From 4cfcf842fe6bdd0535cdc64683d3eb79e74bbceb Mon Sep 17 00:00:00 2001
From: Christian Grothoff <christian@grothoff.org>
Date: Sun, 12 Jul 2020 19:19:13 +0200
Subject: add reserve state machine

---
 doc/system/taler/design.tex  |  18 ++++++++++++++++--
 doc/system/taler/reserve.dot |  14 ++++++++++++++
 doc/system/taler/reserve.pdf | Bin 0 -> 14395 bytes
 3 files changed, 30 insertions(+), 2 deletions(-)
 create mode 100644 doc/system/taler/reserve.dot
 create mode 100644 doc/system/taler/reserve.pdf

(limited to 'doc/system')

diff --git a/doc/system/taler/design.tex b/doc/system/taler/design.tex
index 36d167742..ff43376a9 100644
--- a/doc/system/taler/design.tex
+++ b/doc/system/taler/design.tex
@@ -125,8 +125,8 @@ payment.
 
 % FIXME:  who needs to be trusted for anonymity?
 
-
 \subsection{Reserves}
+
 A \emph{reserve} refers to a customer's non-anonymous funds at an exchange,
 identified by a reserve public key.  Suppose a customer wants to convert money
 into anonymized digital coins.  To do that, the customer first creates a
@@ -149,7 +149,20 @@ customers that lost their reserve private key to eventually recover their
 funds.  If a wire transfer to the exchange does not include a valid reserve public key,
 the exchange transfers the money back to the sender.
 
-% FIXME:  this really needs a diagram
+Figure~\ref{fig:reserve:state} illustrates the state machine for a reserve.
+Long-terms states are shown in boxes, while actions are in circles.  The
+final state is in a double-circle.
+A reserve is first {\em filled} by a wire transfer. The amount in it is reduced
+by withdraw operations. If the balance reaches zero, the reserve is
+{\em drained}. If a reserve is not drained after a certain amount of time,
+it is automatically closed.  A reserve can also be filled via a recoup
+action in case that the denomination of an unspent coin that was
+withdrawn from the reserve is revoked.
+\begin{figure}
+  \includegraphics{reserve.pdf}
+  \caption{State machine of a reserve.}
+  \label{fig:reserve:states}
+\end{figure}
 
 Instead of requiring the customer to manually generate reserve key pairs and
 copy them onto a wire transfer form, banks can offer tight integration with the
@@ -187,6 +200,7 @@ for acceptance as an IETF Internet Standard.
 % FIXME: withdrawal strategy, coin selection
 
 \subsection{Coins and Denominations}
+
 Unlike plain Chaumian e-cash, where a coin just contains a serial number, a
 \emph{coin} in Taler is a public/private key pair where the private key is only
 known to the owner of the coin.
diff --git a/doc/system/taler/reserve.dot b/doc/system/taler/reserve.dot
new file mode 100644
index 000000000..af6e3e86d
--- /dev/null
+++ b/doc/system/taler/reserve.dot
@@ -0,0 +1,14 @@
+digraph Reserve {
+
+  filled [color=blue, label="filled reserve", shape="box"];
+  drained [color=blue, label="drained reserve", shape="doublecircle"];
+
+  transfer->filled;
+  recoup->filled;
+  filled->withdraw;
+  withdraw->drained;
+  withdraw->filled;
+  filled->close;
+  close->drained;
+  drained->recoup;
+}
diff --git a/doc/system/taler/reserve.pdf b/doc/system/taler/reserve.pdf
new file mode 100644
index 000000000..5225bdedb
Binary files /dev/null and b/doc/system/taler/reserve.pdf differ
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