Pub/Sub

Redis Pub/Sub (publish/subscribe) is a messaging pattern where publishers send messages to named channels and subscribers receive them in real time. Publishers and subscribers are decoupled — a publisher does not need to know how many subscribers are listening, and subscribers do not need to know who is publishing.

How Pub/Sub Works

The model has three components:

• Channel — a named message bus (just a string)
• Publisher — a client that sends messages to a channel using PUBLISH
• Subscriber — a client that listens on one or more channels using SUBSCRIBE

When a publisher sends a message, Redis fans it out to every currently-connected subscriber on that channel. Messages are not persisted — if no subscriber is listening when a message is published, it is lost.

SUBSCRIBE

Open a second terminal and subscribe to a channel:

127.0.0.1:6379> SUBSCRIBE notifications
Reading messages... (press Ctrl-C to quit)
1) "subscribe"
2) "notifications"
3) (integer) 1

The client is now in subscriber mode and can only receive messages (no other commands).

PUBLISH

In the first terminal, publish a message:

127.0.0.1:6379> PUBLISH notifications "New order placed: #1042"
(integer) 1   # number of subscribers that received the message

The subscriber terminal immediately displays:

1) "message"
2) "notifications"
3) "New order placed: #1042"

PSUBSCRIBE — Pattern Subscriptions

Subscribe to multiple channels matching a glob-style pattern:

127.0.0.1:6379> PSUBSCRIBE order.*

This matches channels such as order.created, order.shipped, and order.cancelled. Use PUNSUBSCRIBE to unsubscribe from patterns.

UNSUBSCRIBE

127.0.0.1:6379> UNSUBSCRIBE notifications

Limitations of Pub/Sub

• No persistence — messages are fire-and-forget; subscribers that are offline miss all messages
• No acknowledgement — there is no confirmation that a subscriber processed a message
• No replay — you cannot retrieve past messages

For durable, replayable message streams consider using Redis Streams (XADD / XREAD) instead.

Practical Use Cases

# Live dashboard updates
127.0.0.1:6379> PUBLISH dashboard "metrics_updated"

# Chat application rooms
127.0.0.1:6379> PUBLISH room:general "alice: hello everyone"

# Cache invalidation across multiple app servers
127.0.0.1:6379> PUBLISH cache_invalidate "product:99"

Pub/Sub is an excellent fit for lightweight, real-time event broadcasting where at-most-once delivery is acceptable.