.. _guide-extending: ========================== Extensions and Bootsteps ========================== .. contents:: :local: :depth: 2 .. _extending-custom-consumers: Custom Message Consumers ======================== You may want to embed custom Kombu consumers to manually process your messages. For that purpose a special :class:`~celery.bootstep.ConsumerStep` bootstep class exists, where you only need to define the ``get_consumers`` method, which must return a list of :class:`kombu.Consumer` objects to start whenever the connection is established: .. code-block:: python from celery import Celery from celery import bootsteps from kombu import Consumer, Exchange, Queue my_queue = Queue('custom', Exchange('custom'), 'routing_key') app = Celery(broker='amqp://') class MyConsumerStep(bootsteps.ConsumerStep): def get_consumers(self, channel): return [Consumer(channel, queues=[my_queue], callbacks=[self.handle_message], accept=['json'])] def handle_message(self, body, message): print('Received message: {0!r}'.format(body)) message.ack() app.steps['consumer'].add(MyConsumerStep) def send_me_a_message(self, who='world!', producer=None): with app.producer_or_acquire(producer) as producer: producer.send( {'hello': who}, serializer='json', exchange=my_queue.exchange, routing_key='routing_key', declare=[my_queue], retry=True, ) if __name__ == '__main__': send_me_a_message('celery') .. note:: Kombu Consumers can take use of two different message callback dispatching mechanisms. The first one is the ``callbacks`` argument which accepts a list of callbacks with a ``(body, message)`` signature, the second one is the ``on_message`` argument which takes a single callback with a ``(message, )`` signature. The latter will not automatically decode and deserialize the payload which is useful in many cases: .. code-block:: python def get_consumers(self, channel): return [Consumer(channel, queues=[my_queue], on_message=self.on_message)] def on_message(self, message): payload = message.decode() print( 'Received message: {0!r} {props!r} rawlen={s}'.format( payload, props=message.properties, s=len(message.body), )) message.ack() .. _extending-blueprints: Blueprints ========== Bootsteps is a technique to add functionality to the workers. A bootstep is a custom class that defines hooks to do custom actions at different stages in the worker. Every bootstep belongs to a blueprint, and the worker currently defines two blueprints: **Worker**, and **Consumer** ---------------------------------------------------------- **Figure A:** Bootsteps in the Worker and Consumer blueprints. Starting from the bottom up the first step in the worker blueprint is the Timer, and the last step is to start the Consumer blueprint, which then establishes the broker connection and starts consuming messages. .. figure:: ../images/worker_graph_full.png ---------------------------------------------------------- Worker ====== The Worker is the first blueprint to start, and with it starts major components like the event loop, processing pool, the timer, and also optional components like the autoscaler. When the worker is fully started it will continue to the Consumer blueprint. The :class:`~celery.worker.WorkController` is the core worker implementation, and contains several methods and attributes that you can use in your bootstep. Attributes ---------- .. attribute:: app The current app instance. .. attribute:: hostname The workers node name (e.g. `worker1@example.com`) .. attribute:: blueprint This is the worker :class:`~celery.bootsteps.Blueprint`. .. attribute:: hub Event loop object (:class:`~kombu.async.Hub`). You can use this to register callbacks in the event loop. This is only supported by async I/O enabled transports (amqp, redis), in which case the `worker.use_eventloop` attribute should be set. Your worker bootstep must require the Hub bootstep to use this: .. code-block:: python class WorkerStep(bootsteps.StartStopStep): requires = ('celery.worker.components:Hub', ) .. attribute:: pool The current process/eventlet/gevent/thread pool. See :class:`celery.concurrency.base.BasePool`. Your worker bootstep must require the Pool bootstep to use this: .. code-block:: python class WorkerStep(bootsteps.StartStopStep): requires = ('celery.worker.components:Pool', ) .. attribute:: timer :class:`~kombu.async.timer.Timer` used to schedule functions. Your worker bootstep must require the Timer bootstep to use this: .. code-block:: python class WorkerStep(bootsteps.StartStopStep): requires = ('celery.worker.components:Timer', ) .. attribute:: statedb :class:`Database `` to persist state between worker restarts. This is only defined if the ``statedb`` argument is enabled. Your worker bootstep must require the Statedb bootstep to use this: .. code-block:: python class WorkerStep(bootsteps.StartStopStep): requires = ('celery.worker.components:Statedb', ) .. attribute:: autoscaler :class:`~celery.worker.autoscaler.Autoscaler` used to automatically grow and shrink the number of processes