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.

Practice

1.

What does the PUBLISH command do in Redis?

easy

A. Stores a message permanently in the database

B. Sends a message to all clients subscribed to a specific channel

C. Deletes all messages from a channel

D. Creates a new channel for clients to subscribe

Solution

1. Step 1: Understand the purpose of PUBLISH

   The PUBLISH command sends messages to clients currently subscribed to a channel.

2. Step 2: Differentiate from storage commands

   PUBLISH does not store messages permanently; it only broadcasts to active subscribers.

3. Final Answer:

   Sends a message to all clients subscribed to a specific channel -> Option B

4. Quick Check:

   PUBLISH broadcasts messages [OK]

Hint: PUBLISH sends messages only to active subscribers [OK]

Common Mistakes:

• Thinking PUBLISH stores messages permanently
• Confusing PUBLISH with channel creation
• Assuming PUBLISH deletes messages

2.

Which of the following is the correct syntax to publish a message 'Hello World' to a channel named news in Redis?

?

easy

A. PUBLISH news 'Hello World'

B. PUBLISH 'Hello World' news

C. PUBLISH news Hello World

D. PUBLISH news, 'Hello World'

Solution

1. Step 1: Recall Redis PUBLISH syntax

   The syntax is: PUBLISH channel message. Quote the message if it contains spaces.

2. Step 2: Identify correct option

   PUBLISH news 'Hello World' matches the syntax exactly.

3. Final Answer:

   PUBLISH news 'Hello World' -> Option A

4. Quick Check:

   Correct Redis PUBLISH syntax = PUBLISH news 'Hello World' [OK]

Hint: Quote messages with spaces: PUBLISH channel 'msg with spaces' [OK]

Common Mistakes:

• Not quoting messages with spaces
• Swapping channel and message order
• Using commas or extra symbols

3.

Given the following Redis commands executed in order:

SUBSCRIBE sports
PUBLISH sports "Game tonight"
PUBLISH news "Weather update"

What messages will the subscriber receive?

medium

A. Only "Game tonight"

B. Both "Game tonight" and "Weather update"

C. Only "Weather update"

D. No messages

Solution

1. Step 1: Identify subscribed channel

   The subscriber is subscribed to the 'sports' channel only.

2. Step 2: Check published messages

   Messages published to 'sports' are received; messages to 'news' are ignored by this subscriber.

3. Final Answer:

   Only "Game tonight" -> Option A

4. Quick Check:

   Subscribers get messages only from their channels [OK]

Hint: Subscribers get messages only from their subscribed channels [OK]

Common Mistakes:

• Assuming subscriber gets all published messages
• Confusing channel names
• Ignoring subscription context

4.

What is wrong with this Redis command to publish a message?

PUBLISH

Choose the best explanation.

medium

A. PUBLISH cannot be used without SUBSCRIBE

B. Channel name is invalid

C. Message is too long

D. Missing channel and message arguments

Solution

1. Step 1: Check required arguments for PUBLISH

   PUBLISH requires two arguments: channel and message.

2. Step 2: Identify missing parts in the command

   The command only has PUBLISH with no arguments, so it is incomplete.

3. Final Answer:

   Missing channel and message arguments -> Option D

4. Quick Check:

   PUBLISH needs channel and message [OK]

Hint: PUBLISH needs channel and message arguments [OK]

Common Mistakes:

• Thinking PUBLISH works without arguments
• Assuming message length causes error here
• Believing SUBSCRIBE is required before PUBLISH

5.

You want to send a notification to multiple channels news and alerts at once. Which Redis approach is best?

hard

A. Create a new channel combining both names and publish once

B. Publish to one channel and expect all subscribers to receive it

C. Use multiple PUBLISH commands, one per channel

D. Use a Redis command to publish to multiple channels simultaneously

Solution

1. Step 1: Understand Redis PUBLISH limitations

   Redis PUBLISH sends messages to one channel at a time; no built-in multi-channel publish exists.

2. Step 2: Choose correct method to notify multiple channels

   Send separate PUBLISH commands for each channel to reach all subscribers.

3. Final Answer:

   Use multiple PUBLISH commands, one per channel -> Option C

4. Quick Check:

   Multiple channels need multiple PUBLISH calls [OK]

Hint: Publish separately to each channel for multiple notifications [OK]

Common Mistakes:

• Expecting one PUBLISH to reach multiple channels
• Combining channel names incorrectly
• Assuming Redis supports multi-channel publish natively