Redis Pub/Sub
Pub/Sub is a messaging communication pattern. This lesson covers Redis publish/subscribe functionality.
What is Pub/Sub?
Pub/Sub pattern consists of:
- Publisher: sends messages to a channel
- Subscriber: receives messages from a channel
- Channel: the message conduit
Publisher → Channel → Subscriber 1
→ Subscriber 2
→ Subscriber 3
Basic Commands
SUBSCRIBE: Subscribe to a Channel
REDIS
# Subscribe to a single channel
SUBSCRIBE channel1
# Subscribe to multiple channels
SUBSCRIBE channel1 channel2 channel3
After subscribing, the client enters a waiting state to receive messages.
PUBLISH: Publish a Message
REDIS
# Publish a message to a channel
PUBLISH channel1 "Hello, Redis!"
(integer) 2 # Returns the number of subscribers that received the message
UNSUBSCRIBE: Unsubscribe
REDIS
# Unsubscribe from all channels
UNSUBSCRIBE
# Unsubscribe from a specific channel
UNSUBSCRIBE channel1
Pattern Subscription
PSUBSCRIBE: Pattern-Matched Subscription
REDIS
# Subscribe to channels matching a pattern
PSUBSCRIBE news.* # Subscribe to all channels starting with news.
# Example
PSUBSCRIBE user:*:notifications # Subscribe to all user notification channels
PUNSUBSCRIBE: Cancel Pattern Subscription
REDIS
# Cancel all pattern subscriptions
PUNSUBSCRIBE
# Cancel a specific pattern subscription
PUNSUBSCRIBE news.*
Use Cases
1. Real-Time Message Push
Example: Real-Time Notifications
BASH
# Terminal 1: subscribe to notifications
SUBSCRIBE notifications
# Terminal 2: publish a notification
PUBLISH notifications "New user registered"
2. Chat Room
Example: Chat Room
BASH
# User 1: subscribe to chat room
SUBSCRIBE chatroom:general
# User 2: send a message
PUBLISH chatroom:general "Hello everyone!"
3. System Notifications
Example: System Alerts
BASH
# Admin: subscribe to system alerts
SUBSCRIBE system:alerts
# System: send an alert
PUBLISH system:alerts "High CPU usage detected"
Message Format
Message format received by subscribers:
TEXT
1) "message" # Message type
2) "channel1" # Channel name
3) "Hello, Redis!" # Message content
Pattern subscription message format:
TEXT
1) "pmessage" # Message type
2) "news.*" # Subscribed pattern
3) "news:sports" # Actual channel name
4) "Game result!" # Message content
Important Notes
⚠️ Messages are not persisted: Pub/Sub messages are not stored. Subscribers only receive messages sent after they subscribe.
⚠️ Message loss: If a subscriber disconnects, messages sent during that period are lost.
💡 Alternative: When message persistence is needed, use Redis Streams or Lists for message queues.
Redis Stream (Recommended)
Introduced in Redis 5.0, Streams are a better message queue solution:
REDIS
# Add a message
XADD mystream * name "Alice" action "login"
"1638361451780-0"
# Read messages
XREAD COUNT 1 STREAMS mystream 0
# Consumer groups
XGROUP CREATE mystream mygroup 0
XREADGROUP GROUP mygroup consumer1 COUNT 1 STREAMS mystream >
Stream vs Pub/Sub
| Feature | Pub/Sub | Stream |
|---|---|---|
| Message persistence | ❌ Not persisted | ✅ Persisted |
| Message acknowledgment | ❌ No ACK | ✅ ACK supported |
| Consumer groups | ❌ Not supported | ✅ Supported |
| Historical messages | ❌ Cannot retrieve | ✅ Queryable |
| Use case | Real-time notifications | Message queues |
❓ FAQ
Q Are Pub/Sub messages stored?
A No. Pub/Sub is "fire and forget" — messages are not persisted.
Q Can a disconnected subscriber receive historical messages?
A No. Use Streams or Lists for message persistence.
Q How many subscribers can a channel have?
A Theoretically unlimited — depends on server performance.
📖 Summary
- Pub/Sub uses PUBLISH to publish and SUBSCRIBE to subscribe
- Supports pattern matching with PSUBSCRIBE
- Messages are not persisted, suitable for real-time notifications
- For message persistence, use Redis Streams
- Streams support consumer groups, message acknowledgment, and history queries
📝 Exercises
- Subscription practice: Use two terminals — one subscribes, the other publishes
- Pattern subscription: Use PSUBSCRIBE to subscribe to multiple matching channels
- Stream practice: Use XADD/XREAD to implement a simple message queue
Next Lesson
In the next lesson, we will learn Redis Transactions, covering transaction usage and caveats.