprintf (&ho->url,
-                     "%s?date=%llu&instance=%s&delta=%lld",
+                     "%s?date=%llu&delta=%lld",
                      base,
                      seconds,
-                     instance,
                      delta);
   else
     GNUNET_asprintf (&ho->url,
-                     "%s?date=%llu&instance=%s&delta=%lld&start=%llu",
+                     "%s?date=%llu&delta=%lld&start=%llu",
                      base,
                      seconds,
-                     instance,
                      delta,
                      start);
     
@@ -232,7 +228,6 @@ TALER_MERCHANT_history2 (struct GNUNET_CURL_Context *ctx,
  *
  * @param ctx execution context
  * @param backend_url base URL of the merchant backend
- * @param instance which merchant instance is performing this call
  * @param start return `delta` records starting from position
  *        `start`.  If given as zero, then no initial skip of
  *        `start` records is done.
@@ -249,7 +244,6 @@ struct TALER_MERCHANT_HistoryOperation *
 TALER_MERCHANT_history_default_start
   (struct GNUNET_CURL_Context *ctx,
    const char *backend_url,
-   const char *instance,
    long long delta,
    struct GNUNET_TIME_Absolute date,
    TALER_MERCHANT_HistoryOperationCallback history_cb,
@@ -257,7 +251,6 @@ TALER_MERCHANT_history_default_start
 {
   return TALER_MERCHANT_history2 (ctx,
                                   backend_url,
-                                  instance,
                 /* fake 'start' argument: will NOT be used */
                                   -1, 
                 /* Specifies "no start argument" in final URL */
@@ -274,7 +267,6 @@ TALER_MERCHANT_history_default_start
  *
  * @param ctx execution context
  * @param backend_url base URL of the merchant backend
- * @param instance which merchant instance is performing this call
  * @param start return `delta` records starting from position
  *        `start`.  If given as zero, then no initial skip of
  *        `start` records is done.
@@ -291,7 +283,6 @@ struct TALER_MERCHANT_HistoryOperation *
 TALER_MERCHANT_history
   (struct GNUNET_CURL_Context *ctx,
    const char *backend_url,
-   const char *instance,
    unsigned long long start,
    long long delta,
    struct GNUNET_TIME_Absolute date,
@@ -300,7 +291,6 @@ TALER_MERCHANT_history
 {
   return TALER_MERCHANT_history2 (ctx,
                                   backend_url,
-                                  instance,
                                   start,
                                   GNUNET_NO,
                                   delta,
diff --git a/src/lib/merchant_api_pay.c b/src/lib/merchant_api_pay.c
index 0505e950..c7919a04 100644
--- a/src/lib/merchant_api_pay.c
+++ b/src/lib/merchant_api_pay.c
@@ -598,7 +598,7 @@ request_pay_generic
   ph->abort_cb_cls = abort_cb_cls;
   ph->pay_cb = pay_cb;
   ph->pay_cb_cls = pay_cb_cls;
-  ph->url = TALER_url_join (merchant_url, "/public/pay", NULL);
+  ph->url = TALER_url_join (merchant_url, "public/pay", NULL);
   ph->num_coins = num_coins;
   ph->coins = GNUNET_new_array (num_coins,
                                 struct TALER_MERCHANT_PaidCoin);
@@ -639,8 +639,6 @@ request_pay_generic
  *
  * @param ctx the execution loop context
  * @param merchant_url base URL of the merchant's backend
- * @param instance which merchant instance will receive this
- *        payment
  * @param h_contract_terms hashcode of the proposal being paid
  * @param amount total value of the contract to be paid to the
  *        merchant
@@ -668,7 +666,6 @@ request_pay_generic
 static struct TALER_MERCHANT_Pay *
 prepare_pay_generic (struct GNUNET_CURL_Context *ctx,
                      const char *merchant_url,
-                     const char *instance,
                      const struct GNUNET_HashCode *h_contract_terms,
                      const struct TALER_Amount *amount,
                      const struct TALER_Amount *max_fee,
@@ -774,7 +771,6 @@ prepare_pay_generic (struct GNUNET_CURL_Context *ctx,
  *
  * @param ctx the execution loop context
  * @param merchant_url base URL of the merchant's backend
- * @param instance which merchant instance will receive this payment
  * @param h_contract_terms hashcode of the proposal being paid
  * @param amount total value of the contract to be paid to the merchant
  * @param max_fee maximum fee covered by the merchant (according to the contract)
@@ -794,7 +790,6 @@ prepare_pay_generic (struct GNUNET_CURL_Context *ctx,
 struct TALER_MERCHANT_Pay *
 TALER_MERCHANT_pay_wallet (struct GNUNET_CURL_Context *ctx,
                            const char *merchant_url,
-                           const char *instance,
                            const struct GNUNET_HashCode *h_contract_terms,
                            const struct TALER_Amount *amount,
                            const struct TALER_Amount *max_fee,
@@ -812,7 +807,6 @@ TALER_MERCHANT_pay_wallet (struct GNUNET_CURL_Context *ctx,
 {
   return prepare_pay_generic (ctx,
                               merchant_url,
-                              instance,
                               h_contract_terms,
                               amount,
                               max_fee,
@@ -839,7 +833,6 @@ TALER_MERCHANT_pay_wallet (struct GNUNET_CURL_Context *ctx,
  *
  * @param ctx execution context
  * @param merchant_url base URL of the merchant
- * @param instance which merchant instance will receive this payment
  * @param h_wire hash of the merchant’s account details
  * @param h_contract hash of the contact of the merchant with the customer
  * @param transaction_id transaction id for the transaction between merchant and customer
@@ -860,7 +853,6 @@ TALER_MERCHANT_pay_wallet (struct GNUNET_CURL_Context *ctx,
 struct TALER_MERCHANT_Pay *
 TALER_MERCHANT_pay_abort (struct GNUNET_CURL_Context *ctx,
                           const char *merchant_url,
-                          const char *instance,
                           const struct GNUNET_HashCode *h_contract,
                           const struct TALER_Amount *amount,
                           const struct TALER_Amount *max_fee,
@@ -880,7 +872,6 @@ TALER_MERCHANT_pay_abort (struct GNUNET_CURL_Context *ctx,
 
