From 9939f9465b72780427897a201a37dcf58bada207 Mon Sep 17 00:00:00 2001
From: Marcello Stanisci <marcello.stanisci@inria.fr>
Date: Mon, 19 Sep 2016 11:52:32 +0200
Subject: documenting configuration facility

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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 Marcello Stanisci
+
+======================
+Configuration in Taler
+======================
+
+In Taler realm, any component obeys to the same pattern to get configuration
+values.  According to this pattern, once the component has been installed, the
+installation deploys default values in `${prefix}/share/taler/config.d/`, in
+`.conf` files.  In order to override these defaults, the user can write a custom
+`.conf` file and either pass it to the component at execution time, or name it
+`taler.conf` and place it under `$HOME/.config/`.
+
+
+--------------------
+Configuration format
+--------------------
+
+A config file is a text file containing `sections`, and each section contains
+its `values`. The right format follows::
+
+  [section1]
+  value1 = string
+  value2 = 23
+
+  [section2]
+  value21 = string
+  value22 = /path22
+
+Throught any configuration file, it is possible to use ``$``-prefixed variables,
+especially when a value needs a pathname. Some of them are automatically resolved,
+like ``${TALER_DATA_HOME}``, ``${TALER_RUNTIME_DIR}``, or ``${HOME}``. (FIXME: explain
+how/where/when those defaults get resolved) Other variables can be defined under the
+`paths` section. For example::
+
+  [paths]
+  TALER_DEPLOYMENT_SHARED = ${HOME}/shared-data
+  ..
+  [section-x]
+  path-x = ${TALER_DEPLOYMENT_SHARED}/x
+
+The utility ``taler-config``, which gets installed along with the exchange, serves to get and
+set configuration values without directly editing the `.conf`.  See ``taler-config --help``.
+
+Note that, in this stage of development, the file ``$HOME/.config/taler.conf``
+can contain sections for *all* the component. For example, both an exchange and
+a bank can read values from it.
+
+The repository ``git://taler.net/deployment`` contains examples of configuration
+file used in our demos. See under ``deployment/config``.
+
+.. note::
+
+  Expectably, some components will not work just by using default values, as their
+  work is often interdependent. For example, a merchant needs to know an exchange
+  URL, or a database name.
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