Pub/Sub

How Pub/Sub works

Redis Pub/Sub follows the classic publish-subscribe pattern:

1. A client calls SUBSCRIBE channel and enters listening mode. It can no longer issue regular commands — it only receives push messages from Redis.
2. Any other client calls PUBLISH channel message. Redis immediately forwards that message to every currently connected subscriber of that channel.
3. The message is never stored. If no subscribers are connected, the message is silently discarded. If a subscriber was connected ten seconds ago but disconnected, it gets nothing.

Each channel is just a name — Redis creates and destroys channels implicitly as subscribers come and go. There is no need to create a channel before publishing to it.

SUBSCRIBE and PUBLISH — two-terminal demo

The interaction below requires two separate redis-cli sessions running at the same time. Start Terminal 1 first, then send from Terminal 2.

# Terminal 1 — subscriber
127.0.0.1:6379> SUBSCRIBE news
Reading messages... (press Ctrl-C to quit or any command to unsubscribe)
1) "subscribe"
2) "news"
3) (integer) 1

# Terminal 2 — publisher (in a second redis-cli session)
127.0.0.1:6379> PUBLISH news "Breaking: Redis is fast"
(integer) 1

# Back in Terminal 1 — message arrives automatically
1) "message"
2) "news"
3) "Breaking: Redis is fast"

The confirmation message Redis sends back to the subscriber on SUBSCRIBE has three parts:

Part	Value	Meaning
1	"subscribe"	Event type
2	"news"	Channel name
3	(integer) 1	Total channels this client is now subscribed to

When a published message arrives, it also has three parts: the event type "message", the channel name, and the payload.

The integer returned by PUBLISH is the number of subscribers who received the message. A return value of 0 means nobody was listening.

PUBLISH news "Breaking: Redis is fast"
PUBLISH news "Another update"

Copy PUBLISH command ⧉

UNSUBSCRIBE

To leave a channel, a subscribed client sends UNSUBSCRIBE. You can name a specific channel or omit the channel name to unsubscribe from all channels at once.

# Unsubscribe from one channel
127.0.0.1:6379> UNSUBSCRIBE news
1) "unsubscribe"
2) "news"
3) (integer) 0

# Unsubscribe from all channels (no argument)
127.0.0.1:6379> UNSUBSCRIBE
1) "unsubscribe"
2) (nil)
3) (integer) 0

Once the subscription count drops to 0, the client exits listening mode and can issue normal commands again.

SUBSCRIBE to multiple channels

A single SUBSCRIBE call accepts any number of channel names. The client receives messages from all of them on the same connection.

127.0.0.1:6379> SUBSCRIBE sports weather finance
1) "subscribe"
2) "sports"
3) (integer) 1
1) "subscribe"
2) "weather"
3) (integer) 2
1) "subscribe"
2) "finance"
3) (integer) 3

Redis sends one confirmation reply per channel. The third field increments as each subscription is added, giving you the running total.

🟥 Pub/Sub is fire-and-forget — offline subscribers miss messages

There is no buffer, no retry, and no history. The moment PUBLISH returns, the message is gone. A subscriber that was offline for a single millisecond during that window will never see it. If your application cannot accept message loss, use Redis Streams instead.

What does `PUBLISH channel message` return?

OK The message ID The number of subscribers who received the message The number of channels the publisher is connected to

A client runs SUBSCRIBE news. Can it also run GET mykey without unsubscribing first?

Yes, any command works while subscribed Yes, but only read commands No — a subscribed client can only use SUBSCRIBE, UNSUBSCRIBE, PING, and RESET No — it must open a new connection first

A publisher sends a message to a channel with zero subscribers. What happens to the message?

Redis stores it and delivers it when the next subscriber connects Redis returns an error Redis discards it immediately Redis puts it in a dead-letter queue

Which reply field in a push message contains the channel name?

Field 1 Field 2 Field 3 Field 0

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