   ph = prepare_pay_generic (ctx,
                             merchant_url,
-                            instance,
                             h_contract,
                             amount,
                             max_fee,
diff --git a/src/lib/merchant_api_proposal.c b/src/lib/merchant_api_proposal.c
index 25552bc0..83371eda 100644
--- a/src/lib/merchant_api_proposal.c
+++ b/src/lib/merchant_api_proposal.c
@@ -229,7 +229,7 @@ TALER_MERCHANT_order_put
   po->ctx = ctx;
   po->cb = proposal_cb;
   po->cb_cls = proposal_cb_cls;
-  po->url = TALER_url_join (backend_url, "/order", NULL);
+  po->url = TALER_url_join (backend_url, "order", NULL);
   req = json_pack ("{s:O}",
                    "order", (json_t *) order);
   eh = curl_easy_init ();
@@ -366,7 +366,6 @@ struct TALER_MERCHANT_ProposalLookupOperation *
 TALER_MERCHANT_proposal_lookup (struct GNUNET_CURL_Context *ctx,
                                 const char *backend_url,
                                 const char *order_id,
-                                const char *instance,
                                 const struct
                                 GNUNET_CRYPTO_EddsaPublicKey *nonce,
                                 TALER_MERCHANT_ProposalLookupOperationCallback
@@ -375,38 +374,25 @@ TALER_MERCHANT_proposal_lookup (struct GNUNET_CURL_Context *ctx,
 {
   struct TALER_MERCHANT_ProposalLookupOperation *plo;
   CURL *eh;
-  char *base;
+  char *nonce_str = NULL;
 
   plo = GNUNET_new (struct TALER_MERCHANT_ProposalLookupOperation);
   plo->ctx = ctx;
   plo->cb = plo_cb;
   plo->cb_cls = plo_cb_cls;
-  base = TALER_url_join (backend_url, "/public/proposal", NULL);
   if (NULL != nonce)
   {
-    char *nonce_str;
     plo->has_nonce = GNUNET_YES;
     plo->nonce = *nonce;
     nonce_str = GNUNET_STRINGS_data_to_string_alloc (nonce, sizeof (struct
                                                                     GNUNET_CRYPTO_EddsaPublicKey));
-    GNUNET_assert (NULL != nonce_str);
-    GNUNET_asprintf (&plo->url,
-                     "%s?order_id=%s&instance=%s&nonce=%s",
-                     base,
-                     order_id,
-                     instance,
-                     nonce_str);
-    GNUNET_free (nonce_str);
   }
-  else
-  {
-    GNUNET_asprintf (&plo->url,
-                     "%s?order_id=%s&instance=%s",
-                     base,
-                     order_id,
-                     instance);
-  }
-  GNUNET_free (base);
+  plo->url = TALER_url_join (backend_url, "public/proposal",
+                             "order_id",
+                             order_id,
+                             "nonce",
+                             nonce_str,
+                             NULL);
   GNUNET_log (GNUNET_ERROR_TYPE_INFO,
               "looking up proposal from %s\n",
               plo->url);
diff --git a/src/lib/merchant_api_refund.c b/src/lib/merchant_api_refund.c
index 76a01e5d..3f888099 100644
--- a/src/lib/merchant_api_refund.c
+++ b/src/lib/merchant_api_refund.c
@@ -190,7 +190,6 @@ TALER_MERCHANT_refund_increase_cancel (struct TALER_MERCHANT_RefundIncreaseOpera
  * @param order_id id of the order whose refund is to be increased
  * @param refund amount to which increase the refund
  * @param reason human-readable reason justifying the refund
- * @param instance id of the merchant instance issuing the request
  * @param cb callback processing the response from /refund
  * @param cb_cls closure for cb
  */
@@ -200,7 +199,6 @@ TALER_MERCHANT_refund_increase (struct GNUNET_CURL_Context *ctx,
                                 const char *order_id,
                                 const struct TALER_Amount *refund,
                                 const char *reason,
-                                const char *instance,
                                 TALER_MERCHANT_RefundIncreaseCallback cb,
                                 void *cb_cls)
 {
@@ -212,12 +210,11 @@ TALER_MERCHANT_refund_increase (struct GNUNET_CURL_Context *ctx,
   rio->ctx = ctx;
   rio->cb = cb;
   rio->cb_cls = cb_cls;
-  rio->url = TALER_url_join (backend_url, "/refund", NULL);
-  req = json_pack ("{s:o, s:s, s:s, s:s}",
+  rio->url = TALER_url_join (backend_url, "refund", NULL);
+  req = json_pack ("{s:o, s:s, s:s}",
                    "refund", TALER_JSON_from_amount (refund),
                    "order_id", order_id,
-                   "reason", reason,
-                   "instance", instance);
+                   "reason", reason);
   eh = curl_easy_init ();
 
   if (GNUNET_OK != TALER_curl_easy_post (&rio->post_ctx,
@@ -333,26 +330,18 @@ struct TALER_MERCHANT_RefundLookupOperation *
 TALER_MERCHANT_refund_lookup (struct GNUNET_CURL_Context *ctx,
                               const char *backend_url,
                               const char *order_id,
-                              const char *instance,
                               TALER_MERCHANT_RefundLookupCallback cb,
                               void *cb_cls)
 {
   struct TALER_MERCHANT_RefundLookupOperation *rlo;
   CURL *eh;
-  char *base;
 
   rlo = GNUNET_new (struct TALER_MERCHANT_RefundLookupOperation);
   rlo->ctx = ctx;
   rlo->cb = cb;
   rlo->cb_cls = cb_cls;
 
