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+..
+  This file is part of GNU TALER.
+  Copyright (C) 2014, 2015, 2016 INRIA
+  TALER is free software; you can redistribute it and/or modify it under the
+  terms of the GNU General Public License as published by the Free Software
+  Foundation; either version 2.1, or (at your option) any later version.
+  TALER is distributed in the hope that it will be useful, but WITHOUT ANY
+  WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR
+  A PARTICULAR PURPOSE.  See the GNU Lesser General Public License for more details.
+  You should have received a copy of the GNU Lesser General Public License along with
+  TALER; see the file COPYING.  If not, see <http://www.gnu.org/licenses/>
+
+  @author Christian Grothoff
+  @author Marcello Stanisci
+
+========
+Exchange
+========
+
+.. _keys-duration:
+
+-------------
+Keys duration
+-------------
+
+`signkeys`. The option `lookahead_sign` is such that, being `t` the time when `taler-exchange-keyup`
+is run, `taler-exchange-keyup` will generate `n` `signkeys`, where `t + (n * signkey_duration) = t +
+lookahead_sign`. In other words, we generate a number of keys which is sufficient to cover a period of
+`lookahead_sign`. As for the starting date, the first generated key will get a starting time of `t`,
+and the `j`-th key will get a starting time of `x + signkey_duration`, where `x` is the starting time
+of the `(j-1)`-th key.
+
+`denom keys`. The option `lookahead_sign` is such that, being `t` the time when `taler-exchange-keyup`
+is run, `taler-exchange-keyup` will generate `n` `denom keys` for each denomination, where
+`t + (n * duration_withdraw) = t + lookahead_sign`. In other words, for each denomination, we generate a
+number of keys which is sufficient to cover a period of `lookahead_sign`. As for the starting date, the
+first generated key will get a starting time of `t`, and the `j`-th key will get a starting time of
+`x + duration_withdraw`, where `x` is the starting time of the `(j-1)`-th key. 
+
+
+
+---------------
+Database Scheme
+---------------
+
+The exchange database must be initialized using `taler-exchange-dbinit`.  This
+tool creates the tables required by the Taler exchange to operate.  The
+tool also allows you to reset the Taler exchange database, which is useful
+for test cases but should never be used in production.  Finally,
+`taler-exchange-dbinit` has a function to garbage collect a database,
+allowing administrators to purge records that are no longer required.
+
+The database scheme used by the exchange look as follows:
+
+.. image:: exchange-db.png