in the pool. This is only defined if the ``autoscale`` argument is enabled. Your worker bootstep must require the `Autoscaler` bootstep to use this: .. code-block:: python class WorkerStep(bootsteps.StartStopStep): requires = ('celery.worker.autoscaler:Autoscaler', ) .. attribute:: autoreloader :class:`~celery.worker.autoreloder.Autoreloader` used to automatically reload use code when the filesystem changes. This is only defined if the ``autoreload`` argument is enabled. Your worker bootstep must require the `Autoreloader` bootstep to use this; .. code-block:: python class WorkerStep(bootsteps.StartStopStep): requires = ('celery.worker.autoreloader:Autoreloader', ) An example Worker bootstep could be: .. code-block:: python from celery import bootsteps class ExampleWorkerStep(bootsteps.StartStopStep): requires = ('Pool', ) def __init__(self, worker, **kwargs): print('Called when the WorkController instance is constructed') print('Arguments to WorkController: {0!r}'.format(kwargs)) def create(self, worker): # this method can be used to delegate the action methods # to another object that implements ``start`` and ``stop``. return self def start(self, worker): print('Called when the worker is started.') def stop(self, worker): print("Called when the worker shuts down.") def terminate(self, worker): print("Called when the worker terminates") Every method is passed the current ``WorkController`` instance as the first argument. Another example could use the timer to wake up at regular intervals: .. code-block:: python from celery import bootsteps class DeadlockDetection(bootsteps.StartStopStep): requires = ('Timer', ) def __init__(self, worker, deadlock_timeout=3600): self.timeout = deadlock_timeout self.requests = [] self.tref = None def start(self, worker): # run every 30 seconds. self.tref = worker.timer.call_repeatedly( 30.0, self.detect, (worker, ), priority=10, ) def stop(self, worker): if self.tref: self.tref.cancel() self.tref = None def detect(self, worker): # update active requests for req in self.worker.active_requests: if req.time_start and time() - req.time_start > self.timeout: raise SystemExit() Consumer ======== The Consumer blueprint establishes a connection to the broker, and is restarted every time this connection is lost. Consumer bootsteps include the worker heartbeat, the remote control command consumer, and importantly, the task consumer. When you create consumer bootsteps you must take into account that it must be possible to restart your blueprint. An additional 'shutdown' method is defined for consumer bootsteps, this method is called when the worker is shutdown. Attributes ---------- .. attribute:: app The current app instance. .. attribute:: controller The parent :class:`~@WorkController` object that created this consumer. .. attribute:: hostname The workers node name (e.g. `worker1@example.com`) .. attribute:: blueprint This is the worker :class:`~celery.bootsteps.Blueprint`. .. attribute:: hub Event loop object (:class:`~kombu.async.Hub`). You can use this to register callbacks in the event loop. This is only supported by async I/O enabled transports (amqp, redis), in which case the `worker.use_eventloop` attribute should be set. Your worker bootstep must require the Hub bootstep to use this: .. code-block:: python class WorkerStep(bootsteps.StartStopStep): requires = ('celery.worker:Hub', ) .. attribute:: connection The current broker connection (:class:`kombu.Connection`). A consumer bootstep must require the 'Connection' bootstep to use this: .. code-block:: python class Step(bootsteps.StartStopStep): requires = ('celery.worker.consumer:Connection', ) .. attribute:: event_dispatcher A :class:`@events.Dispatcher` object that can be used to send events. A consumer bootstep must require the `Events` bootstep to use this. .. code-block:: python class Step(bootsteps.StartStopStep): requires = ('celery.worker.consumer:Events', ) .. attribute:: gossip Worker to worker broadcast communication (class:`~celery.worker.consumer.Gossip`). A consumer bootstep must require the `Gossip` bootstep to use this. .. code-block:: python class Step(bootsteps.StartStopStep): requires = ('celery.worker.consumer:Events', ) .. attribute:: pool The current process/eventlet/gevent/thread pool. See :class:`celery.concurrency.base.BasePool`. .. attribute:: timer :class:`Timer >> app = Celery() >>> app.steps['worker'].add(MyWorkerStep) # < add class, do not instantiate >>> app.steps['consumer'].add(MyConsumerStep) >>> app.steps['consumer'].update([StepA, StepB]) >>> app.steps['consumer'] {step:proj.StepB{()}, step:proj.MyConsumerStep{()}, step:proj.StepA{()} The order of steps is not important here as the order is decided by the resulting dependency graph (``Step.requires``). To illustrate how you can install bootsteps and how they work, this is an example step that prints some useless debugging information. It can be added both as a worker and consumer bootstep: .. code-block:: python from celery import Celery from celery