notes to DD50 about fetching payments

1 files changed, 17 insertions, 0 deletions

diff --git a/design-documents/050-libeufin-nexus.rst b/design-documents/050-libeufin-nexus.rst
index 272e7a7b..82982dc3 100644
--- a/design-documents/050-libeufin-nexus.rst
+++ b/design-documents/050-libeufin-nexus.rst
@@ -196,6 +196,23 @@ nexus-ebics-fetch
     over (before committing any transactions with the next day!) also fetches a
     final statement of the previous day, thus ensuring we get a statement
     every day plus intra-day reports.
+    
+    .. note::
+      
+      (1) "from that day forward (inclusive)" must **not** rely on EBICS returning
+      the unseen messages: that's because they might **already** be downloaded but
+      never made it to the database.
+      
+      (2) "and when the day rolls over".  When does a day roll over? => A day rolls
+      over when the current time is at least on the day after the transaction with the
+      most recent timestamp that's stored in the database.
+
+      (3) "Afterwards, fetches reports".  This must happen **only after** any possible
+      previous statement got downloaded.
+
+      To summarize: at any point in time the database must contain (the content of) any
+      possible statement up to the current time, plus any possible report up to the current
+      time (in case that's not covered by any statement so far).
 
   * Bounces transactions with mal-formed wire transfer subjects.