-  base = TALER_url_join (backend_url, "/public/refund", NULL);
-  GNUNET_asprintf (&rlo->url,
-                   "%s?instance=%s&order_id=%s",
-                   base,
-                   instance,
-                   order_id);
-  GNUNET_free (base);
+  rlo->url = TALER_url_join (backend_url, "public/refund", "order_id", order_id, NULL);
   eh = curl_easy_init ();
   if (CURLE_OK != curl_easy_setopt (eh,
                                     CURLOPT_URL,
diff --git a/src/lib/merchant_api_tip_authorize.c b/src/lib/merchant_api_tip_authorize.c
index f79064a6..582c9a4e 100644
--- a/src/lib/merchant_api_tip_authorize.c
+++ b/src/lib/merchant_api_tip_authorize.c
@@ -179,7 +179,6 @@ handle_tip_authorize_finished (void *cls,
  * @param pickup_url frontend URL for where the tip can be picked up
  * @param next_url where the browser should proceed after picking up the tip
  * @param amount amount to be handed out as a tip
- * @param instance which backend instance should create the tip (identifies the reserve and exchange)
  * @param justification which justification should be stored (human-readable reason for the tip)
  * @param authorize_cb callback which will work the response gotten from the backend
  * @param authorize_cb_cls closure to pass to @a authorize_cb
@@ -191,7 +190,6 @@ TALER_MERCHANT_tip_authorize (struct GNUNET_CURL_Context *ctx,
                               const char *pickup_url,
                               const char *next_url,
                               const struct TALER_Amount *amount,
-                              const char *instance,
                               const char *justification,
                               TALER_MERCHANT_TipAuthorizeCallback authorize_cb,
                               void *authorize_cb_cls)
@@ -204,16 +202,14 @@ TALER_MERCHANT_tip_authorize (struct GNUNET_CURL_Context *ctx,
   tao->ctx = ctx;
   tao->cb = authorize_cb;
   tao->cb_cls = authorize_cb_cls;
-  tao->url = TALER_url_join (backend_url, "/tip-authorize", NULL);
+  tao->url = TALER_url_join (backend_url, "tip-authorize", NULL);
   te_obj = json_pack ("{"
                       " s:o," /* amount */
-                      " s:s," /* instance */
                       " s:s," /* justification */
                       " s:s," /* pickup_url */
                       " s:s," /* next_url */
                       "}",
                       "amount", TALER_JSON_from_amount (amount),
-                      "instance", instance,
                       "justification", justification,
                       "pickup_url", pickup_url,
                       "next_url", next_url);
diff --git a/src/lib/merchant_api_tip_pickup.c b/src/lib/merchant_api_tip_pickup.c
index e49151bc..116b994c 100644
--- a/src/lib/merchant_api_tip_pickup.c
+++ b/src/lib/merchant_api_tip_pickup.c
@@ -288,7 +288,7 @@ TALER_MERCHANT_tip_pickup (struct GNUNET_CURL_Context *ctx,
   tpo->cb_cls = pickup_cb_cls;
 
   tpo->url = TALER_url_join (backend_url,
-                             "/public/tip-pickup",
+                             "public/tip-pickup",
                              NULL);
   eh = curl_easy_init ();
   if (GNUNET_OK != TALER_curl_easy_post (&tpo->post_ctx,
diff --git a/src/lib/merchant_api_tip_query.c b/src/lib/merchant_api_tip_query.c
index d02de8b1..3887390b 100644
--- a/src/lib/merchant_api_tip_query.c
+++ b/src/lib/merchant_api_tip_query.c
@@ -187,13 +187,11 @@ handle_tip_query_finished (void *cls,
  *
  * @param ctx execution context
  * @param backend_url base URL of the merchant backend
- * @param instance instance to query
  * @return handle for this operation, NULL upon errors
  */
 struct TALER_MERCHANT_TipQueryOperation *
 TALER_MERCHANT_tip_query (struct GNUNET_CURL_Context *ctx,
                           const char *backend_url,
-                          const char *instance,
                           TALER_MERCHANT_TipQueryCallback query_cb,
                           void *query_cb_cls)
 {
@@ -204,8 +202,7 @@ TALER_MERCHANT_tip_query (struct GNUNET_CURL_Context *ctx,
   tqo->ctx = ctx;
   tqo->cb = query_cb;
   tqo->cb_cls = query_cb_cls;
-  tqo->url = TALER_url_join (backend_url, "/tip-query",
-                             "instance", instance,
+  tqo->url = TALER_url_join (backend_url, "tip-query",
                              NULL);
 
   GNUNET_log (GNUNET_ERROR_TYPE_INFO,
diff --git a/src/lib/merchant_api_track_transaction.c b/src/lib/merchant_api_track_transaction.c
index 32393ece..e5cac2bb 100644
--- a/src/lib/merchant_api_track_transaction.c
+++ b/src/lib/merchant_api_track_transaction.c
@@ -134,7 +134,6 @@ handle_track_transaction_finished (void *cls,
  *
  * @param ctx execution context
  * @param backend_url base URL of the backend
- * @param instance which merchant instance is going to be tracked
  * @param order_id order id pointing to the transaction being tracked
  * @param track_transaction_cb the callback to call when a reply for this request is available
  * @param track_transaction_cb_cls closure for @a track_transaction_cb
@@ -143,26 +142,19 @@ handle_track_transaction_finished (void *cls,
 struct TALER_MERCHANT_TrackTransactionHandle *
 TALER_MERCHANT_track_transaction (struct GNUNET_CURL_Context *ctx,
                                   const char *backend_url,
-                                  const char *instance,
                                   const char *order_id,
                                   TALER_MERCHANT_TrackTransactionCallback track_transaction_cb,
                                   void *track_transaction_cb_cls)
 {
   struct TALER_MERCHANT_TrackTransactionHandle *tdo;
   CURL *eh;
-  char *base;
 
   tdo = GNUNET_new (struct TALER_MERCHANT_TrackTransactionHandle);
   tdo->ctx = ctx;
   tdo->cb = track_transaction_cb;
   tdo->cb_cls = track_transaction_cb_cls;
-  base = TALER_url_join (backend_url, "/track/transaction", NULL);
-  GNUNET_asprintf (&tdo->url,
-                   "%s?order_id=%s&instance=%s",
-                   base,
-                   order_id,
-                   instance);
-  GNUNET_free (base);
+  tdo->url = TALER_url_join (backend_url, "track/transaction",
+                             "order_id", order_id, NULL);
   GNUNET_log (GNUNET_ERROR_TYPE_DEBUG,
               "Requesting URL '%s'\n",
               tdo->url);
diff --git a/src/lib/merchant_api_track_transfer.c b/src/lib/merchant_api_track_transfer.c
index b5105c54..df303b1b 100644
--- a/src/lib/merchant_api_track_transfer.c
+++ b/src/lib/merchant_api_track_transfer.c
@@ -219,7 +219,6 @@ handle_track_transfer_finished (void *cls,
  *
  * @param ctx execution context
  * @param backend_url base URL of the backend
- * @param instance which merchant instance is going to be tracked
  * @param wire_method wire method used for the wire transfer
  * @param wtid base32 string indicating a wtid
  * @param exchange_url base URL of the exchange in charge of returning the wanted information
@@ -230,7 +229,6 @@ handle_track_transfer_finished (void *cls,
 struct TALER_MERCHANT_TrackTransferHandle *
 TALER_MERCHANT_track_transfer (struct GNUNET_CURL_Context *ctx,
                                const char *backend_url,
-                               const char *instance,
                                const char *wire_method,
                                const struct
                                TALER_WireTransferIdentifierRawP *wtid,
@@ -250,10 +248,9 @@ TALER_MERCHANT_track_transfer (struct GNUNET_CURL_Context *ctx,
   tdo->ctx = ctx;
   tdo->cb = track_transfer_cb; // very last to be called
   tdo->cb_cls = track_transfer_cb_cls;
-  tdo->url = TALER_url_join (backend_url, "/track/transfer",
+  tdo->url = TALER_url_join (backend_url, "track/transfer",
                              "wtid", wtid_str,
                              "exchange", exchange_url,
-                             "instance", instance,
                              "wire_method", wire_method,
                              NULL);
   GNUNET_free (wtid_str);
diff --git a/src/lib/test_merchant_api.c b/src/lib/test_merchant_api.c
index 67df2b93..fa7a385d 100644
--- a/src/lib/test_merchant_api.c
+++ b/src/lib/test_merchant_api.c
@@ -62,6 +62,26 @@ static char *fakebank_url;
 static char *merchant_url;
 