import bootsteps class InfoStep(bootsteps.Step): def __init__(self, parent, **kwargs): # here we can prepare the Worker/Consumer object # in any way we want, set attribute defaults and so on. print('{0!r} is in init'.format(parent)) def start(self, parent): # our step is started together with all other Worker/Consumer # bootsteps. print('{0!r} is starting'.format(parent)) def stop(self, parent): # the Consumer calls stop every time the consumer is restarted # (i.e. connection is lost) and also at shutdown. The Worker # will call stop at shutdown only. print('{0!r} is stopping'.format(parent)) def shutdown(self, parent): # shutdown is called by the Consumer at shutdown, it's not # called by Worker. print('{0!r} is shutting down'.format(parent)) app = Celery(broker='amqp://') app.steps['worker'].add(InfoStep) app.steps['consumer'].add(InfoStep) Starting the worker with this step installed will give us the following logs:: is in init is in init [2013-05-29 16:18:20,544: WARNING/MainProcess] is starting [2013-05-29 16:18:21,577: WARNING/MainProcess] is starting is stopping is stopping is shutting down The ``print`` statements will be redirected to the logging subsystem after the worker has been initialized, so the "is starting" lines are timestamped. You may notice that this does no longer happen at shutdown, this is because the ``stop`` and ``shutdown`` methods are called inside a *signal handler*, and it's not safe to use logging inside such a handler. Logging with the Python logging module is not :term:`reentrant`, which means that you cannot interrupt the function and call it again later. It's important that the ``stop`` and ``shutdown`` methods you write is also :term:`reentrant`. Starting the worker with ``--loglevel=debug`` will show us more information about the boot process:: [2013-05-29 16:18:20,509: DEBUG/MainProcess] | Worker: Preparing bootsteps. [2013-05-29 16:18:20,511: DEBUG/MainProcess] | Worker: Building graph... is in init [2013-05-29 16:18:20,511: DEBUG/MainProcess] | Worker: New boot order: {Hub, Queues (intra), Pool, Autoreloader, Timer, StateDB, Autoscaler, InfoStep, Beat, Consumer} [2013-05-29 16:18:20,514: DEBUG/MainProcess] | Consumer: Preparing bootsteps. [2013-05-29 16:18:20,514: DEBUG/MainProcess] | Consumer: Building graph... is in init [2013-05-29 16:18:20,515: DEBUG/MainProcess] | Consumer: New boot order: {Connection, Mingle, Events, Gossip, InfoStep, Agent, Heart, Control, Tasks, event loop} [2013-05-29 16:18:20,522: DEBUG/MainProcess] | Worker: Starting Hub [2013-05-29 16:18:20,522: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:20,522: DEBUG/MainProcess] | Worker: Starting Pool [2013-05-29 16:18:20,542: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:20,543: DEBUG/MainProcess] | Worker: Starting InfoStep [2013-05-29 16:18:20,544: WARNING/MainProcess] is starting [2013-05-29 16:18:20,544: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:20,544: DEBUG/MainProcess] | Worker: Starting Consumer [2013-05-29 16:18:20,544: DEBUG/MainProcess] | Consumer: Starting Connection [2013-05-29 16:18:20,559: INFO/MainProcess] Connected to amqp://guest@127.0.0.1:5672// [2013-05-29 16:18:20,560: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:20,560: DEBUG/MainProcess] | Consumer: Starting Mingle [2013-05-29 16:18:20,560: INFO/MainProcess] mingle: searching for neighbors [2013-05-29 16:18:21,570: INFO/MainProcess] mingle: no one here [2013-05-29 16:18:21,570: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:21,571: DEBUG/MainProcess] | Consumer: Starting Events [2013-05-29 16:18:21,572: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:21,572: DEBUG/MainProcess] | Consumer: Starting Gossip [2013-05-29 16:18:21,577: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:21,577: DEBUG/MainProcess] | Consumer: Starting InfoStep [2013-05-29 16:18:21,577: WARNING/MainProcess] is starting [2013-05-29 16:18:21,578: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:21,578: DEBUG/MainProcess] | Consumer: Starting Heart [2013-05-29 16:18:21,579: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:21,579: DEBUG/MainProcess] | Consumer: Starting Control [2013-05-29 16:18:21,583: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:21,583: DEBUG/MainProcess] | Consumer: Starting Tasks [2013-05-29 16:18:21,606: DEBUG/MainProcess] basic.qos: prefetch_count->80 [2013-05-29 16:18:21,606: DEBUG/MainProcess] ^-- substep ok [2013-05-29 16:18:21,606: DEBUG/MainProcess] | Consumer: Starting event loop [2013-05-29 16:18:21,608: WARNING/MainProcess] celery@example.com ready. .. _extending-programs: Command-line programs ===================== .. _extending-commandoptions: Adding new command-line options ------------------------------- .. _extending-command-options: Command-specific options ~~~~~~~~~~~~~~~~~~~~~~~~ You can add additional command-line options to the ``worker``, ``beat`` and ``events`` commands by modifying the :attr:`~@Celery.user_options` attribute of the application instance. Celery commands uses the :mod:`optparse` module to parse command-line arguments, and so you have to use :mod:`optparse` specific option instances created using :func:`optparse.make_option`. Please see the :mod:`optparse` documentation to read about the fields supported. Example adding a custom option to the :program:`celery worker` command: .. code-block:: python from celery import Celery from celery.bin import Option # <-- alias to optparse.make_option app = Celery(broker='amqp://') app.user_options['worker'].add( Option('--enable-my-option', action='store_true', default=False, help='Enable custom option.'), ) All bootsteps will now receive this argument as a keyword argument to ``Bootstep.