Publishing Basics#

FastStream is broker-agnostic and easy to use, even as a client in non-FastStream applications.

It offers several use cases for publishing messages:

• Using broker.publish(...)
• Using the @broker.publisher(...) decorator
• Using a publisher object decorator
• Using a publisher object directly

All of these variants have their own advantages and limitations, so you can choose what you want based on your requirements. Please visit the following pages for details.

Serialization#

FastStream allows you to publish any JSON-serializable messages (Python types, Pydantic models, etc.) or raw bytes.

It automatically sets up all required headers, especially the correlation_id, which is used to trace message processing pipelines across all services.

The content-type is a meaningfull header for FastStream services. It helps the framework serialize messages faster, selecting the right serializer based on the header. This header is automatically set by FastStream too, but you should set it up manually using other libraries to interact with FastStream applications.

Content-Type can be:

• text/plain
• application/json
• empty with bytes content

By the way, you can use application/json for all of your messages if they are not raw bytes. You can even omit using any header at all, but it makes serialization slightly slower.

Publishing#

FastStream can also be used as a Broker client to send messages in other applications. It is quite straightforward and similar to aiohttp or requests.

You just need to connect your broker, and you are ready to send a message. Additionally, you can use Broker as an async context manager to establish a connection and disconnect when leaving the scope.

To publish a message, simply set up the message content and a routing key:

async with KafkaBroker() as br:
    await br.publish("message", "topic")

async with RabbitBroker() as br:
    await br.publish("message", "queue")

async with NatsBroker() as br:
    await br.publish("message", "subject")