Pub/Sub

Pub/Sub commands for real-time messaging between clients. Messages are broadcast to all subscribers of a channel.

Commands

Command	Syntax	Description
SUBSCRIBE	`SUBSCRIBE channel [channel ...]`	Subscribe to channels
UNSUBSCRIBE	`UNSUBSCRIBE [channel ...]`	Unsubscribe from channels
PUBLISH	`PUBLISH channel message`	Publish message to channel
PSUBSCRIBE	`PSUBSCRIBE pattern [pattern ...]`	Subscribe to pattern-matched channels
PUNSUBSCRIBE	`PUNSUBSCRIBE [pattern ...]`	Unsubscribe from patterns

How Pub/Sub Works

Clients subscribe to channels using SUBSCRIBE or PSUBSCRIBE
Publishers send messages with PUBLISH
All subscribed clients receive the message
Messages are not persisted - if no subscribers, message is lost

Examples

Basic Subscribe/Publish

Terminal 1 (Subscriber):

127.0.0.1:6379> SUBSCRIBE news
Reading messages... (press Ctrl-C to quit)
1) "subscribe"
2) "news"
3) (integer) 1

# When message arrives:
1) "message"
2) "news"
3) "Breaking: Redlite 1.0 released!"

Terminal 2 (Publisher):

127.0.0.1:6379> PUBLISH news "Breaking: Redlite 1.0 released!"
(integer) 1  # Number of subscribers who received it

Multiple Channels

Subscriber:

127.0.0.1:6379> SUBSCRIBE sports weather news
Reading messages...
1) "subscribe"
2) "sports"
3) (integer) 1
1) "subscribe"
2) "weather"
3) (integer) 2
1) "subscribe"
4) "news"
3) (integer) 3

# Messages from any channel:
1) "message"
2) "sports"
3) "Goal!"

1) "message"
2) "weather"
3) "Sunny, 72F"

Pattern Subscriptions

Subscribe to all channels matching a pattern:

Subscriber:

127.0.0.1:6379> PSUBSCRIBE news:*
Reading messages...
1) "psubscribe"
2) "news:*"
3) (integer) 1

# Matches news:sports, news:tech, news:local, etc.
1) "pmessage"
2) "news:*"
3) "news:tech"
4) "New iPhone announced"

1) "pmessage"
2) "news:*"
3) "news:sports"
4) "Championship finals tonight"

Publisher:

127.0.0.1:6379> PUBLISH news:tech "New iPhone announced"
(integer) 1

127.0.0.1:6379> PUBLISH news:sports "Championship finals tonight"
(integer) 1

127.0.0.1:6379> PUBLISH weather "Sunny"
(integer) 0  # No pattern match

Unsubscribe

# Unsubscribe from specific channels
127.0.0.1:6379> UNSUBSCRIBE news weather
1) "unsubscribe"
2) "news"
3) (integer) 1
1) "unsubscribe"
2) "weather"
3) (integer) 0

# Unsubscribe from all channels
127.0.0.1:6379> UNSUBSCRIBE
1) "unsubscribe"
2) (nil)
3) (integer) 0

# Unsubscribe from patterns
127.0.0.1:6379> PUNSUBSCRIBE news:*
1) "punsubscribe"
2) "news:*"
3) (integer) 0

Check Subscriber Count

# PUBLISH returns number of subscribers
127.0.0.1:6379> PUBLISH mychannel "test"
(integer) 3  # 3 clients received the message

127.0.0.1:6379> PUBLISH emptychannel "test"
(integer) 0  # No subscribers

Pattern Syntax

Pattern	Matches
`*`	Any sequence of characters
`?`	Any single character
`[abc]`	Any character in brackets

Examples:

news:* matches news:sports, news:tech, news:local
user:? matches user:1, user:a but not user:12
log:[we]rror matches log:error, log:wrror

Message Format

1) "subscribe"      # Message type
2) "channel-name"   # Channel subscribed to
3) (integer) N      # Total subscriptions for this client

Regular Message

1) "message"        # Message type
2) "channel-name"   # Source channel
3) "message-body"   # The message content

Pattern Message

1) "pmessage"       # Message type (pattern)
2) "pattern"        # Pattern that matched
3) "channel-name"   # Actual channel
4) "message-body"   # The message content

Important Notes

At-Most-Once Delivery

Messages are delivered to current subscribers only
If no subscribers exist, the message is lost
No message persistence or history
No acknowledgment or retry mechanism

Subscription Mode

Once a client enters subscription mode:

Only SUBSCRIBE, UNSUBSCRIBE, PSUBSCRIBE, PUNSUBSCRIBE, and QUIT are allowed
Regular commands like GET, SET are not available
Use a separate connection for commands

Server Mode Only

Pub/Sub requires server mode because:

Requires persistent TCP connections
Real-time message delivery
Multiple client coordination

Use Cases

Real-Time Notifications

# User activity channel
SUBSCRIBE user:123:notifications

# Send notification
PUBLISH user:123:notifications '{"type":"message","from":"user:456"}'

Chat Rooms

# Join room
SUBSCRIBE room:general

# Send message
PUBLISH room:general '{"user":"alice","text":"Hello everyone!"}'

Cache Invalidation

# All app instances subscribe
PSUBSCRIBE cache:invalidate:*

# When data changes
PUBLISH cache:invalidate:users "user:123"
PUBLISH cache:invalidate:products "product:456"

Event Broadcasting

# Subscribe to all events
PSUBSCRIBE events:*

# Publish events
PUBLISH events:user:created '{"id":123}'
PUBLISH events:order:completed '{"id":456}'

Distributed Coordination

# Leader election channel
SUBSCRIBE leader:election

# Announce leadership
PUBLISH leader:election '{"node":"node-1","action":"claim"}'

Comparison with Streams

Feature	Pub/Sub	Streams
Persistence	No	Yes
Message history	No	Yes
Consumer groups	No	Yes
Acknowledgment	No	Yes
Blocking read	Subscribe-based	XREAD BLOCK
Use case	Real-time broadcast	Reliable queuing

Use Pub/Sub for fire-and-forget broadcasting. Use Streams when you need reliability and history.