Redis Pub/Sub provides a lightweight messaging system that enables clients to communicate through channels. Publishers send messages to channels, and subscribers receive all messages sent to the channels they’re subscribed to.
Overview
Pub/Sub in Redis implements a fire-and-forget messaging pattern with three key components:
- Publishers send messages to channels
- Channels are named message conduits
- Subscribers receive messages from channels they subscribe to
Messages are not persisted - they’re delivered only to active subscribers at the time of publication.
Basic Commands
SUBSCRIBE
Subscribe to one or more channels:
SUBSCRIBE channel1 channel2 channel3
- Subscription confirmations
- Messages published to subscribed channels
- Unsubscription confirmations
PUBLISH
Publish a message to a channel:
PUBLISH channel1 "Hello, subscribers!"
UNSUBSCRIBE
Unsubscribe from channels:
# Unsubscribe from specific channels
UNSUBSCRIBE channel1 channel2
# Unsubscribe from all channels
UNSUBSCRIBE
Pattern Matching
Redis supports pattern-based subscriptions using glob-style patterns.
PSUBSCRIBE
Subscribe to channels matching a pattern:
# Subscribe to all news channels
PSUBSCRIBE news.*
# Subscribe to all channels starting with 'user:'
PSUBSCRIBE user:*
* matches any characters? matches a single character[abc] matches one character from the set
PUNSUBSCRIBE
Unsubscribe from pattern-based subscriptions:
# Unsubscribe from specific patterns
PUNSUBSCRIBE news.*
# Unsubscribe from all patterns
PUNSUBSCRIBE
Shard Pub/Sub (Cluster Mode)
In cluster mode, shard pub/sub provides channel messaging scoped to specific hash slots.
SSUBSCRIBE
Subscribe to shard channels:
SSUBSCRIBE {user:1000}:notifications {user:1001}:notifications
Shard channels use the same hash slot calculation as keys. Channels with the same hash tag will be routed to the same shard.
SPUBLISH
Publish to a shard channel:
SPUBLISH {user:1000}:notifications "New message"
SUNSUBSCRIBE
Unsubscribe from shard channels:
SUNSUBSCRIBE {user:1000}:notifications
Introspection Commands
PUBSUB CHANNELS
List active channels:
# List all active channels
PUBSUB CHANNELS
# List channels matching a pattern
PUBSUB CHANNELS news.*
PUBSUB NUMSUB
Get subscriber count for channels:
PUBSUB NUMSUB channel1 channel2
PUBSUB NUMPAT
Get the count of pattern subscriptions:
PUBSUB SHARDCHANNELS
List active shard channels:
PUBSUB SHARDCHANNELS
PUBSUB SHARDCHANNELS {user:*}:notifications
PUBSUB SHARDNUMSUB
Get subscriber count for shard channels:
PUBSUB SHARDNUMSUB {user:1000}:notifications
Use Cases
Real-time Notifications
import redis
r = redis.Redis()
# Publisher
r.publish('notifications', 'New order received')
# Subscriber
pubsub = r.pubsub()
pubsub.subscribe('notifications')
for message in pubsub.listen():
if message['type'] == 'message':
print(f"Received: {message['data']}")
Chat Application
# Join a chat room
pubsub.subscribe('chatroom:general')
# Send a message
r.publish('chatroom:general', 'Hello everyone!')
Event Broadcasting
# Subscribe to multiple event types
pubsub.psubscribe('events:*')
# Publish different events
r.publish('events:user_login', 'user123')
r.publish('events:order_created', 'order456')
1) "subscribe"
2) "channel1"
3) (integer) 1 # Total subscriptions
1) "message"
2) "channel1"
3) "Hello, world!"
1) "pmessage"
2) "news.*" # Pattern matched
3) "news.tech" # Actual channel
4) "New article"
Important Considerations
No Message Persistence: Messages are only delivered to active subscribers. If no subscribers are listening when a message is published, the message is lost.
Client State: Once a client subscribes to a channel, it enters pub/sub mode and can only execute pub/sub-related commands until it unsubscribes from all channels.
- Pub/Sub has minimal memory overhead - only active subscriptions are tracked
- Pattern matching adds computational overhead proportional to the number of patterns
- In cluster mode, global pub/sub messages are broadcast to all cluster nodes
- Shard pub/sub is more efficient in clusters as messages are only sent to relevant shards
Limitations
- No Message History: Past messages cannot be retrieved
- No Guaranteed Delivery: Slow subscribers may miss messages
- No Acknowledgments: Publishers don’t know if messages were processed
- Fire-and-Forget: No built-in retry mechanism
For reliable messaging with persistence, consider using Redis Streams instead.
RESP3 Support
With RESP3 protocol, pub/sub messages are delivered as push events, allowing clients to receive messages while executing other commands.
# Enable RESP3
HELLO 3
# Subscribe and continue using other commands
SUBSCRIBE notifications
GET mykey # This works in RESP3