Add some basic info on overnight luigi tasks

docs/base/cron.rst

@@ -2,10 +2,67 @@
 Scheduled Tasks
 ===============
 
-TODO
+By "scheduled task" we mean, a command which runs automatically at a
+pre-determined time.  See also :doc:`commands` and :doc:`scripts`.
+
+For now, Rattail assumes that any scheduled tasks will be manually
+configured to run via `cron`_.  See your operating system's
+documentation for details.
+
+.. _cron: https://en.wikipedia.org/wiki/Cron
+
+(In the future, Rattail may add a `dedicated scheduler daemon`_, but
+it is not yet a priority.)
+
+.. _dedicated scheduler daemon: https://redmine.rattailproject.org/issues/13
 
 
 Luigi
 -----
 
-TODO
+`Luigi`_ is a "task runner" of sorts.  Rattail uses it to help
+orchestrate a "batch" of commands such as is often needed for
+overnight automation (aka. "EOD").
+
+.. _Luigi: https://luigi.readthedocs.io/en/stable/
+
+The general idea is to maintain a Python module containing all of the
+individual tasks which need to run.  Rattail can then invoke the
+module in such a way that Luigi runs all the tasks, keeping track of
+which tasks have (not yet) ran, and in which order they should run
+etc.
+
+
+Overnight
+~~~~~~~~~
+
+Often there is just one "overnight" module, which contains all tasks.
+But in some cases you may want a couple of separate modules, to run at
+different times.  A common example is like:
+
+* run "all" (typical EOD) tasks, starting at 1am
+* run "backups" for all servers, starting at 4am
+
+In this example the "all" task may include a dozen or more individual
+"steps" (which, confusingly, are also called "tasks").  Maybe data is
+exported from one system to another, reports are ran, etc.  Then the
+"backups" task runs later, and may itself also contain multiple steps,
+or perhaps just one.
+
+There are two main benefits from wiring up overnight tasks via modules
+to be ran by Luigi.
+
+The first is that each individual step will (typically) start
+immediately after the previous one completes.  This means all steps
+are "packed in" as tightly as possible, to help keep overall run time
+down.  Whereas scheduling each step separately often involves
+"guessing" how long each will take, and for example you might schedule
+them 5 minutes apart just to be safe, even if a given step only needs
+2 minutes.  (So another benefit really, is just avoiding the hassle of
+scheduling all those steps separately to begin with.)
+
+The other benefit is to allow for *restarting* the overnight tasks.
+Luigi can keep track of which steps have ran thus far, so if there is
+a failure, which you then address, you can restart the overnight task
+and Luigi will pick up where it left off.  (Steps which already ran
+successfully will be skipped over.)