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, that must
return a list of :class:`kombu.Consumer` objects to start
whenever the connection is established:
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(who, producer=None):
with app.producer_or_acquire(producer) as producer:
producer.publish(
{'hello': who},
serializer='json',
exchange=my_queue.exchange,
routing_key='routing_key',
declare=[my_queue],
retry=True,
)
if __name__ == '__main__':
send_me_a_message('world!')Note
Kombu Consumers can take use of two different message callback dispatching
mechanisms. The first one is the callbacks argument that accepts
a list of callbacks with a (body, message) signature,
the second one is the on_message argument that takes a single
callback with a (message,) signature. The latter won't
automatically decode and deserialize the payload.
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()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, that then establishes the broker connection and starts consuming messages.
The Worker is the first blueprint to start, and with it starts major components like the event loop, processing pool, and the timer used for ETA tasks and other timed events.
When the worker is fully started it continues with the Consumer blueprint, that sets up how tasks are executed, connects to the broker and starts the message consumers.
The :class:`~celery.worker.WorkController` is the core worker implementation, and contains several methods and attributes that you can use in your bootstep.
.. 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.asynchronous.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.asynchronous.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 <celery.worker.state.Persistent>`` 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 file-system 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:
from celery import bootsteps
class ExampleWorkerStep(bootsteps.StartStopStep):
requires = {'celery.worker.components: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:
from celery import bootsteps
class DeadlockDetection(bootsteps.StartStopStep):
requires = {'celery.worker.components: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 worker.active_requests:
if req.time_start and time() - req.time_start > self.timeout:
raise SystemExit()The Celery worker emits messages to the Python logging subsystem for various
events throughout the lifecycle of a task.
These messages can be customized by overriding the LOG_<TYPE> format
strings which are defined in :file:`celery/app/trace.py`.
For example:
import celery.app.trace
celery.app.trace.LOG_SUCCESS = "This is a custom message"The various format strings are all provided with the task name and ID for
% formatting, and some of them receive extra fields like the return value
or the exception which caused a task to fail.
These fields can be used in custom format strings like so:
import celery.app.trace
celery.app.trace.LOG_REJECTED = "%(name)r is cursed and I won't run it: %(exc)s"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.
.. 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.asynchronous.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:: 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: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:Events'}
.. attribute:: gossip
Worker to worker broadcast communication
(:class:`~celery.worker.consumer.gossip.Gossip`).
A consumer bootstep must require the `Gossip` bootstep to use this.
.. code-block:: python
class RatelimitStep(bootsteps.StartStopStep):
"""Rate limit tasks based on the number of workers in the
cluster."""
requires = {'celery.worker.consumer.gossip:Gossip'}
def start(self, c):
self.c = c
self.c.gossip.on.node_join.add(self.on_cluster_size_change)
self.c.gossip.on.node_leave.add(self.on_cluster_size_change)
self.c.gossip.on.node_lost.add(self.on_node_lost)
self.tasks = [
self.app.tasks['proj.tasks.add']
self.app.tasks['proj.tasks.mul']
]
self.last_size = None
def on_cluster_size_change(self, worker):
cluster_size = len(list(self.c.gossip.state.alive_workers()))
if cluster_size != self.last_size:
for task in self.tasks:
task.rate_limit = 1.0 / cluster_size
self.c.reset_rate_limits()
self.last_size = cluster_size
def on_node_lost(self, worker):
# may have processed heartbeat too late, so wake up soon
# in order to see if the worker recovered.
self.c.timer.call_after(10.0, self.on_cluster_size_change)
**Callbacks**
- ``<set> gossip.on.node_join``
Called whenever a new node joins the cluster, providing a
:class:`~celery.events.state.Worker` instance.
- ``<set> gossip.on.node_leave``
Called whenever a new node leaves the cluster (shuts down),
providing a :class:`~celery.events.state.Worker` instance.
- ``<set> gossip.on.node_lost``
Called whenever heartbeat was missed for a worker instance in the
cluster (heartbeat not received or processed in time),
providing a :class:`~celery.events.state.Worker` instance.
This doesn't necessarily mean the worker is actually offline, so use a time
out mechanism if the default heartbeat timeout isn't sufficient.
.. attribute:: pool
The current process/eventlet/gevent/thread pool.
See :class:`celery.concurrency.base.BasePool`.
.. attribute:: timer
:class:`Timer <celery.utils.timer2.Schedule` used to schedule functions.
.. attribute:: heart
Responsible for sending worker event heartbeats
(:class:`~celery.worker.heartbeat.Heart`).
Your consumer bootstep must require the `Heart` bootstep to use this:
.. code-block:: python
class Step(bootsteps.StartStopStep):
requires = {'celery.worker.consumer.heart:Heart'}
.. attribute:: task_consumer
The :class:`kombu.Consumer` object used to consume task messages.
Your consumer bootstep must require the `Tasks` bootstep to use this:
.. code-block:: python
class Step(bootsteps.StartStopStep):
requires = {'celery.worker.consumer.tasks:Tasks'}
.. attribute:: strategies
Every registered task type has an entry in this mapping,
where the value is used to execute an incoming message of this task type
(the task execution strategy). This mapping is generated by the Tasks
bootstep when the consumer starts:
.. code-block:: python
for name, task in app.tasks.items():
strategies[name] = task.start_strategy(app, consumer)
task.__trace__ = celery.app.trace.build_tracer(
name, task, loader, hostname
)
Your consumer bootstep must require the `Tasks` bootstep to use this:
.. code-block:: python
class Step(bootsteps.StartStopStep):
requires = {'celery.worker.consumer.tasks:Tasks'}
.. attribute:: task_buckets
A :class:`~collections.defaultdict` used to look-up the rate limit for
a task by type.
