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+Periodic Taskgraphs
+===================
+
+The cron functionality allows in-tree scheduling of task graphs that run
+periodically, instead of on a push.
+
+Cron.yml
+--------
+
+In the root of the Gecko directory, you will find `.cron.yml`.  This defines
+the periodic tasks ("cron jobs") run for Gecko.  Each specifies a name, what to
+do, and some parameters to determine when the cron job should occur.
+
+See `the scema <https://hg.mozilla.org/ci/ci-configuration/file/tip/build-decision/src/build_decision/cron/schema.yml>`_
+for details on the format and meaning of this file.
+
+How It Works
+------------
+
+The `TaskCluster Hooks Service <https://firefox-ci-tc.services.mozilla.com/hooks>`_
+has a hook configured for each repository supporting periodic task graphs.  The
+hook runs every 15 minutes, and the resulting task is referred to as a "cron task".
+That cron task runs the `build-decision
+<https://hg.mozilla.org/ci/ci-admin/file/default/build-decision>`_ image in a
+checkout of the Gecko source tree.
+
+The task reads ``.cron.yml``, then consults the current time (actually the time
+the cron task was created, rounded down to the nearest 15 minutes) and creates
+tasks for any cron jobs scheduled at that time.
+
+Each cron job in ``.cron.yml`` specifies a ``job.type``, corresponding to a
+function responsible for creating TaskCluster tasks when the job runs.
+
+Describing Time
+---------------
+
+This cron implementation understands the following directives when
+describing when to run:
+
+* ``minute``: The minute in which to run, must be in 15 minute increments (see above)
+* ``hour``: The hour of the day in which to run, in 24 hour time.
+* ``day``: The day of the month as an integer, such as `1`, `16`. Be cautious above `28`, remember February.
+* ``weekday``: The day of the week, `Monday`, `Tuesday`, etc. Full length ISO compliant words.
+
+Setting both 'day' and 'weekday' will result in a cron job that won't run very often,
+and so is undesirable.
+
+*Examples*
+
+.. code-block:: yaml
+
+    # Never
+    when: []
+
+    # 4 AM and 4 PM, on the hour, every day.
+    when:
+        - {hour: 16, minute: 0}
+        - {hour: 4, minute: 0}
+
+    # The same as above, on a single line
+    when: [{hour: 16, minute: 0}, {hour: 4, minute: 0}]
+
+    # 4 AM on the second day of every month.
+    when:
+        - {day: 2, hour: 4, minute: 0}
+
+    # Mondays and Thursdays at 10 AM
+    when:
+        - {weekday: 'Monday', hour: 10, minute: 0}
+        - {weekday: 'Thursday', hour: 10, minute: 0}
+
+.. note::
+
+   Times are expressed in UTC (Coordinated Universal Time)
+
+
+Decision Tasks
+..............
+
+For ``job.type`` "decision-task", tasks are created based on
+``.taskcluster.yml`` just like the decision tasks that result from a push to a
+repository.  They run with a distinct ``taskGroupId``, and are free to create
+additional tasks comprising a task graph.
+
+Scopes
+------
+
+The cron task runs with the sum of all cron job scopes for the given repo.  For
+example, for the "sequoia" project, the scope would be
+``assume:repo:hg.mozilla.org/projects/sequoia:cron:*``.  Each cron job creates
+tasks with scopes for that particular job, by name.  For example, the
+``check-frob`` cron job on that repo would run with
+``assume:repo:hg.mozilla.org/projects/sequoia:cron:check-frob``.
+
+.. important::
+
+    The individual cron scopes are a useful check to ensure that a job is not
+    accidentally doing something it should not, but cannot actually *prevent* a
+    job from using any of the scopes afforded to the cron task itself (the
+    ``..cron:*`` scope).  This is simply because the cron task runs arbitrary
+    code from the repo, and that code can be easily modified to create tasks
+    with any scopes that it possesses.