Pub/Sub

Redis Pub/Sub (Publish/Subscribe) is a messaging paradigm where:

• Senders (called publishers) send messages to channels.
• Receivers (called subscribers) listen to channels and receive messages in real time.

This is asynchronous, fire-and-forget messaging. Messages are not stored in Redis — they are only delivered to currently active subscribers.

Key Concepts

Term	Description
PUBLISH	Sends a message to a channel.
SUBSCRIBE	Listens for messages on a channel.
UNSUBSCRIBE	Stops listening to a channel.
PSUBSCRIBE	Subscribe to channel(s) using pattern matching.
PUBSUB	Inspect the state of Pub/Sub (like list of channels, etc.).

Example

Let's simulate a basic chat system using Redis Pub/Sub. Step-by-step in Redis CLI

Terminal 1: Subscriber

SUBSCRIBE chatroom

Output

Reading messages... (blocking)

1) "subscribe"
2) "chatroom"
3) "Hello, world!"

Terminal 1 is now listening for all messages on the chatroom channel.

Index	What it represents	Meaning
1)	Event type	"subscribe" = you subscribed to a channel
2)	Channel name	"chatroom"
3)	Number of channels you’re subscribed to now	1

Terminal 2: Publisher

PUBLISH chatroom "Hello, world!"

Output:

(integer) 1

It means 1 subscriber received the message.

Explanation

• The subscriber was already listening on chatroom.
• When the publisher sent "Hello, world!" to chatroom, Redis delivered it instantly.
• If no subscribers are listening, the message is discarded.

Pattern Subscription

Redis provides two kinds of subscriptions:

1. Direct subscriptions → using SUBSCRIBE channel (exact channel name).
2. Pattern-based subscriptions → using PSUBSCRIBE pattern (wildcard matching).

With PSUBSCRIBE, a client can subscribe to multiple channels at once using glob-style patterns (*, ?, []).

How It Works

• A pattern is matched against channel names.
• When a PUBLISH command sends a message to a channel, Redis checks if the channel name matches any active patterns.
• If it matches, all pattern-subscribed clients receive the message, in addition to any direct subscribers.

Supported Wildcards in Patterns

• * → matches zero or more characters.
  • Example: news.* matches news.sports, news.politics.
• ? → matches exactly one character.
  • Example: user.? matches user.1, user.a, but not user.10.
• [] → matches one character from the set.
  • Example: room[123] matches room1, room2, room3.

Example Walkthrough

1. Subscribe with a Pattern

   In one terminal:

   redis-cli
   127.0.0.1:6379> PSUBSCRIBE news.*

   Output:

   Reading messages... (press Ctrl-C to quit)
   1) "psubscribe"
   2) "news.*"
   3) (integer) 1

2. Publish to Matching Channels

   In another terminal:

   127.0.0.1:6379> PUBLISH news.sports "Sports update: Redis is awesome!"
   (integer) 1
   
   127.0.0.1:6379> PUBLISH news.weather "Weather update: Sunny day!"
   (integer) 1

3. Subscriber Receives Messages

   Back in the subscriber terminal:

   1) "pmessage"
   2) "news.*"
   3) "news.sports"
   4) "Sports update: Redis is awesome!"
   
   1) "pmessage"
   2) "news.*"
   3) "news.weather"
   4) "Weather update: Sunny day!"

   • pmessage → type of message (pattern-based).
   • news.* → the pattern that matched.
   • news.sports → actual channel.
   • "Sports update: Redis is awesome!" → message content.