Pub/sub limitations (no persistence) in Redis

Introduction

Pub/sub in Redis lets programs send messages instantly, but it does not save messages for later. This means if a program is not listening when a message is sent, it will miss that message.

You want to send real-time chat messages between users.

You need to notify multiple services immediately about an event.

You want to update live dashboards with fresh data.

You have temporary alerts that don't need to be saved.

You want to broadcast messages to many clients at once.

Syntax

Redis

SUBSCRIBE channel_name
PUBLISH channel_name message

SUBSCRIBE listens to messages on a channel.

PUBLISH sends a message to all subscribers of that channel.

Examples

Start listening to messages sent to the 'news' channel.

Redis

SUBSCRIBE news

Send the message 'Hello subscribers!' to everyone listening on 'news'.

Redis

PUBLISH news "Hello subscribers!"

Subscribe to 'sports' channel and publish a message to it.

Redis

SUBSCRIBE sports
PUBLISH sports "Game starts now!"

Sample Program

One client listens to 'updates' channel. Another client sends a message. The listening client receives it only if it is connected at that moment.

Redis

# In one Redis client:
SUBSCRIBE updates

# In another Redis client:
PUBLISH updates "New update available!"

Important Notes

If no client is subscribed when a message is published, that message is lost forever.

Pub/sub does not store messages; it only delivers them live.

For message persistence, consider Redis Streams or other message queues.

Summary

Redis Pub/sub sends messages instantly but does not save them.

Subscribers must be connected to receive messages.

Use Pub/sub for real-time notifications without needing history.