Core Publish-Subcribe (CLI)

Core Publish-Subcribe 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/cli

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

Code

#!/bin/sh

The nats CLI utilizes the NATS_URL environment variable if set. However, if you want to manage different contexts for connecting or authenticating, check out the nats context commands. For example:

nats context save --server=$NATS_URL local

NATS_URL="${NATS_URL:-nats://localhost:4222}"

Publish a message to the subject ‘greet.joe’. Nothing will happen since the subscription is not yet setup.

nats pub 'greet.joe' 'hello'

Let’s start a subscription in the background that will print the output to stdout.

nats sub 'greet.*' &

This just captures the process ID of the previous command in this shell.

SUB_PID=$!

Tiny sleep to ensure the subscription connected.

sleep 0.5

Now we can publish a couple times..

nats pub 'greet.joe' 'hello'
nats pub 'greet.pam' 'hello'
nats pub 'greet.bob' 'hello'

Remove the subscription.

kill $SUB_PID

Publishing again will not result in anything.

nats pub 'greet.bob' 'hello'

Output

Network e113dd2b_default  Creating
Network e113dd2b_default  Created
Container e113dd2b-nats-1  Creating
Container e113dd2b-nats-1  Created
Container e113dd2b-nats-1  Starting
Container e113dd2b-nats-1  Started
             _             _               
 _ __   __ _| |_ ___      | |__   _____  __
| '_ \ / _` | __/ __|_____| '_ \ / _ \ \/ /
| | | | (_| | |_\__ \_____| |_) | (_) >  < 
|_| |_|\__,_|\__|___/     |_.__/ \___/_/\_\
                                           
nats-box v0.12.0
12:08:34 Published 5 bytes to "greet.joe"
12:08:34 Subscribing on greet.*
12:08:34 Published 5 bytes to "greet.joe"
[#1] Received on "greet.joe"
hello

[#2] Received on "greet.pam"
hello

12:08:34 Published 5 bytes to "greet.pam"
[#3] Received on "greet.bob"
hello

12:08:34 Published 5 bytes to "greet.bob"
12:08:35 Published 5 bytes to "greet.bob"

Recording

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