Adding documentation on automatic purge

services/alarm-logger/doc/index.rst

@@ -32,3 +32,19 @@ Examples:
 * **Commands** 
 
  e.g. a user actions to *Acknowledge* an alarm
+
+****************************************
+Automatic purge of Elasticsearch indices
+****************************************
+
+To avoid issues related to a high number of Elasticsearch indices, automatic purge can be enabled in order to delete
+indices considered obsolete. This is done by setting the preference ``retention_period_days`` to a number larger
+or equal to 100. The default value is 0, i.e. automatic purge is disabled by default.
+
+The automatic purge is run using a cron expression defined in preference ``purge_cron_expr``, default is
+``0 0 0 * * SUN``, i.e. midnight each Sunday. See the SpringDocumentation_ on how to define the cron expression.
+
+An Elasticsearch index is considered eligible for deletion if the last inserted message date is before current time
+minus ``retention_period_days``.
+
+.. _SpringDocumentation: https://docs.spring.io/spring-framework/docs/current/javadoc-api/org/springframework/scheduling/support/CronExpression.html