Added Configuration Template Developer Docs

Add some basic documentation about the datastore configuration template
files. Includes a brief description of how the templates are located,
details of the context exposed to the templates and some basic examples.

Change-Id: Iff43ea2c7912f749cb14eb89fd495eafb67e6bd9

doc/source/contributor/datastore-configuration-templates.rst

@@ -0,0 +1,111 @@
+=======================================
+Trove Datastore Configuration Templates
+=======================================
+
+In order to support the injecting of dynamic configuration into database
+configuration files Trove uses the Jinja2 templating engine.
+
+A general guide to writing Jinja2 templates can be found at
+https://jinja.palletsprojects.com/en/stable/templates/
+
+Location and Searching
+======================
+
+The templates used to configure database instances are located under
+`trove/templates` with a directory for each database support by Trove.
+
+The Datastore Version is used to determine which set of templates are used. A
+set of locations are search in a priority order with the most specific location
+that exists used.  The locations search are based upon the Datastore Version
+name, version and manager as well as the Datastore name.
+
+For example consider the following layout under `trove/templates`:
+
+.. code-block::
+
+    mysql/
+        mysql-test/
+            config.template
+        5.5/
+            config.template
+        config.template
+
+If the Datastore Version for `mysql` has the name `mysql-test` then the template
+file `mysql/mysql-test/config.template` will be used.
+
+If the Datastore Version for `mysql` has the name `5.5.62` then Trove will look
+for `mysql/5.5.62`, then `mysql/5.5`, `mysql/5` and finally `mysql/`.  Based on
+the above layout Trove will use `mysql/5.5/config.template`.
+
+
+Variables available in the templates
+====================================
+
+The following variables are exposed to the templates and may be useful in
+making the template adapt to different database sizes or versions.
+
+datastore
+~~~~~~~~~
+
+Contains details of the Datastore and Datastore Version and has the following
+attributes:
+
+``name``
+    The Datastore name, for example "mysql", "postgresql" or "MySQL".  It is
+    used in locating the template to used as described above.
+    The value is set by the operators so may be difficult to rely on in a
+    template.
+
+``manager``
+    The Datastore Version manager - this is the type of the datastore and
+    corresponds to the name used in the Trove code for example "mysql" or
+    "postgresql".  See trove/common/constants.py for a list of managers.
+    Like `name` it is used in locating templates files.
+
+``version``
+    This variable is the value of the Datastore Version name.  Usually it will
+    be the release version of the database such as "5.7.30", but can be a string
+    like "mysql-test".  The value is configured by the operator.
+
+``semantic_version``
+    A parsed version from the Datastore Version version or name.  Trove will first
+    attempt to parse the Datastore Version version, then the Datastore Version name.
+    The `datastore.semantic_version` makes the components of a version string
+    available as attributes `major`, `minor`, `patch`.
+
+    In the following example the major version of a PostgreSQL database is used to
+    select between different configuration options for retaining the binary logs
+
+    .. code-block::
+
+        {% if datastore.semantic_version.major >= 13 %}
+        wal_keep_size = 80 # in MB = wal_keep_segments x wal_keep_size
+        {% else %}
+        wal_keep_segments = 5 # in logfile segments; 0 disables
+        {% endif %}
+
+flavor
+~~~~~~
+The flavor variable is a dictionary of the resource sizes of the compute server
+that will be hosting the database.  It includes details on the number of VCPUs
+and the maximum amount of RAM.
+
+The following example configures the thread cache size for a mysql database
+based on the maximum RAM of the server:
+
+.. code-block::
+
+  thread_cache_size = {{ (4 * flavor['ram']/512)|int }}
+
+server_id
+~~~~~~~~~
+
+A unique ID based on a hash of the instance ID and it is useful to use in the
+configuration of a replication set to distinguish between the primary and
+secondary instance.
+
+The following example is from a MySQL configuration template:
+
+.. code-block::
+
+    server_id = {{server_id}}

doc/source/contributor/index.rst

@@ -13,4 +13,5 @@ functionality, the following resources are provided.
    design
    functional_test
    testing
+   datastore-configuration-templates
    release-notes