dispatcher<Dispatcher> is a processor that converts a given action in Senlin engine into certain format and then persists it into some storage or sends it to downstream processing software.
Since version 3.0.0, Senlin comes with some built-in dispatchers that can dump event records into database and/or send event notifications via the default message queue. The former is referred to as the
database dispatcher which is enabled by default; the latter is referred to as the
message dispatcher which has to be manually enabled by adding the following line to the
event_dispatchers = message
However, the distributors or the users can always add their own event dispatchers easily when needed.
Event dispatchers are managed as Senlin plugins. Once a new event dispatcher is implemented, a deployer can enable it by first adding a new item to the
senlin.dispatchers entries in the
entry_points section of the
setup.cfg file, followed by a reinstall of the Senlin service, i.e.
sudo pip install command.
The Base Class
All event dispatchers are expected to subclass the base class
EventBackend in the
senlin.events.base module. The only requirement for a dispatcher subclass is to override the
dump() method that implements the processing logic.
Providing New Dispatchers
Developing A New Event Dispatcher
The first step for adding a new dispatcher is to create a new file containing a subclass of
EventBackend. In this new class, say
JsonDispatcher, you will need to implement the
dump() class method as exemplified below:
class JsonDispatcher(base.EventBackend): """Dispatcher for dumping events to a JSON file.""" @classmethod def dump(cls, level, action, **kwargs): # Your logic goes here ...
level parameter for the method is identical to that defined by the
logging module of Python. It is an integer representing the criticality of an event. The
action parameter is an instance of Senlin action class, which is defined in the
senlin.engine.actions.base module. There is virtually no constraints on which properties you will pick and how you want to process them.
**kwargs parameter may provide some useful fields for you to use:
timestamp: A datetime value that indicates when the event was generated.
phase: A string value indicating the phase an action is in. Most of the time this can be safely ignored.
reason: There are some rare cases where an event comes with a textual description. Most of the time, this is empty.
extra: There are even rarer cases where an event comes with some additional fields for attention. This can be safely ignored most of the time.
Registering the New Dispatcher
For Senlin service to be aware of and thus to make use of the new dispatcher, you will register it to the Senlin engine service. This is done by editing the
setup.cfg file in the root directory of the code base, for example:
[entry_points] senlin.dispatchers = database = senlin.events.database:DBEvent message = senlin.events.message:MessageEvent jsonfile = <path to the dispatcher module>:<dispatch class name>
Finally, save that file and do a reinstall of the Senlin service, followed by a restart of the
$ sudo pip install -e .
Dynamically Enabling/Disabling a Dispatcher
All dispatchers are loaded when the Senlin engine is started, however, they can be dynamically enabled or disabled by editing the
senlin.conf file. The option
event_dispatchers in the
[DEFAULT] section is a multi-string value option for this purpose. To enable your dispatcher (i.e.
jsonfile), you will need to add the following line to the
event_dispatchers = jsonfile