Add Sahara log guideliness

During Kilo cycle we design and implement the new way of
creating logs in Sahara project. Now we add a document which
describe our vision on it.

Change-Id: I5f3c215600475a012d335d415fa826e1d4aa2cd8

doc/source/devref/log.guidelines.rst

@@ -0,0 +1,53 @@
+
+Log Guidelines
+======================
+
+Levels Guidelines
+-----------------
+
+During Kilo release cycle Sahara community defined the following log levels:
+
+
+* Debug: Shows everything and is likely not suitable for normal production
+  operation due to the sheer size of logs generated (e.g. scripts executions,
+  process execution, etc.).
+* Info: Usually indicates successful service start/stop, versions and such
+  non-error related data. This should include largely positive units of work
+  that are accomplished (e.g. service setup, cluster start, successful job
+  execution).
+* Warning: Indicates that there might be a systemic issue;
+  potential predictive failure notice (e.g. job execution failed).
+* Error: An error has occurred and an administrator should research the event
+  (e.g. cluster failed to start, plugin violations of operation).
+* Critical: An error has occurred and the system might be unstable, anything
+  that eliminates part of Sahara's intended functionality; immediately get
+  administrator assistance (e.g. failed to access keystone/database, plugin
+  load failed).
+
+
+Formatting Guidelines
+----------------------
+
+Now Sahara uses string formatting defined in `PEP 3101`_ for logs.
+
+.. code:: python
+
+    LOG.warning(_LW("Incorrect path: {path}").format(path=path))
+
+
+..
+
+
+Translation Guidelines
+----------------------
+
+All log levels except Debug requires translation. None of the separate cli tools packaged
+with Sahara contain log translations.
+
+* Debug: no translation
+* Info: _LI
+* Warning: _LW
+* Error: _LE
+* Critical: _LC
+
+.. _PEP 3101: https://www.python.org/dev/peps/pep-3101/

doc/source/index.rst

@@ -92,6 +92,7 @@ Developer Guide
     devref/how_to_build_oozie
     devref/adding_database_migrations
     devref/testing
+    devref/log.guidelines
 
 **Background Concepts for Sahara**