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Fanout Exchange#

The Fanout Exchange is an even simpler, but slightly less popular way of routing in RabbitMQ. This type of exchange sends messages to all queues subscribed to it, ignoring any arguments of the message.

At the same time, if the queue listens to several consumers, messages will also be distributed among them (default scaling mechanism).

Example#

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from faststream import FastStream, Logger
from faststream.rabbit import ExchangeType, RabbitBroker, RabbitExchange, RabbitQueue

broker = RabbitBroker()
app = FastStream(broker)

exch = RabbitExchange("exchange", auto_delete=True, type=ExchangeType.FANOUT)

queue_1 = RabbitQueue("test-q-1", auto_delete=True)
queue_2 = RabbitQueue("test-q-2", auto_delete=True)


@broker.subscriber(queue_1, exch)
async def base_handler1(logger: Logger):
    logger.info("base_handler1")


@broker.subscriber(queue_1, exch)  # another service
async def base_handler2(logger: Logger):
    logger.info("base_handler2")


@broker.subscriber(queue_2, exch)
async def base_handler3(logger: Logger):
    logger.info("base_handler3")


@app.after_startup
async def send_messages():
    await broker.publish(exchange=exch)  # handlers: 1, 3
    await broker.publish(exchange=exch)  # handlers: 2, 3
    await broker.publish(exchange=exch)  # handlers: 1, 3
    await broker.publish(exchange=exch)  # handlers: 2, 3

Consumer Announcement#

To begin with, we announced our Fanout exchange and several queues that will listen to it:

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exch = RabbitExchange("exchange", auto_delete=True, type=ExchangeType.FANOUT)

queue_1 = RabbitQueue("test-q-1", auto_delete=True)
queue_2 = RabbitQueue("test-q-2", auto_delete=True)

Then we signed up several consumers using the advertised queues to the exchange we created:

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@broker.subscriber(queue_1, exch)
async def base_handler1(logger: Logger):
    logger.info("base_handler1")


@broker.subscriber(queue_1, exch)  # another service
async def base_handler2(logger: Logger):
    logger.info("base_handler2")


@broker.subscriber(queue_2, exch)
async def base_handler3(logger: Logger):
    logger.info("base_handler3")

Note

handler1 and handler2 are subscribed to the same exchange using the same queue: within a single service, this does not make sense, since messages will come to these handlers in turn. Here we emulate the work of several consumers and load balancing between them.

Message Distribution#

Now the all messages will be send to all subscribers due they are binded to the same FANOUT exchange:

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await broker.publish(exchange=exch)  # handlers: 1, 3
await broker.publish(exchange=exch)  # handlers: 2, 3
await broker.publish(exchange=exch)  # handlers: 1, 3
await broker.publish(exchange=exch)  # handlers: 2, 3

Note

When sending messages to Fanout exchange, it makes no sense to specify the arguments queue or routing_key, because they will be ignored.