 /**
+ * Merchant base URL for the tipping instance.
+ */
+static char *merchant_tip_instance_url;
+
+/**
+ * Merchant base URL for the tipping instance.
+ */
+static char *merchant_tip_instance_2_url;
+
+/**
+ * Merchant base URL for the tipping instance.
+ */
+static char *merchant_tip_instance_nulltip_url;
+
+/**
+ * Merchant base URL for a non-existent instance.
+ */
+static char *merchant_tip_unknown_instance_url;
+
+/**
  * Merchant process.
  */
 static struct GNUNET_OS_Process *merchantd;
@@ -212,8 +232,7 @@ run (void *cls,
         \"summary\": \"merchant-lib testcase\",\
         \"fulfillment_url\": \"https://example.com/\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:5}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:5}\"} ] }"),
 
     TALER_TESTING_cmd_check_payment ("check-payment-1",
                                      merchant_url,
@@ -287,8 +306,7 @@ run (void *cls,
         \"summary\": \"useful product\",\
         \"fulfillment_url\": \"https://example.com/\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:5}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:5}\"} ] }"),
 
     TALER_TESTING_cmd_proposal_lookup ("fetch-proposal-2",
                                        merchant_url,
@@ -485,8 +503,7 @@ run (void *cls,
         \"summary\": \"useful product\",\
         \"fulfillment_url\": \"https://example.com/\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:5}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:5}\"} ] }"),
 
     /* Try to increase a non paid proposal.  */
     TALER_TESTING_cmd_refund_increase
@@ -551,8 +568,7 @@ run (void *cls,
         \"summary\": \"merchant-lib testcase\",\
         \"fulfillment_url\": \"https://example.com/\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:5}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:5}\"} ] }"),
 
     TALER_TESTING_cmd_pay ("pay-unincreased-proposal",
                            merchant_url,
@@ -613,18 +629,16 @@ run (void *cls,
        "EUR:20.04", USER_ACCOUNT_NO, EXCHANGE_ACCOUNT_NO),
 
     TALER_TESTING_cmd_tip_authorize ("authorize-tip-1",
-                                     merchant_url,
+                                     merchant_tip_instance_url,
                                      exchange_url,
                                      MHD_HTTP_OK,
-                                     "tip",
                                      "tip 1",
                                      "EUR:5.01"),
 
     TALER_TESTING_cmd_tip_authorize ("authorize-tip-2",
-                                     merchant_url,
+                                     merchant_tip_instance_url,
                                      exchange_url,
                                      MHD_HTTP_OK,
-                                     "tip",
                                      "tip 2",
                                      "EUR:5.01"),
 
@@ -635,51 +649,46 @@ run (void *cls,
      * actually create a reserve.  */
     TALER_TESTING_cmd_tip_authorize_with_ec
       ("authorize-tip-null",
-       merchant_url,
+       merchant_tip_instance_nulltip_url,
        exchange_url,
        MHD_HTTP_NOT_FOUND,
-       "nulltip",
        "tip 2",
        "EUR:5.01",
        TALER_EC_RESERVE_STATUS_UNKNOWN),
 
     TALER_TESTING_cmd_tip_query ("query-tip-1",
-                                 merchant_url,
-                                 MHD_HTTP_OK,
-                                 "tip"),
+                                 merchant_tip_instance_url,
+                                 MHD_HTTP_OK),
 
     TALER_TESTING_cmd_tip_query_with_amounts ("query-tip-2",
-                                              merchant_url,
+                                              merchant_tip_instance_url,
                                               MHD_HTTP_OK,
-                                              "tip",
                                               "EUR:0.0", // picked
                                               "EUR:10.02", // auth
                                               "EUR:20.04"),// ava
 
     TALER_TESTING_cmd_tip_pickup ("pickup-tip-1",
-                                  merchant_url,
+                                  merchant_tip_instance_url,
                                   MHD_HTTP_OK,
                                   "authorize-tip-1",
                                   pickup_amounts_1),
 
     TALER_TESTING_cmd_tip_query_with_amounts ("query-tip-3",
-                                              merchant_url,
+                                              merchant_tip_instance_url,
                                               MHD_HTTP_OK,
-                                              "tip",
                                               "EUR:5.01", // picked
                                               NULL, // auth
                                               "EUR:15.03"),// ava
 
     TALER_TESTING_cmd_tip_pickup ("pickup-tip-2",
-                                  merchant_url,
+                                  merchant_tip_instance_url,
                                   MHD_HTTP_OK,
                                   "authorize-tip-2",
                                   pickup_amounts_1),
 
     TALER_TESTING_cmd_tip_query_with_amounts ("query-tip-4",
-                                              merchant_url,
+                                              merchant_tip_instance_url,
                                               MHD_HTTP_OK,
-                                              "tip",
                                               "EUR:10.02", // pick
                                               "EUR:10.02", // auth
                                               "EUR:10.02"), // ava
@@ -708,20 +717,18 @@ run (void *cls,
 
     TALER_TESTING_cmd_tip_authorize_with_ec
       ("authorize-tip-3-insufficient-funds",
-       merchant_url,
+       merchant_tip_instance_2_url,
        exchange_url,
        MHD_HTTP_PRECONDITION_FAILED,
-       "dtip",
        "tip 3",
        "EUR:2.02",
        TALER_EC_TIP_AUTHORIZE_INSUFFICIENT_FUNDS),
 
     TALER_TESTING_cmd_tip_authorize_with_ec
       ("authorize-tip-4-unknown-instance",
-       merchant_url,
+       merchant_tip_unknown_instance_url,
        exchange_url,
        MHD_HTTP_NOT_FOUND,
-       "unknown",
        "tip 4",
        "EUR:5.01",
        TALER_EC_TIP_AUTHORIZE_INSTANCE_UNKNOWN),
@@ -731,14 +738,13 @@ run (void *cls,
        merchant_url,
        exchange_url,
        MHD_HTTP_NOT_FOUND,
-       "default",
        "tip 5",
        "EUR:5.01",
        TALER_EC_TIP_AUTHORIZE_INSTANCE_DOES_NOT_TIP),
 
