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RPC over NATS#

Because NATS has zero cost for creating new subjects, we can easily set up a new subject consumer just for the one response message. This way, your request message will be published to one topic, and the response message will be consumed from another one (temporary subject), which allows you to use regular FastStream RPC syntax in the NATS case too.


FastStream RPC over NATS works in both the NATS-Core and NATS-JS cases as well, but in the NATS-JS case, you have to specify the expected stream as a publish argument.

Blocking Request#

FastStream provides you with the ability to send a blocking RPC request over NATS in a very simple way.

Just send a message like a regular one and get a response synchronously.

It is very close to the common requests syntax:

msg = await broker.publish(

Also, you have two extra options to control this behavior:

  • rpc_timeout: Optional[float] = 30.0 - controls how long you are waiting for a response.
  • raise_timeout: bool = False - by default, a timeout request returns None, but if you need to raise a TimeoutException directly, you can specify this option.


Also, if you want to create a permanent request-reply data flow, probably, you should create a permanent subject to consume responses.

So, if you have such one, you can specify it with the reply_to argument. This way, FastStream will send a response to this subject automatically.

async def consume_responses(msg):

msg = await broker.publish(