__init__``: .. code-block:: python from celery import bootsteps class MyBootstep(bootsteps.Step): def __init__(self, worker, enable_my_option=False, **options): if enable_my_option: party() app.steps['worker'].add(MyBootstep) .. _extending-preload_options: Preload options ~~~~~~~~~~~~~~~ The :program:`celery` umbrella command supports the concept of 'preload options', which are special options passed to all subcommands and parsed outside of the main parsing step. The list of default preload options can be found in the API reference: :mod:`celery.bin.base`. You can add new preload options too, e.g. to specify a configuration template: .. code-block:: python from celery import Celery from celery import signals from celery.bin import Option app = Celery() app.user_options['preload'].add( Option('-Z', '--template', default='default', help='Configuration template to use.'), ) @signals.user_preload_options.connect def on_preload_parsed(options, **kwargs): use_template(options['template']) .. _extending-subcommands: Adding new :program:`celery` sub-commands ----------------------------------------- New commands can be added to the :program:`celery` umbrella command by using `setuptools entry-points`_. .. _`setuptools entry-points`: http://reinout.vanrees.org/weblog/2010/01/06/zest-releaser-entry-points.html Entry-points is special metadata that can be added to your packages ``setup.py`` program, and then after installation, read from the system using the :mod:`pkg_resources` module. Celery recognizes ``celery.commands`` entry-points to install additional subcommands, where the value of the entry-point must point to a valid subclass of :class:`celery.bin.base.Command`. There is limited documentation, unfortunately, but you can find inspiration from the various commands in the :mod:`celery.bin` package. This is how the Flower_ monitoring extension adds the :program:`celery flower` command, by adding an entry-point in :file:`setup.py`: .. code-block:: python setup( name='flower', entry_points={ 'celery.commands': [ 'flower = flower.command.FlowerCommand', ], } ) .. _Flower: http://pypi.python.org/pypi/flower The command definition is in two parts separated by the equal sign, where the first part is the name of the subcommand (flower), then the fully qualified module path to the class that implements the command (``flower.command.FlowerCommand``). In the module :file:`flower/command.py`, the command class is defined something like this: .. code-block:: python from celery.bin.base import Command, Option class FlowerCommand(Command): def get_options(self): return ( Option('--port', default=8888, type='int', help='Webserver port', ), Option('--debug', action='store_true'), ) def run(self, port=None, debug=False, **kwargs): print('Running our command') Worker API ========== :class:`~kombu.async.Hub` - The workers async event loop. --------------------------------------------------------- :supported transports: amqp, redis .. versionadded:: 3.0 The worker uses asynchronous I/O when the amqp or redis broker transports are used. The eventual goal is for all transports to use the eventloop, but that will take some time so other transports still use a threading-based solution. .. method:: hub.add(fd, callback, flags) .. method:: hub.add_reader(fd, callback, \*args) Add callback to be called when ``fd`` is readable. The callback will stay registered until explictly removed using :meth:`hub.remove(fd) `, or the fd is automatically discarded because it's no longer valid. Note that only one callback can be registered for any given fd at a time, so calling ``add`` a second time will remove any callback that was previously registered for that fd. A file descriptor is any file-like object that supports the ``fileno`` method, or it can be the file descriptor number (int). .. method:: hub.add_writer(fd, callback, \*args) Add callback to be called when ``fd`` is writable. See also notes for :meth:`hub.add_reader` above. .. method:: hub.remove(fd) Remove all callbacks for ``fd`` from the loop. Timer - Scheduling events ------------------------- .. method:: timer.call_after(secs, callback, args=(), kwargs=(), priority=0) .. method:: timer.call_repeatedly(secs, callback, args=(), kwargs=(), priority=0) .. method:: timer.call_at(eta, callback, args=(), kwargs=(), priority=0)