Core Publish-Subscribe (Python)

Core Publish-Subscribe in Messaging

This example demonstrates the core NATS publish-subscribe behavior. This is the fundamental pattern that all other NATS patterns and higher-level APIs build upon. There are a few takeaways from this example:

Delivery is an at-most-once. For MQTT users, this is referred to as Quality of Service (QoS) 0.
There are two circumstances when a published message won’t be delivered to a subscriber:
- The subscriber does not have an active connection to the server (i.e. the client is temporarily offline for some reason)
- There is a network interruption where the message is ultimately dropped
Messages are published to subjects which can be one or more concrete tokens, e.g. greet.bob. Subscribers can utilize wildcards to show interest on a set of matching subjects.

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$ nbe run messaging/pub-sub/python

View the source code or learn how to run this example yourself

Code

import os
import asyncio


import nats
from nats.errors import TimeoutError

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)

Messages are published to subjects. Although there are no subscribers, this will be published successfully.

    await nc.publish("greet.joe", b"hello")

Let’s create a subscription on the greet.* wildcard.

    sub = await nc.subscribe("greet.*")

For a synchronous subscription, we need to fetch the next message. However.. since the publish occured before the subscription was established, this is going to timeout.

    try:
        msg = await sub.next_msg(timeout=0.1)
    except TimeoutError:
        pass

Publish a couple messages.

    await nc.publish("greet.joe", b"hello")
    await nc.publish("greet.pam", b"hello")

Since the subscription is established, the published messages will immediately be broadcasted to all subscriptions. They will land in # their buffer for subsequent NextMsg calls.

    msg = await sub.next_msg(timeout=0.1)
    print(f"{msg.data} on subject {msg.subject}")


    msg = await sub.next_msg(timeout=0.1)
    print(f"{msg.data} on subject {msg.subject}")

One more for good measures..

    await nc.publish("greet.bob", b"hello")


    msg = await sub.next_msg(timeout=0.1)
    print(f"{msg.data} on subject {msg.subject}")

Drain the subscription and connection. In contrast to unsubscribe, drain will process any queued messages before removing interest.

    await sub.unsubscribe()
    await nc.drain()




if __name__ == '__main__':
    asyncio.run(main())

Output

b'hello' on subject greet.joe
b'hello' on subject greet.pam
b'hello' on subject greet.bob

Recording

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