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diff --git a/src/ceph/doc/mgr/plugins.rst b/src/ceph/doc/mgr/plugins.rst new file mode 100644 index 0000000..a75c14c --- /dev/null +++ b/src/ceph/doc/mgr/plugins.rst @@ -0,0 +1,215 @@ + +ceph-mgr plugin author guide +============================ + +Creating a plugin +----------------- + +In pybind/mgr/, create a python module. Within your module, create a class +named ``Module`` that inherits from ``MgrModule``. + +The most important methods to override are: + +* a ``serve`` member function for server-type modules. This + function should block forever. +* a ``notify`` member function if your module needs to + take action when new cluster data is available. +* a ``handle_command`` member function if your module + exposes CLI commands. + +Installing a plugin +------------------- + +Once your module is present in the location set by the +``mgr module path`` configuration setting, you can enable it +via the ``ceph mgr module enable`` command:: + + ceph mgr module enable mymodule + +Note that the MgrModule interface is not stable, so any modules maintained +outside of the Ceph tree are liable to break when run against any newer +or older versions of Ceph. + +Logging +------- + +MgrModule instances have a ``log`` property which is a logger instance that +sends log messages into the Ceph logging layer where they will be recorded +in the mgr daemon's log file. + +Use it the same way you would any other python logger. The python +log levels debug, info, warn, err are mapped into the Ceph +severities 20, 4, 1 and 0 respectively. + +Exposing commands +----------------- + +Set the ``COMMANDS`` class attribute of your plugin to a list of dicts +like this:: + + COMMANDS = [ + { + "cmd": "foobar name=myarg,type=CephString", + "desc": "Do something awesome", + "perm": "rw" + } + ] + +The ``cmd`` part of each entry is parsed in the same way as internal +Ceph mon and admin socket commands (see mon/MonCommands.h in +the Ceph source for examples) + +Config settings +--------------- + +Modules have access to a simple key/value store (keys and values are +byte strings) for storing configuration. Don't use this for +storing large amounts of data. + +Config values are stored using the mon's config-key commands. + +Hints for using these: + +* Reads are fast: ceph-mgr keeps a local in-memory copy +* Don't set things by hand with "ceph config-key", the mgr doesn't update + at runtime (only set things from within modules). +* Writes block until the value is persisted, but reads from another + thread will see the new value immediately. + +Any config settings you want to expose to users from your module will +need corresponding hooks in ``COMMANDS`` to expose a setter. + +Accessing cluster data +---------------------- + +Modules have access to the in-memory copies of the Ceph cluster's +state that the mgr maintains. Accessor functions as exposed +as members of MgrModule. + +Calls that access the cluster or daemon state are generally going +from Python into native C++ routines. There is some overhead to this, +but much less than for example calling into a REST API or calling into +an SQL database. + +There are no consistency rules about access to cluster structures or +daemon metadata. For example, an OSD might exist in OSDMap but +have no metadata, or vice versa. On a healthy cluster these +will be very rare transient states, but plugins should be written +to cope with the possibility. + +``get(self, data_name)`` + +Fetch named cluster-wide objects such as the OSDMap. Valid things +to fetch are osd_crush_map_text, osd_map, osd_map_tree, +osd_map_crush, config, mon_map, fs_map, osd_metadata, pg_summary, +df, osd_stats, health, mon_status. + +All these structures have their own JSON representations: experiment +or look at the C++ dump() methods to learn about them. + +``get_server(self, hostname)`` + +Fetch metadata about a particular hostname. This is information +that ceph-mgr has gleaned from the daemon metadata reported +by daemons running on a particular server. + +``list_servers(self)`` + +Like ``get_server``, but gives information about all servers (i.e. all +unique hostnames that have been mentioned in daemon metadata) + +``get_metadata(self, svc_type, svc_id)`` + +Fetch the daemon metadata for a particular service. svc_type is one +of osd or mds, and svc_id is a string (convert OSD integer IDs to strings +when calling this). + +``get_counter(self, svc_type, svc_name, path)`` + +Fetch the latest performance counter data for a particular counter. The +path is a period-separated concatenation of the subsystem and the counter +name, for example "mds.inodes". + +A list of two-tuples of (timestamp, value) is returned. This may be +empty if no data is available. + +Sending commands +---------------- + +A non-blocking facility is provided for sending monitor commands +to the cluster. + +``send_command(self, result, command_str, tag)`` + +The ``result`` parameter should be an instance of the CommandResult +class, defined in the same module as MgrModule. This acts as a +completion and stores the output of the command. Use CommandResult.wait() +if you want to block on completion. + +The ``command_str`` parameter is a JSON-serialized command. This +uses the same format as the ceph command line, which is a dictionary +of command arguments, with the extra ``prefix`` key containing the +command name itself. Consult MonCommands.h for available commands +and their expected arguments. + +The ``tag`` parameter is used for nonblocking operation: when +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 +------- + +Use your module's ``log`` attribute as your logger. This is a logger +configured to output via the ceph logging framework, to the local ceph-mgr +log files. + +Python log severities are mapped to ceph severities as follows: + +* DEBUG is 20 +* INFO is 4 +* WARN is 1 +* ERR is 0 + +Shutting down cleanly +--------------------- + +If a module implements the ``serve()`` method, it should also implement +the ``shutdown()`` method to shutdown cleanly: misbehaving modules +may otherwise prevent clean shutdown of ceph-mgr. + +Is something missing? +--------------------- + +The ceph-mgr python interface is not set in stone. If you have a need +that is not satisfied by the current interface, please bring it up +on the ceph-devel mailing list. While it is desired to avoid bloating +the interface, it is not generally very hard to expose existing data +to the Python code when there is a good reason. + |