Managing Subscriptions

Introduction

Redis offers two subscription strategies: exact channel subscriptions and pattern subscriptions. Understanding both lets you build flexible message routing.

Key Concepts

• SUBSCRIBE: registers exact channel names; a client can subscribe to multiple channels at once
• PSUBSCRIBE: registers glob-style patterns that match many channels in one call
• pmessage: the message type delivered when a pattern subscription matches (includes the matched pattern)

Real World Context

Pattern subscriptions are useful when channel names are dynamic at runtime — for example, subscribing to all user-specific notification channels with PSUBSCRIBE notifications:user:* from a logging or monitoring service.

Deep Dive

bash
PSUBSCRIBE news.*
# Matches: news.sports, news.tech, news.weather

PSUBSCRIBE user.?.events
# Matches: user.A.events, user.1.events

Messages delivered via pattern match arrive as pmessage (not message), and include the matched pattern:

1) "pmessage"
2) "news.*"       <- the pattern that matched
3) "news.sports"  <- the actual channel
4) "Goal!"

Inspecting active subscriptions:

bash
PUBSUB CHANNELS            # list all active channels with subscribers
PUBSUB CHANNELS news.*     # filter by pattern
PUBSUB NUMSUB news sports  # count subscribers per channel
PUBSUB NUMPAT              # count active pattern subscriptions

Common Pitfalls

• Overlapping patterns: if two patterns match the same channel, the subscriber receives two pmessage events for a single publish
• Performance at scale: Redis checks every published message against every registered pattern; thousands of patterns create CPU overhead
• Forgetting PUNSUBSCRIBE: unsubscribing from patterns requires PUNSUBSCRIBE, not UNSUBSCRIBE

Best Practices

• Prefer exact SUBSCRIBE when the channel set is known and bounded
• Use PSUBSCRIBE only when channels are truly dynamic and the pattern set is small
• Monitor PUBSUB NUMPAT in production to detect pattern subscription leaks

Summary

Use SUBSCRIBE for precise channels and PSUBSCRIBE for dynamic routing based on naming conventions. Pattern subscriptions are powerful but add slight overhead — Redis must check every published message against every active pattern.

# Subscribe to all news sub-channels
redis-cli PSUBSCRIBE 'news.*'

# In another terminal:
redis-cli PUBLISH news.sports "Goal!"
# Subscriber receives:
# 1) "pmessage"
# 2) "news.*"
# 3) "news.sports"
# 4) "Goal!"

# Inspect active channels
redis-cli PUBSUB CHANNELS
redis-cli PUBSUB NUMSUB news.sports