     TALER_TESTING_cmd_tip_pickup_with_ec
       ("pickup-tip-3-too-much",
-       merchant_url,
+       merchant_tip_instance_url,
        MHD_HTTP_CONFLICT,
        "authorize-tip-1",
        pickup_amounts_1,
@@ -749,7 +755,7 @@ run (void *cls,
 
     TALER_TESTING_cmd_tip_pickup_with_ec
       ("pickup-non-existent-id",
-       merchant_url,
+       merchant_tip_instance_url,
        MHD_HTTP_NOT_FOUND,
        "fake-tip-authorization",
        pickup_amounts_1,
@@ -757,7 +763,7 @@ run (void *cls,
 
     TALER_TESTING_cmd_proposal
       ("create-proposal-tip-1",
-       merchant_url,
+       merchant_tip_instance_url,
        MHD_HTTP_OK,
        "{\"max_fee\":\
           {\"currency\":\"EUR\",\
@@ -773,11 +779,10 @@ run (void *cls,
         \"summary\": \"useful product\",\
         \"fulfillment_url\": \"https://example.com/\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:5}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:5}\"} ] }"),
 
     TALER_TESTING_cmd_pay ("deposit-tip-simple",
-                           merchant_url,
+                           merchant_tip_instance_url,
                            MHD_HTTP_OK,
                            "create-proposal-tip-1",
                            "pickup-tip-1",
@@ -849,8 +854,7 @@ run (void *cls,
         \"summary\": \"merchant-lib testcase\",\
         \"fulfillment_url\": \"https://example.com/\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:10}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:10}\"} ] }"),
 
     TALER_TESTING_cmd_pay ("pay-fail-partial-double-10",
                            merchant_url,
@@ -929,8 +933,7 @@ run (void *cls,
         \"summary\": \"merchant-lib testcase\",\
         \"fulfillment_url\": \"https://example.com/\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:10}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:10}\"} ] }"),
 
     TALER_TESTING_cmd_pay ("pay-fail-partial-double-11-good",
                            merchant_url,
@@ -1008,7 +1011,7 @@ run (void *cls,
        merchant_url,
        MHD_HTTP_OK,
        GNUNET_TIME_UNIT_ZERO_ABS,
-       5, /* Expected number of records */
+       4, /* Expected number of records */
        -100), /* Delta */
     /**
      * End the suite.  Fixme: better to have a label for this
@@ -1043,6 +1046,19 @@ main (int argc,
       (merchant_url = TALER_TESTING_prepare_merchant (CONFIG_FILE)))
     return 77;
 
+  merchant_tip_instance_url = TALER_url_join (merchant_url,
+                                              "instances/tip/",
+                                              NULL);
+  merchant_tip_instance_2_url = TALER_url_join (merchant_url,
+                                                "instances/dtip/",
+                                                NULL);
+  merchant_tip_instance_nulltip_url = TALER_url_join (merchant_url,
+                                                      "instances/nulltip/",
+                                                      NULL);
+  merchant_tip_unknown_instance_url = TALER_url_join (merchant_url,
+                                                      "instances/foo/",
+                                                      NULL);
+
   TALER_TESTING_cleanup_files (CONFIG_FILE);
 
   switch (TALER_TESTING_prepare_exchange (CONFIG_FILE,
diff --git a/src/lib/test_merchant_api_twisted.c b/src/lib/test_merchant_api_twisted.c
index 43f3ef6f..901823f4 100644
--- a/src/lib/test_merchant_api_twisted.c
+++ b/src/lib/test_merchant_api_twisted.c
@@ -75,6 +75,16 @@ static char *twister_exchange_url;
 static char *twister_merchant_url;
 
 /**
+ * Twister URL that proxies the merchant.
+ */
+static char *twister_merchant_url_instance_nonexistent;
+
+/**
+ * Twister URL that proxies the merchant.
+ */
+static char *twister_merchant_url_instance_tor;
+
+/**
  * URL of the fakebank.  Obtained from CONFIG_FILE's
  * "exchange-wire-test:BANK_URI" option.
  */
@@ -238,8 +248,7 @@ run (void *cls,
            \"fraction\":0},\
         \"summary\": \"merchant-lib testcase\",\
         \"products\": [ {\"description\":\"triggering bug 5719\",\
-                         \"value\":\"{EUR:1}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:1}\"} ] }"),
 
     /**
      * Instruct the Twister to malform the response given by
@@ -285,8 +294,7 @@ run (void *cls,
            \"fraction\":0},\
         \"summary\": \"merchant-lib testcase\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:3}\"} ] }",
-       NULL),
+                         \"value\":\"{EUR:3}\"} ] }"),
 
     /* Need any response code != 200.  */
     TALER_TESTING_cmd_hack_response_code
@@ -344,8 +352,7 @@ run (void *cls,
        MHD_HTTP_BAD_REQUEST,
        /* giving a valid JSON to not make it fail before
         * data reaches the merchant.  */
-       "{\"not\": \"used\"}",
-       NULL),
+       "{\"not\": \"used\"}"),
   
     TALER_TESTING_cmd_hack_response_code
       ("proposal-500",
@@ -360,8 +367,7 @@ run (void *cls,
        MHD_HTTP_INTERNAL_SERVER_ERROR,
        /* giving a valid JSON to not make it fail before
         * data reaches the merchant.  */
-       "{\"not\": \"used\"}",
-       NULL),
+       "{\"not\": \"used\"}"),
   
     /**
      * Cause the PUT /proposal callback to be called
@@ -390,8 +396,7 @@ run (void *cls,
            \"fraction\":0},\
         \"summary\": \"merchant-lib testcase\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:5}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:5}\"} ] }"),
     /**
      * Cause proposal to be invalid: this is achieved
      * by deleting the "order_id" field of it.
@@ -417,20 +422,18 @@ run (void *cls,
             \"fraction\":0},\
          \"summary\": \"merchant-lib testcase\",\
          \"products\": [ {\"description\":\"ice cream\",\
-                          \"value\":\"{EUR:5}\"} ] }",
-         NULL),
+                          \"value\":\"{EUR:5}\"} ] }"),
     /**
      * Cause a 404 Not Found response code,
      * due to a non existing merchant instance.
      */
     TALER_TESTING_cmd_proposal
       ("create-proposal-4",
-       twister_merchant_url,
+       twister_merchant_url_instance_nonexistent,
        MHD_HTTP_NOT_FOUND,
        "{\"amount\":\"EUR:5\",\
          \"fulfillment_url\": \"https://example.com/\",\
-         \"summary\": \"merchant-lib testcase\"}",
-       "non-existent-instance"),
+         \"summary\": \"merchant-lib testcase\"}"),
 
