Factory

Although you could set up your own topics and queues in your infrastructure (e.g. by using terraform) you can rely on

Personally I have no strong feelings over defining your queues and topics through an infrastructure-as-code framework or letting the application create it's own queues and topics (as long as it has the appropriate permissions to do so). In any case, Melange offers a factory to create queues and topics for you with the MessagingBackendFactory.

The factory initialization methods are idempotent. If a queue or a topic already exist, they will keep the same queue or topic, but override any settings or customizations that you might have manually set in your PaaS platform.

Creating a queue

Let's say you'd wish to create an Amazon SQS queue to listen to the events for a payment service. You could invoke the factory as follows:

from melange import MessagingBackendFactory
from melange.backends import LocalSQSBackend

backend = LocalSQSBackend()
factory = MessagingBackendFactory(backend)
factory.init_queue("payment-updates.fifo")

This will create a FIFO queue payment-updates in your AWS account (remember to appropriately set the AWS variables since the SQS backend uses boto behind the scenes).

You could also define a dead letter queue for messages that could not be delivered successfully:

factory.init_queue("payment-updates.fifo", dead_letter_queue_name="payment-updates.fifo")

Creating a topic

Topics apply the fan-out pattern to send the message to anyone who is subscribed to them. They are useful to decouple your consumers from your application so that they don't need to know who they are sending their messages to. With the factory you could create a topic like this:

from melange import MessagingBackendFactory
from melange.backends import AWSBackend

backend = AWSBackend()
factory = MessagingBackendFactory(backend)
factory.init_topic("my-topic")

Creating a queue and subscribing it to several topics

You could create a queue and immediately subscribe it to a number of topics:

from melange import MessagingBackendFactory
from melange.backends import LocalSQSBackend

backend = LocalSQSBackend()
factory = MessagingBackendFactory(backend)
factory.init_queue(
    "payment-updates.fifo",
    ["my-topic-1", "my-topic-2", "my-topic-3"],
    dead_letter_queue_name="payment-updates.fifo",
)

This will create the topics my-topic-1, my-topic-2 and my-topic-3, then create the payment-updates.fifo queue, and subscribe it to the aforementioned topics. It will create the dead letter queue too.