Publish–Subscribe in NATS: Simple and Powerful Messaging

In the previous article, I wrote an overview of NATS and NATS JetStream. Before we go deeper into advanced topics like scalability and reliability, it’s important to understand the most common messaging pattern in NATS: Publish–Subscribe.

What is Publish–Subscribe?

Publish–Subscribe (or pub/sub) is a way of sending messages where:

• Publishers send messages to a subject.
• Subscribers listen to that subject.
• Publishers don’t know who the subscribers are.
• Subscribers don’t know where the messages come from.

It’s a decoupled system: the publisher just fires a message, and anyone who is interested can subscribe and receive it.

Pub/Sub in NATS

In NATS, everything revolves around subjects. A subject is like a channel or a topic.

• A publisher publishes a message to a subject.
• A subscriber subscribes to that subject and receives the message in real time.

Example:

// Publisher
nc.Publish("updates", []byte("Hello subscribers!"))

// Subscriber
nc.Subscribe("updates", func(msg *nats.Msg) {
    fmt.Printf("Received: %s\n", string(msg.Data))
})

When the publisher sends "Hello subscribers!", all subscribers of updates will get that message instantly.

Using Wildcards

NATS subjects support wildcards, which makes pub/sub flexible.

• * matches one token.
• > matches one or more tokens.

Example:

• orders.* → matches orders.created, orders.paid, but not orders.created.new.
• orders.> → matches everything that starts with orders.

This is useful when you want to organize your events with patterns.

Real-World Examples

1. Order Processing

• Subjects:
  • orders.created
  • orders.shipped
• Services can subscribe only to the events they care about. For example, the shipping service subscribes to orders.created.

2. Chat Application

• Subject: chat.room1
• Every user in room1 subscribes to chat.room1.
• Messages are instantly broadcast to everyone in that room.

3. System Monitoring

• Subject: metrics.cpu.usage
• Monitoring tools subscribe to all metrics using metrics.>.
• This way, they receive all metrics without needing separate subscriptions for each one.