PUBLISH to channels in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the PUBLISH command do in Redis?

The PUBLISH command sends a message to all subscribers of a given channel. It is used to broadcast messages in Redis's pub/sub system.

beginner

How do subscribers receive messages in Redis?

Subscribers use the SUBSCRIBE command to listen to channels. When a message is published to those channels, subscribers receive the message instantly.

beginner

What is the syntax of the PUBLISH command?

PUBLISH <channel> <message> — where <channel> is the name of the channel and <message> is the text to send.

intermediate

Can PUBLISH send messages to multiple channels at once?

No, PUBLISH sends a message to one channel at a time. To send to multiple channels, you must call PUBLISH separately for each channel.

intermediate

What happens if no clients are subscribed to a channel when a message is published?

The message is simply discarded. Redis does not store messages for channels; only active subscribers receive messages.

What command do you use to send a message to subscribers in Redis?

A PUBLISH

B SUBSCRIBE

C GET

D SET

PUBLISH sends messages to subscribers on a channel.

If no clients are subscribed to a channel, what happens to a published message?

A It is stored for later delivery

B It is discarded

C It is sent to all channels

D It triggers an error

Redis does not store messages; they are discarded if no subscribers exist.

Which command do clients use to listen to messages on a channel?

A SUBSCRIBE

B PUBLISH

C UNSUBSCRIBE

D DEL

SUBSCRIBE lets clients listen to messages on channels.

Can the PUBLISH command send a message to multiple channels in one call?

A Yes, by listing channels separated by commas

B Yes, but only in Redis Cluster

C Yes, by using a wildcard

D No, only one channel per PUBLISH call

PUBLISH sends to one channel at a time.

What is the correct syntax to publish 'hello' to channel 'news'?

A PUBLISH 'hello' news

B SUBSCRIBE news hello

C PUBLISH news hello

D SEND news hello

PUBLISH is the correct syntax.

Explain how Redis pub/sub works with PUBLISH and SUBSCRIBE commands.

Think of a radio station broadcasting to listeners.

Describe what happens when you publish a message to a channel with no subscribers.

Imagine shouting in an empty room.