     /* Cause a 404 Not Found from /proposal/lookup,
      * due to a non existing order id being queried.  */
@@ -476,8 +479,7 @@ run (void *cls,
         \"fulfillment_url\": \"https://example.com/\",\
         \"summary\": \"merchant-lib testcase\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:5}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:5}\"} ] }"),
 
     /* Remove expected field.  */
     TALER_TESTING_cmd_delete_object ("remove-contract-terms",
@@ -569,7 +571,11 @@ run (void *cls,
 
     TALER_TESTING_cmd_proposal
       ("create-proposal-unaggregation",
-       twister_merchant_url,
+       /* Need a fresh instance in order to associate this
+        * proposal with a fresh h_wire;  this way, this proposal
+        * won't get hooked by the aggregator gathering same-H_wire'd
+        * transactions.  */
+       twister_merchant_url_instance_tor,
        MHD_HTTP_OK,
        "{\"max_fee\":\
           {\"currency\":\"EUR\",\
@@ -578,7 +584,6 @@ run (void *cls,
         \"refund_deadline\":\"\\/Date(2)\\/\",\
         \"pay_deadline\":\"\\/Date(1)\\/\",\
         \"wire_transfer_delay\":\"\\/Delay(30000)\\/\",\
-        \"instance\":\"tor\",\
         \"amount\":\
           {\"currency\":\"EUR\",\
            \"value\":5,\
@@ -586,12 +591,7 @@ run (void *cls,
         \"summary\": \"unaggregated product\",\
         \"fulfillment_url\": \"https://example.com/\",\
         \"products\": [ {\"description\":\"unaggregated cream\",\
-                         \"value\":\"{EUR:5}\"} ] }",
-        /* Need a fresh instance in order to associate this
-         * proposal with a fresh h_wire;  this way, this proposal
-         * won't get hooked by the aggregator gathering same-H_wire'd
-         * transactions.  */
-        "tor"),
+                         \"value\":\"{EUR:5}\"} ] }"),
 
     TALER_TESTING_cmd_pay
       ("pay-unaggregation",
@@ -652,8 +652,7 @@ run (void *cls,
            \"fraction\":0},\
         \"summary\": \"merchant-lib testcase\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:2}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:2}\"} ] }"),
     TALER_TESTING_cmd_pay ("deposit-simple-5383",
                            twister_merchant_url,
                            MHD_HTTP_OK,
@@ -744,8 +743,7 @@ run (void *cls,
            \"fraction\":0},\
         \"summary\": \"merchant-lib testcase\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:3}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:3}\"} ] }"),
 
     TALER_TESTING_cmd_check_payment ("check-payment-1",
                                      twister_merchant_url,
@@ -862,8 +860,7 @@ run (void *cls,
            \"fraction\":0},\
         \"summary\": \"merchant-lib testcase\",\
         \"products\": [ {\"description\":\"ice cream\",\
-                         \"value\":\"{EUR:3}\"} ] }",
-        NULL),
+                         \"value\":\"{EUR:3}\"} ] }"),
 
     /* Will only pay _half_ the supposed price,
      * so we'll then have the right to abort.  */
@@ -948,8 +945,7 @@ run (void *cls,
            \"value\":1,\
            \"fraction\":0},\
         \"summary\": \"merchant-lib testcase\",\
-        \"products\": [ {\"description\": \"will succeed\"}] }",
-        NULL),
+        \"products\": [ {\"description\": \"will succeed\"}] }"),
 
     TALER_TESTING_cmd_proposal
       ("create-proposal-double-spend-1",
@@ -968,8 +964,7 @@ run (void *cls,
            \"value\":1,\
            \"fraction\":0},\
         \"summary\": \"merchant-lib testcase\",\
-        \"products\": [ {\"description\": \"will fail\"}] }",
-        NULL),
+        \"products\": [ {\"description\": \"will fail\"}] }"),
 
     TALER_TESTING_cmd_withdraw_amount
       ("withdraw-coin-double-spend",
@@ -1087,6 +1082,9 @@ main (int argc,
       (PROXY_MERCHANT_CONFIG_FILE)))
     return 77;
 
+  twister_merchant_url_instance_nonexistent = TALER_url_join (twister_exchange_url, "instances/foo/", NULL);
+  twister_merchant_url_instance_tor = TALER_url_join (twister_exchange_url, "instances/tor/", NULL);
+
   TALER_TESTING_cleanup_files (CONFIG_FILE);
 
   switch (TALER_TESTING_prepare_exchange (CONFIG_FILE,
diff --git a/src/lib/testing_api_cmd_history.c b/src/lib/testing_api_cmd_history.c
index c9792680..6704fb93 100644
--- a/src/lib/testing_api_cmd_history.c
+++ b/src/lib/testing_api_cmd_history.c
@@ -43,11 +43,6 @@ struct HistoryState
   unsigned int http_status;
 
   /**
-   * The merchant instance executing this CMD.
-   */
-  const char *instance;
-
-  /**
    * URL of the merchant backend serving the /history request.
    */
   const char *merchant_url;
@@ -272,7 +267,6 @@ history_run (void *cls,
     hs->ho = TALER_MERCHANT_history_default_start
       (is->ctx,
        hs->merchant_url,
-       "default",
        hs->nrows,
        hs->time,
        &history_cb,
@@ -282,7 +276,6 @@ history_run (void *cls,
     case GNUNET_NO:
     hs->ho = TALER_MERCHANT_history (is->ctx,
                                      hs->merchant_url,
-                                     "default",
                                      hs->start,
                                      hs->nrows,
                                      hs->time,
diff --git a/src/lib/testing_api_cmd_pay.c b/src/lib/testing_api_cmd_pay.c
index 7f1c82a7..3be5d8dd 100644
--- a/src/lib/testing_api_cmd_pay.c
+++ b/src/lib/testing_api_cmd_pay.c
@@ -386,7 +386,6 @@ check_payment_run (void *cls,
   cps->cpo = TALER_MERCHANT_check_payment
                (is->ctx,
                cps->merchant_url,
-               "default", // only default instance for now.
                order_id,
                NULL,
                check_payment_cb,
@@ -854,7 +853,6 @@ _pay_run (const char *merchant_url,
 
   ret = api_func (is->ctx,
                   merchant_url,
-                  "default", // instance
                   h_proposal,
                   &total_amount,
                   &max_fee,
diff --git a/src/lib/testing_api_cmd_proposal.c b/src/lib/testing_api_cmd_proposal.c
index f095f38f..f5e282c9 100644
--- a/src/lib/testing_api_cmd_proposal.c
+++ b/src/lib/testing_api_cmd_proposal.c
@@ -79,11 +79,6 @@ struct ProposalState
   struct GNUNET_CRYPTO_EddsaPublicKey nonce;
 
