This document is for Kombu's development version, which can be significantly different from previous releases. Get the stable docs here: 3.0.
Consumers¶
Basics¶
The Consumer
takes a connection (or channel) and a list of queues to
consume from. Several consumers can be mixed to consume from different
channels, as they all bind to the same connection, and drain_events
will
drain events from all channels on that connection.
Note
Kombu since 3.0 will only accept json/binary or text messages by default,
to allow deserialization of other formats you have to specify them
in the accept
argument:
Consumer(conn, accept=['json', 'pickle', 'msgpack', 'yaml'])
Draining events from a single consumer:
with Consumer(connection, queues, accept=['json']):
connection.drain_events(timeout=1)
Draining events from several consumers:
from kombu.utils.compat import nested
with connection.channel(), connection.channel() as (channel1, channel2):
with nested(Consumer(channel1, queues1, accept=['json']),
Consumer(channel2, queues2, accept=['json'])):
connection.drain_events(timeout=1)
Or using ConsumerMixin
:
from kombu.mixins import ConsumerMixin
class C(ConsumerMixin):
def __init__(self, connection):
self.connection = connection
def get_consumers(self, Consumer, channel):
return [
Consumer(queues, callbacks=[self.on_message], accept=['json']),
]
def on_message(self, body, message):
print('RECEIVED MESSAGE: {0!r}'.format(body))
message.ack()
C(connection).run()
and with multiple channels again:
from kombu import Consumer
from kombu.mixins import ConsumerMixin
class C(ConsumerMixin):
channel2 = None
def __init__(self, connection):
self.connection = connection
def get_consumers(self, _, default_channel):
self.channel2 = default_channel.connection.channel()
return [Consumer(default_channel, queues1,
callbacks=[self.on_message],
accept=['json']),
Consumer(self.channel2, queues2,
callbacks=[self.on_special_message],
accept=['json'])]
def on_consumer_end(self, connection, default_channel):
if self.channel2:
self.channel2.close()
C(connection).run()
There’s also a ConsumerProducerMixin
for consumers
that need to also publish messages on a separate connection (e.g. sending rpc
replies, streaming results):
from kombu import Producer, Queue
from kombu.mixins import ConsumerProducerMixin
rpc_queue = Queue('rpc_queue')
class Worker(ConsumerProducerMixin):
def __init__(self, connection):
self.connection = connection
def get_consumers(self, Consumer, channel):
return [Consumer(
queues=[rpc_queue],
on_message=self.on_request,
accept={'application/json'},
prefetch_count=1,
)]
def on_request(self, message):
n = message.payload['n']
print(' [.] fib({0})'.format(n))
result = fib(n)
self.producer.publish(
{'result': result},
exchange='', routing_key=message.properties['reply_to'],
correlation_id=message.properties['correlation_id'],
serializer='json',
retry=True,
)
message.ack()
See also
examples/rpc-tut6/
in the Github repository.
Advanced Topics¶
RabbitMQ¶
Consumer Priorities¶
RabbitMQ defines a consumer priority extension to the amqp protocol,
that can be enabled by setting the x-priority
argument to
basic.consume
.
In kombu you can specify this argument on the Queue
, like
this:
queue = Queue('name', Exchange('exchange_name', type='direct'),
consumer_arguments={'x-priority': 10})
Read more about consumer priorities here: https://www.rabbitmq.com/consumer-priority.html
Reference¶
-
class
kombu.
Consumer
(channel, queues=None, no_ack=None, auto_declare=None, callbacks=None, on_decode_error=None, on_message=None, accept=None, prefetch_count=None, tag_prefix=None)[source] Message consumer.
Parameters: - channel (kombu.Connection, ChannelT) – see
channel
. - queues (Sequence[kombu.Queue]) – see
queues
. - no_ack (bool) – see
no_ack
. - auto_declare (bool) – see
auto_declare
- callbacks (Sequence[Callable]) – see
callbacks
. - on_message (Callable) – See
on_message
- on_decode_error (Callable) – see
on_decode_error
. - prefetch_count (int) – see
prefetch_count
.
-
exception
ContentDisallowed
Consumer does not allow this content-type.
-
Consumer.
accept
= None List of accepted content-types.
An exception will be raised if the consumer receives a message with an untrusted content type. By default all content-types are accepted, but not if
kombu.disable_untrusted_serializers()
was called, in which case only json is allowed.
-
Consumer.
add_queue
(queue)[source] Add a queue to the list of queues to consume from.
Note
This will not start consuming from the queue, for that you will have to call
consume()
after.
