[taler-docs] 02/05: add frag: configuration-format.rst

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commit f72a46f462f9070fa4a48b240c309f7554e735f6
Author: Thien-Thi Nguyen <ttn@gnuvola.org>
AuthorDate: Fri Dec 11 00:52:04 2020 -0500

    add frag: configuration-format.rst
---
 frags/configuration-format.rst | 69 ++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 69 insertions(+)

diff --git a/frags/configuration-format.rst b/frags/configuration-format.rst
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+Configuration format
+--------------------
+
+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 $HOME/.config/.
+
+A config file is a text file containing sections, and each section
+contains its values. The right format follows:
+
+.. code-block:: ini
+
+   [section1]
+   value1 = string
+   value2 = 23
+
+   [section2]
+   value21 = string
+   value22 = /path22
+
+Throughout any configuration file, it is possible to use ``$``-prefixed
+variables, like ``$VAR``, especially when they represent filesystem
+paths. It is also possible to provide defaults values for those
+variables that are unset, by using the following syntax:
+``${VAR:-default}``. However, there are two ways a user can set
+``$``-prefixable variables:
+
+by defining them under a ``[paths]`` section, see example below,
+
+.. code-block:: ini
+
+   [paths]
+   TALER_DEPLOYMENT_SHARED = ${HOME}/shared-data
+   ..
+   [section-x]
+   path-x = ${TALER_DEPLOYMENT_SHARED}/x
+
+or by setting them in the environment:
+
+.. code-block:: console
+
+   $ export VAR=/x
+
+The configuration loader will give precedence to variables set under
+``[path]``, though.
+
+The utility ``taler-config``, which gets installed along with the
+exchange, serves to get and set configuration values without directly
+editing the .conf. The option ``-f`` is particularly useful to resolve
+pathnames, when they use several levels of ``$``-expanded variables. See
+``taler-config --help``.
+
+Note that, in this stage of development, the file
+``$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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