📝 [#403] Document Notificaties API retry behavior

docs/admin/notifications.rst

@@ -61,6 +61,51 @@ please refer to the `Open Zaak authorization documentation`_.
 For more information on how to set up subscriptions in Open Notificaties, please refer
 to the `Open Notificaties subscription documentation`_.
 
+Automatic retry for notifications
+=================================
+
+By default, sending notifications has automatic retry behaviour, i.e. if the notification
+publishing task has failed, it will automatically be rescheduled/tried again until the maximum
+retry limit has been reached.
+
+Autoretry explanation and configuration
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+Retry behaviour is implemented using binary exponential backoff with a delay factor,
+the formula to calculate the time to wait until the next retry is as follows:
+
+.. math::
+
+    t = \text{backoff_factor} * 2^c
+
+where `t` is time in seconds and  `c` is the number of retries that have been performed already.
+
+In the **Configuratie > Notificatiescomponentconfiguratie** admin page the autoretry settings
+can be configured:
+
+* **Notification delivery max retries**: the maximum number of retries the task queue
+  will do if sending a notification has failed. Default is ``5``.
+* **Notification delivery retry backoff**: a boolean or a number. If this option is set to
+  ``True``, autoretries will be delayed following the rules of binary exponential backoff. If
+  this option is set to a number, it is used as a delay factor. Default is ``3``.
+* **Notification delivery retry backoff max**: an integer, specifying number of seconds.
+  If ``Notification delivery retry backoff`` is enabled, this option will set a maximum
+  delay in seconds between task autoretries. Default is ``48`` seconds.
+
+With the assumption that the requests are done immediately we can model the notification
+tasks schedule with the default configurations:
+
+1. At 0s the request to create an Object is made, the notification task is scheduled, picked up
+   by worker and failed
+2. At 3s with 3s delay the first retry happens (``2^0`` * ``Notification delivery retry backoff``)
+3. At 9s with 6s delay - the second retry (``2^1`` * ``Notification delivery retry backoff``)
+4. At 21s with 12s delay - the third retry
+5. At 45s with 24s delay - the fourth retry
+6. At 1m33s with 48s delay - the fifth retry, which is the last one.
+
+So if the Notification API is up after 1 min of downtime the default configuration can handle it
+automatically.
+
 .. _documentation: https://open-notificaties.readthedocs.io
 .. _Open Zaak authorization documentation: https://open-zaak.readthedocs.io/en/stable/manual/api-authorizations.html
 .. _Open Notificaties subscription documentation: https://open-notificaties.readthedocs.io/en/stable/manual/subscriptions.html