   /**
-   * The merchant instance.
-   */
-  const char *instance;
-
-  /**
    * URL of the merchant backend.
    */
   const char *merchant_url;
@@ -318,7 +313,6 @@ proposal_cb (void *cls,
                    (ps->is->ctx,
                    ps->merchant_url,
                    ps->order_id,
-                   ps->instance,
                    &ps->nonce,
                    &proposal_lookup_initial_cb,
                    ps)))
@@ -376,18 +370,6 @@ proposal_run (void *cls,
     (GNUNET_CRYPTO_QUALITY_WEAK,
     &ps->nonce,
     sizeof (struct GNUNET_CRYPTO_EddsaPublicKey));
-  if (NULL != ps->instance)
-  {
-    json_t *merchant;
-
-    merchant = json_object ();
-    json_object_set_new (merchant,
-                         "instance",
-                         json_string (ps->instance));
-    json_object_set_new (order,
-                         "merchant",
-                         merchant);
-  }
 
   ps->po = TALER_MERCHANT_order_put (is->ctx,
                                      ps->merchant_url,
@@ -468,7 +450,6 @@ proposal_lookup_cleanup (void *cls,
  *        the proposal request.
  * @param http_status expected HTTP status.
  * @param order the order to PUT to the merchant.
- * @param instance merchant instance performing the operation.
  *
  * @return the command
  */
@@ -476,8 +457,7 @@ struct TALER_TESTING_Command
 TALER_TESTING_cmd_proposal (const char *label,
                             const char *merchant_url,
                             unsigned int http_status,
-                            const char *order,
-                            const char *instance)
+                            const char *order)
 {
   struct ProposalState *ps;
 
@@ -485,7 +465,6 @@ TALER_TESTING_cmd_proposal (const char *label,
   ps->order = order;
   ps->http_status = http_status;
   ps->merchant_url = merchant_url;
-  ps->instance = (NULL == instance) ? "default" : instance;
 
   struct TALER_TESTING_Command cmd = {
     .cls = ps,
@@ -580,7 +559,6 @@ proposal_lookup_run (void *cls,
   pls->plo = TALER_MERCHANT_proposal_lookup (is->ctx,
                                              pls->merchant_url,
                                              order_id,
-                                             "default",
                                              nonce,
                                              &proposal_lookup_cb,
                                              pls);
diff --git a/src/lib/testing_api_cmd_refund.c b/src/lib/testing_api_cmd_refund.c
index abe68f8e..f0ee31b5 100644
--- a/src/lib/testing_api_cmd_refund.c
+++ b/src/lib/testing_api_cmd_refund.c
@@ -221,7 +221,6 @@ refund_increase_run (void *cls,
                                              ris->order_id,
                                              &refund_amount,
                                              ris->reason,
-                                             "default",
                                              &refund_increase_cb,
                                              ris);
   GNUNET_assert (NULL != ris->rio);
@@ -450,7 +449,6 @@ refund_lookup_run (void *cls,
   rls->rlo = TALER_MERCHANT_refund_lookup (is->ctx,
                                            rls->merchant_url,
                                            rls->order_id,
-                                           "default",
                                            &refund_lookup_cb,
                                            rls);
   GNUNET_assert (NULL != rls->rlo);
diff --git a/src/lib/testing_api_cmd_tip.c b/src/lib/testing_api_cmd_tip.c
index 652bd95d..a5bbf2a1 100644
--- a/src/lib/testing_api_cmd_tip.c
+++ b/src/lib/testing_api_cmd_tip.c
@@ -139,11 +139,6 @@ struct TipQueryState
   unsigned int http_status;
 
   /**
-   * Which merchant instance is running this CMD.
-   */
-  const char *instance;
-
-  /**
    * The handle to the current /tip-query request.
    */
   struct TALER_MERCHANT_TipQueryOperation *tqo;
@@ -188,11 +183,6 @@ struct TipAuthorizeState
   unsigned int http_status;
 
   /**
-   * Merchant instance running this CMD.
-   */
-  const char *instance;
-
-  /**
    * Human-readable justification for the
    * tip authorization carried on by this CMD.
    */
@@ -345,7 +335,6 @@ tip_authorize_run (void *cls,
      "http://merchant.com/pickup",
      "http://merchant.com/continue",
      &amount,
-     tas->instance,
      tas->justification,
      tip_authorize_cb,
      tas);
@@ -411,7 +400,6 @@ tip_authorize_cleanup (void *cls,
  *        the reserve from which the tip is going to be gotten.
  * @param http_status the HTTP response code which is expected
  *        for this operation.
- * @param instance which merchant instance is running this CMD.
  * @param justification human-readable justification for this
  *        tip authorization.
  * @param amount the amount to authorize for tipping.
@@ -423,7 +411,6 @@ TALER_TESTING_cmd_tip_authorize_with_ec
    const char *merchant_url,
    const char *exchange_url,
    unsigned int http_status,
-   const char *instance,
    const char *justification,
    const char *amount,
    enum TALER_ErrorCode ec)
@@ -432,7 +419,6 @@ TALER_TESTING_cmd_tip_authorize_with_ec
 
   tas = GNUNET_new (struct TipAuthorizeState);
   tas->merchant_url = merchant_url;
-  tas->instance = instance;
   tas->justification = justification;
   tas->amount = amount;
   tas->http_status = http_status;
@@ -461,7 +447,6 @@ TALER_TESTING_cmd_tip_authorize_with_ec
  *        the reserve from which the tip is going to be gotten.
  * @param http_status the HTTP response code which is expected
  *        for this operation.
- * @param instance which merchant instance is running this CMD.
  * @param justification human-readable justification for this
  *        tip authorization.
  * @param amount the amount to authorize for tipping.
@@ -471,7 +456,6 @@ TALER_TESTING_cmd_tip_authorize (const char *label,
                                  const char *merchant_url,
                                  const char *exchange_url,
                                  unsigned int http_status,
-                                 const char *instance,
                                  const char *justification,
                                  const char *amount)
 {
@@ -479,7 +463,6 @@ TALER_TESTING_cmd_tip_authorize (const char *label,
 
