Redis Pub/Sub

13. Redis Pub/Sub

What is Pub/Sub?

Pub/Sub (Publish/Subscribe) is a real-time messaging pattern where:

Publishers send messages to a channel
Subscribers listen to channels and receive messages instantly

Publishers and subscribers are completely decoupled — they do not know about each other.

Analogy: Think of a radio station. The station (publisher) broadcasts on FM 98.5 (channel). Anyone tuning into that frequency (subscriber) receives the broadcast in real time.

Core Commands

Terminal 1 — Subscriber:

# Subscribe to a channel
SUBSCRIBE notifications

# Subscribe to multiple channels
SUBSCRIBE notifications alerts updates

# Subscribe using pattern matching
PSUBSCRIBE news:*      # matches: news:tech, news:sports, etc.

Terminal 2 — Publisher:

# Publish a message to a channel
PUBLISH notifications "New order received: #1042"

# Publisher sees how many subscribers received it
# Output: (integer) 1

Subscriber Terminal output:

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

Pub/Sub Communication Diagram

flowchart TB
    Pub1["📢 Publisher 1<br/>PUBLISH notifications 'New order!'"] --> Channel["Channel: notifications
Messages broadcast to all subscribers"]
    Pub2["📢 Publisher 2<br/>PUBLISH alerts 'Server down!'"] --> Channel

    Channel --> Sub1["👂 Subscriber 1<br/>SUBSCRIBE notifications alerts
Receives: notifications + alerts"]
    Channel --> Sub2["👂 Subscriber 2<br/>SUBSCRIBE notifications
Receives: notifications only"]
    Channel --> Sub3["👂 Subscriber 3<br/>PSUBSCRIBE news:*<br/>Receives: news:tech, news:sports, ..."]

    style Pub1 fill:#7c3aed,color:#fff
    style Pub2 fill:#7c3aed,color:#fff
    style Channel fill:#f59e0b,color:#fff
    style Sub1 fill:#3b82f6,color:#fff
    style Sub2 fill:#3b82f6,color:#fff
    style Sub3 fill:#3b82f6,color:#fff

Key insight: Publishers and subscribers are completely decoupled — they have no knowledge of each other. A publisher sends to a channel, and Redis delivers to every subscriber listening on that channel. If no one is subscribed, the message is simply lost.

Real-World Examples

Chat Application:

# User joins room channel
SUBSCRIBE chat:room:general

# User sends a message
PUBLISH chat:room:general '{"user":"Alice","msg":"Hello everyone!"}'

Live Notifications:

# Notification service publishes
PUBLISH user:notifications:42 '{"type":"order","message":"Your order shipped!"}'

# Frontend WebSocket server subscribed to this channel pushes to browser