Doc entry

docs/concepts.rst

@@ -206,3 +206,28 @@ detailing the list of tasks that missed their SLA. The event is also recorded
 in the database and made available in the web UI under ``Browse->Missed SLAs``
 where events can be analyzed and documented.
 
+
+Trigger Rules
+'''''''''''''
+
+Though the normal workflow behavior is to trigger tasks when all their
+directly upstream tasks have succeeded, Airflow allows for more complex
+dependency settings.
+
+All operators have a ``trigger_rule`` argument which defines the rule by which 
+the generated task get triggered. The default value for ``trigger_rule`` is
+``all_success`` and can be defined as "trigger this task when all directly
+upstream tasks have succeeded". All other rules described here are based
+on direct parent tasks and are values that can be passed to any operator
+while creating tasks:
+
+* ``all_success``: (default) all parents have succeeded
+* ``all_failed``: all parents are in a ``failed`` or ``upstream_failed`` state
+* ``all_done``: all parents are done with their execution
+* ``one_failed``: at least one parent has failed
+* ``one_success``: at least one parent has succeeded
+* ``dummy``: dependencies are just for show, trigger at will
+
+Note that these can be used in conjunction with ``depends_on_past`` (boolean)
+that, when set to ``True``, keeps a task from getting triggered if the 
+previous schedule for the task hasn't succeeded.

docs/faq.rst

@@ -33,3 +33,10 @@ Here are some of the common causes:
 
 You may also want to read the Scheduler section of the docs and make 
 sure you fully understand how it proceeds.
+
+
+**How do I trigger tasks based on another task's failure?**
+
+Check out the ``Trigger Rule`` section in the Concepts section of the 
+documentation
+