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doc: describe how to implement standby modules

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Signed-off-by: John Spray <john.spray@redhat.com>
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John Spray 2017-11-01 06:28:30 -04:00
parent 9988ebed95
commit ceb2a91190
2 changed files with 26 additions and 1 deletions
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doc/mgr/index.rst
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@ -26,11 +26,11 @@ sensible.
:maxdepth: 1
Installation and Configuration <administrator>
Writing plugins <plugins>
Dashboard plugin <dashboard>
Local pool plugin <localpool>
RESTful plugin <restful>
Zabbix plugin <zabbix>
Prometheus plugin <prometheus>
Writing plugins <plugins>
Influx plugin <influx>

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doc/mgr/plugins.rst
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@ -160,6 +160,31 @@ a command completes, the ``notify()`` callback on the MgrModule
instance is triggered, with notify_type set to "command", and
notify_id set to the tag of the command.
Implementing standby mode
-------------------------
For some modules, it is useful to run on standby manager daemons as well
as on the active daemon. For example, an HTTP server can usefully
serve HTTP redirect responses from the standby managers so that
the user can point his browser at any of the manager daemons without
having to worry about which one is active.
Standby manager daemons look for a class called ``StandbyModule``
in each module. If the class is not found then the module is not
used at all on standby daemons. If the class is found, then
its ``serve`` method is called. Implementations of ``StandbyModule``
must inherit from ``mgr_module.MgrStandbyModule``.
The interface of ``MgrStandbyModule`` is much restricted compared to
``MgrModule`` -- none of the Ceph cluster state is available to
the module. ``serve`` and ``shutdown`` methods are used in the same
way as a normal module class. The ``get_active_uri`` method enables
the standby module to discover the address of its active peer in
order to make redirects. See the ``MgrStandbyModule`` definition
in the Ceph source code for the full list of methods.
For an example of how to use this interface, look at the source code
of the ``dashboard`` module.
Logging
-------