SUBSCRIBE & PUBLISH — Pub/Sub Messaging

Redis Pub/Sub lets clients communicate through message channels without knowing about each other — the publish-subscribe pattern.

How It Works

1. Subscriber calls SUBSCRIBE channel — starts listening
2. Publisher calls PUBLISH channel message — sends to all subscribers
3. Subscriber receives: message <channel> <message>

Real-World Uses

• Chat rooms: each room is a channel
• Live notifications: subscribe to notifications:user:123
• Cache invalidation: publish when data changes, subscribers flush their cache
• Event-driven architecture: microservices communicate via channels

Fire and Forget

Pub/Sub is fire-and-forget — if no one is subscribed when a message is published, it's lost forever. This is NOT a queue (use Redis Streams for that).

Our Simplified Model

Since stdin/stdout is single-client, we simulate it: when PUBLISH is called and the client is subscribed to that channel, output the message. PUBLISH returns the number of matching subscriptions.

Implementation

Track subscriptions as a set of channel names. On PUBLISH, check if the channel is in the set.