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Request-Reply in Messaging

The request-reply pattern allows a client to send a message and expect a reply of some kind. In practice, the request message will either be a command, which is an intention for service to carry out some work that results in a state change, or a query, which is a request for information.

Unlike request-reply constrained protocols like HTTP, NATS is not limited to a strict point-to-point interaction between a client and server. The request-reply pattern is built on top of the core publish-subscribe model.

By default, this means that any one of subscribers could be a responder and reply to the client. However, because NATS is not limited to point-to-point interactions, the client could indicate to NATS that multiple replies should be allowed.

This example shows the basics of the request-reply pattern including the standard “no responders” error if there are no subscribers available to handle and reply to the requesting message.

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$ nbe run messaging/request-reply/python
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import os
import asyncio

import nats
from nats.errors import TimeoutError, NoRespondersError

Get the list of servers.

servers = os.environ.get("NATS_URL", "nats://localhost:4222").split(",")

async def main():

Create the connection to NATS which takes a list of servers.

    nc = await nats.connect(servers=servers)

In addition to vanilla publish-request, NATS supports request-reply interactions as well. Under the covers, this is just an optimized pair of publish-subscribe operations. The request handler is a subscription that responds to a message sent to it. This kind of subscription is called a service. We can use the cb argument for asynchronous handling.

    async def greet_handler(msg):

Parse out the second token in the subject (everything after greet.) and use it as part of the response message.

        name = msg.subject[6:]
        reply = f"hello, {name}"
        await msg.respond(reply.encode("utf8"))

    sub = await nc.subscribe("greet.*", cb=greet_handler)

Now we can use the built-in request method to do the service request. We simply pass a empty body since that is being used right now. In addition, we need to specify a timeout since with a request we are waiting for the reply and we likely don’t want to wait forever.

    rep = await nc.request("greet.joe", b'', timeout=0.5)

    rep = await nc.request("greet.sue", b'', timeout=0.5)

    rep = await nc.request("greet.bob", b'', timeout=0.5)

What happens if the service is unavailable? We can simulate this by unsubscribing our handler from above. Now if we make a request, we will expect an error.

    await sub.drain()

        await nc.request("greet.joe", b'', timeout=0.5)
    except NoRespondersError:
        print("no responders")

    await nc.drain()

if __name__ == '__main__':


b'hello, joe'
b'hello, sue'
b'hello, bob'
no responders


Note, playback is half speed to make it a bit easier to follow.