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.
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.
nats context save --server=$NATS_URL local
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.
Tiny sleep to ensure the subscription connected.
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.
Publishing again will not result in anything.
nats pub 'greet.bob' 'hello'
13:57:13 Published 5 bytes to "greet.joe" 13:57:13 Subscribing on greet.* 13:57:13 Published 5 bytes to "greet.joe" [#1] Received on "greet.joe" hello 13:57:13 Published 5 bytes to "greet.pam" [#2] Received on "greet.pam" hello 13:57:13 Published 5 bytes to "greet.bob" [#3] Received on "greet.bob" hello 13:57:13 Published 5 bytes to "greet.bob"