-
Consumer.
auto_declare
= True By default all entities will be declared at instantiation, if you want to handle this manually you can set this to
False
.
-
Consumer.
callbacks
= None List of callbacks called in order when a message is received.
The signature of the callbacks must take two arguments: (body, message), which is the decoded message body and the
Message
instance.
-
Consumer.
cancel
()[source] End all active queue consumers.
Note
This does not affect already delivered messages, but it does mean the server will not send any more messages for this consumer.
-
Consumer.
cancel_by_queue
(queue)[source] Cancel consumer by queue name.
-
Consumer.
channel
= None The connection/channel to use for this consumer.
-
Consumer.
close
() End all active queue consumers.
Note
This does not affect already delivered messages, but it does mean the server will not send any more messages for this consumer.
-
Consumer.
consume
(no_ack=None)[source] Start consuming messages.
Can be called multiple times, but note that while it will consume from new queues added since the last call, it will not cancel consuming from removed queues ( use
cancel_by_queue()
).Parameters: no_ack (bool) – See no_ack
.
-
Consumer.
consuming_from
(queue)[source] Return
True
if currently consuming from queue’.
-
Consumer.
declare
()[source] Declare queues, exchanges and bindings.
Note
This is done automatically at instantiation when
auto_declare
is set.
-
Consumer.
flow
(active)[source] Enable/disable flow from peer.
This is a simple flow-control mechanism that a peer can use to avoid overflowing its queues or otherwise finding itself receiving more messages than it can process.
The peer that receives a request to stop sending content will finish sending the current content (if any), and then wait until flow is reactivated.
-
Consumer.
no_ack
= None Flag for automatic message acknowledgment. If enabled the messages are automatically acknowledged by the broker. This can increase performance but means that you have no control of when the message is removed.
Disabled by default.
-
Consumer.
on_decode_error
= None Callback called when a message can’t be decoded.
The signature of the callback must take two arguments: (message, exc), which is the message that can’t be decoded and the exception that occurred while trying to decode it.
-
Consumer.
on_message
= None Optional function called whenever a message is received.
When defined this function will be called instead of the
receive()
method, andcallbacks
will be disabled.So this can be used as an alternative to
callbacks
when you don’t want the body to be automatically decoded. Note that the message will still be decompressed if the message has thecompression
header set.The signature of the callback must take a single argument, which is the
Message
object.Also note that the
message.body
attribute, which is the raw contents of the message body, may in some cases be a read-onlybuffer
object.
-
Consumer.
prefetch_count
= None Initial prefetch count
If set, the consumer will set the prefetch_count QoS value at startup. Can also be changed using
qos()
.
-
Consumer.
purge
()[source] Purge messages from all queues.
Warning
This will delete all ready messages, there is no undo operation.
-
Consumer.
qos
(prefetch_size=0, prefetch_count=0, apply_global=False)[source] Specify quality of service.
The client can request that messages should be sent in advance so that when the client finishes processing a message, the following message is already held locally, rather than needing to be sent down the channel. Prefetching gives a performance improvement.
The prefetch window is Ignored if the
no_ack
option is set.Parameters: - prefetch_size (int) – Specify the prefetch window in octets. The server will send a message in advance if it is equal to or smaller in size than the available prefetch size (and also falls within other prefetch limits). May be set to zero, meaning “no specific limit”, although other prefetch limits may still apply.
- prefetch_count (int) – Specify the prefetch window in terms of whole messages.
- apply_global (bool) – Apply new settings globally on all channels.
-
Consumer.
queues
A single
Queue
, or a list of queues to consume from.
-
Consumer.
receive
(body, message)[source] Method called when a message is received.
This dispatches to the registered
callbacks
.Parameters: - body (Any) – The decoded message body.
- message (Message) – The message instance.
Raises: NotImplementedError
– If no consumer callbacks have been registered.
-
Consumer.
recover
(requeue=False)[source] Redeliver unacknowledged messages.
Asks the broker to redeliver all unacknowledged messages on the specified channel.
Parameters: requeue (bool) – By default the messages will be redelivered to the original recipient. With requeue set to true, the server will attempt to requeue the message, potentially then delivering it to an alternative subscriber.
-
Consumer.
register_callback
(callback)[source] Register a new callback to be called when a message is received.
Note
The signature of the callback needs to accept two arguments: (body, message), which is the decoded message body and the
Message
instance.
-
Consumer.
revive
(channel)[source] Revive consumer after connection loss.
- channel (kombu.Connection, ChannelT) – see