Entries in this dict may be None (for no limit) or a
:class:`~kombu.utils.limits.TokenBucket` instance implementing
``consume(tokens)`` and ``expected_time(tokens)``.
TokenBucket implements the `token bucket algorithm`_, but any algorithm
may be used as long as it conforms to the same interface and defines the
two methods above.
.. _`token bucket algorithm`: https://en.wikipedia.org/wiki/Token_bucket
.. attribute:: qos
The :class:`~kombu.common.QoS` object can be used to change the
task channels current prefetch_count value:
.. code-block:: python
# increment at next cycle
consumer.qos.increment_eventually(1)
# decrement at next cycle
consumer.qos.decrement_eventually(1)
consumer.qos.set(10)
.. method:: consumer.reset_rate_limits()
Updates the ``task_buckets`` mapping for all registered task types.
.. method:: consumer.bucket_for_task(type, Bucket=TokenBucket)
Creates rate limit bucket for a task using its ``task.rate_limit``
attribute.
.. method:: consumer.add_task_queue(name, exchange=None, exchange_type=None,
routing_key=None, \*\*options):
Adds new queue to consume from. This will persist on connection restart.
.. method:: consumer.cancel_task_queue(name)
Stop consuming from queue by name. This will persist on connection
restart.
.. method:: apply_eta_task(request)
Schedule ETA task to execute based on the ``request.eta`` attribute.
(:class:`~celery.worker.request.Request`)
app.steps['worker'] and app.steps['consumer'] can be modified
to add new bootsteps:
>>> app = Celery()
>>> app.steps['worker'].add(MyWorkerStep) # < add class, don't 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 isn't 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:
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:
<Worker: w@example.com (initializing)> is in init
<Consumer: w@example.com (initializing)> is in init
[2013-05-29 16:18:20,544: WARNING/MainProcess]
<Worker: w@example.com (running)> is starting
[2013-05-29 16:18:21,577: WARNING/MainProcess]
<Consumer: w@example.com (running)> is starting
<Consumer: w@example.com (closing)> is stopping
<Worker: w@example.com (closing)> is stopping
<Consumer: w@example.com (terminating)> 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 time-stamped.
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 isn't :term:`reentrant`:
meaning you cannot interrupt the function then
call it again later. It's important that the stop and shutdown methods
you write is also :term:`reentrant`.
Starting the worker with :option:`--loglevel=debug <celery worker --loglevel>` 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...
<celery.apps.worker.Worker object at 0x101ad8410> is in init
[2013-05-29 16:18:20,511: DEBUG/MainProcess] | Worker: New boot order:
{Hub, Pool, 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...
<celery.worker.consumer.Consumer object at 0x101c2d8d0> 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]
<celery.apps.worker.Worker object at 0x101ad8410> 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]
<celery.worker.consumer.Consumer object at 0x101c2d8d0> 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.
You can add additional command-line options to the worker, beat, and
events commands by modifying the :attr:`~@user_options` attribute of the
application instance.
Celery commands uses the :mod:`click` module to parse command-line arguments, and so to add custom arguments you need to add :class:`click.Option` instances to the relevant set.
Example adding a custom option to the :program:`celery worker` command:
from celery import Celery
from click import Option
app = Celery(broker='amqp://')
app.user_options['worker'].add(Option(('--enable-my-option',),
is_flag=True,
help='Enable custom option.'))All bootsteps will now receive this argument as a keyword argument to
Bootstep.__init__:
from celery import bootsteps
class MyBootstep(bootsteps.Step):
def __init__(self, parent, enable_my_option=False, **options):
super().__init__(parent, **options)
if enable_my_option:
party()
app.steps['worker'].add(MyBootstep)The :program:`celery` umbrella command supports the concept of 'preload options'. These are special options passed to all sub-commands.
You can add new preload options, for example to specify a configuration template:
from celery import Celery
from celery import signals
from click 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'])Adding new :program:`celery` sub-commands
New commands can be added to the :program:`celery` umbrella command by using setuptools entry-points.
Entry-points is special meta-data that can be added to your packages setup.py program,
and then after installation, read from the system using the :mod:`importlib` module.
Celery recognizes celery.commands entry-points to install additional
sub-commands, where the value of the entry-point must point to a valid click
command.
This is how the :pypi:`Flower` monitoring extension may add the :program:`celery flower` command, by adding an entry-point in :file:`setup.py`:
setup(
name='flower',
entry_points={
'celery.commands': [
'flower = flower.command:flower',
],
}
)The command definition is in two parts separated by the equal sign, where the first part is the name of the sub-command (flower), then the second part is the fully qualified symbol path to the function that implements the command:
flower.command:flower
The module path and the name of the attribute should be separated by colon as above.
In the module :file:`flower/command.py`, the command function may be defined as the following:
import click
@click.command()
@click.option('--port', default=8888, type=int, help='Webserver port')
@click.option('--debug', is_flag=True)
def flower(port, debug):
print('Running our command'):class:`~kombu.asynchronous.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 event-loop, 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 explicitly removed using
:meth:`hub.remove(fd) <hub.remove>`, or the file descriptor is
automatically discarded because it's no longer valid.
Note that only one callback can be registered for any given
file descriptor at a time, so calling ``add`` a second time will remove
any callback that was previously registered for that file descriptor.
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 file descriptor ``fd`` from the loop.
.. 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)