   tas = GNUNET_new (struct TipAuthorizeState);
   tas->merchant_url = merchant_url;
-  tas->instance = instance;
   tas->justification = justification;
   tas->amount = amount;
   tas->http_status = http_status;
@@ -631,7 +614,6 @@ tip_query_run (void *cls,
   tqs->is = is;
   tqs->tqo = TALER_MERCHANT_tip_query (is->ctx,
                                        tqs->merchant_url,
-                                       tqs->instance,
                                        &tip_query_cb,
                                        tqs);
   GNUNET_assert (NULL != tqs->tqo);
@@ -646,7 +628,6 @@ tip_query_run (void *cls,
  *        server the /tip-query request.
  * @param http_status expected HTTP response code for the
  *        /tip-query request.
- * @param instance the merchant instance running this CMD.
  * @param expected_amount_picked_up expected amount already
  *        picked up.
  * @param expected_amount_authorized expected amount that was
@@ -660,7 +641,6 @@ TALER_TESTING_cmd_tip_query_with_amounts
   (const char *label,
    const char *merchant_url,
    unsigned int http_status,
-   const char *instance,
    const char *expected_amount_picked_up,
    const char *expected_amount_authorized,
    const char *expected_amount_available)
@@ -669,7 +649,6 @@ TALER_TESTING_cmd_tip_query_with_amounts
 
   tqs = GNUNET_new (struct TipQueryState);
   tqs->merchant_url = merchant_url;
-  tqs->instance = instance;
   tqs->http_status = http_status;
   tqs->expected_amount_picked_up = expected_amount_picked_up;
   tqs->expected_amount_authorized = expected_amount_authorized;
@@ -694,19 +673,16 @@ TALER_TESTING_cmd_tip_query_with_amounts
  *        server the /tip-query request.
  * @param http_status expected HTTP response code for the
  *        /tip-query request.
- * @param instance the merchant instance running this CMD.
  */
 struct TALER_TESTING_Command
 TALER_TESTING_cmd_tip_query (const char *label,
                              const char *merchant_url,
-                             unsigned int http_status,
-                             const char *instance)
+                             unsigned int http_status)
 {
   struct TipQueryState *tqs;
 
   tqs = GNUNET_new (struct TipQueryState);
   tqs->merchant_url = merchant_url;
-  tqs->instance = instance;
   tqs->http_status = http_status;
 
   struct TALER_TESTING_Command cmd = {
diff --git a/src/lib/testing_api_cmd_track.c b/src/lib/testing_api_cmd_track.c
index 9cd4803b..1f1596a4 100644
--- a/src/lib/testing_api_cmd_track.c
+++ b/src/lib/testing_api_cmd_track.c
@@ -373,7 +373,6 @@ track_transfer_run (void *cls,
     TALER_TESTING_FAIL (is);
   tts->tth = TALER_MERCHANT_track_transfer (is->ctx,
                                             tts->merchant_url,
-                                            "default",
                                             "x-taler-bank",
                                             wtid,
                                             exchange_url,
@@ -412,7 +411,6 @@ track_transaction_run (void *cls,
   tts->tth = TALER_MERCHANT_track_transaction
     (is->ctx,
      tts->merchant_url,
-     "default",
      order_id,
      &track_transaction_cb,
      tts);
diff --git a/src/merchant-tools/taler-merchant-benchmark.c b/src/merchant-tools/taler-merchant-benchmark.c
index bf34a7e7..9bd80ff8 100644
--- a/src/merchant-tools/taler-merchant-benchmark.c
+++ b/src/merchant-tools/taler-merchant-benchmark.c
@@ -95,7 +95,12 @@ char *root_help_str = \
 /**
  * Alternative non default instance.
  */
-static char *alt_instance;
+static char *alt_instance_id;
+
+/**
+ * Base URL of the alternative non default instance.
+ */
+static char *alt_instance_url;
 
 /**
  * How many unaggregated payments we want to generate.
@@ -349,8 +354,7 @@ run (void *cls,
       ("create-proposal-1",
        merchant_url,
        MHD_HTTP_OK,
-       order_worth_5,
-       NULL),
+       order_worth_5),
 
     TALER_TESTING_cmd_pay
       ("deposit-simple",
@@ -375,8 +379,7 @@ run (void *cls,
       ("create-proposal-2",
        merchant_url,
        MHD_HTTP_OK,
-       order_worth_5_track,
-       NULL),
+       order_worth_5_track),
 
     TALER_TESTING_cmd_pay
       ("deposit-simple-2",
@@ -432,10 +435,9 @@ run (void *cls,
 
     TALER_TESTING_cmd_proposal
       ("create-unaggregated-proposal",
-       merchant_url,
+       alt_instance_url,
        MHD_HTTP_OK,
-       order_worth_5_unaggregated,
-       alt_instance),
+       order_worth_5_unaggregated),
 
     TALER_TESTING_cmd_pay
       ("deposit-unaggregated",
@@ -476,8 +478,7 @@ run (void *cls,
       ("create-twocoins-proposal",
        merchant_url,
        MHD_HTTP_OK,
-       order_worth_10_2coins,
-       NULL),
+       order_worth_10_2coins),
 
     TALER_TESTING_cmd_pay
       ("deposit-twocoins",
@@ -629,7 +630,7 @@ main (int argc,
        " never author now-deadlined transactions,"
        " as they would get those far future ones"
        " aggregated too.",
-       &alt_instance),
+       &alt_instance_id),
 
     GNUNET_GETOPT_option_string
       ('b',
@@ -755,7 +756,7 @@ main (int argc,
     return 1;
   }
 
-  if ((GNUNET_YES == corner) && (NULL == alt_instance))
+  if ((GNUNET_YES == corner) && (NULL == alt_instance_id))
   {
     fprintf (stderr, "option '-i' is mandatory"
                      " with sub-command 'corner'!\n");
@@ -777,6 +778,14 @@ main (int argc,
     return MISSING_MERCHANT_URL;
   }
 
+  if (NULL != alt_instance_id)
+  {
+    GNUNET_assert (0 < GNUNET_asprintf (&alt_instance_url,
+                                        "%s/instances/%s/",
+                                        merchant_url,
+                                        &alt_instance_id));
+  }
+
   if (NULL == (merchantd = TALER_TESTING_run_merchant
     (cfg_filename, merchant_url)))
   {