PUBLISH to channels in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this PUBLISH command?

Consider a Redis server with no subscribers currently connected. What will be the output of the following command?

PUBLISH news "Breaking news!"

Redis

PUBLISH news "Breaking news!"

A null

B 1

C Error: no subscribers

D 0

💡 Hint

Think about how many clients are subscribed to the channel at the time of publishing.

The PUBLISH command returns the number of clients that received the message. If no clients are subscribed, it returns 0.

❓ query_result

intermediate

How many clients receive the message?

Assume three clients are subscribed to the channel 'sports'. What will be the output of this command?

PUBLISH sports "Game tonight at 8pm"

Redis

PUBLISH sports "Game tonight at 8pm"

A 2

B 3

C 1

D 0

💡 Hint

The output is the count of subscribers that receive the message.

The PUBLISH command returns the number of clients subscribed to the channel, which is 3 in this case.

📝 Syntax

advanced

Which PUBLISH command syntax is correct?

Identify the correct syntax to publish the message "Hello" to the channel "chat" in Redis.

A PUBLISH chat "Hello"

B PUBLISH "chat" "Hello"

C PUBLISH chat Hello

D PUBLISH "chat" Hello

💡 Hint

Channel names do not require quotes but messages with spaces or special characters do.

The correct syntax is PUBLISH channel message. The channel name can be unquoted if it has no spaces, but the message should be quoted if it contains spaces or special characters.

🧠 Conceptual

advanced

What happens if a client subscribes to multiple channels and a message is published?

If a client is subscribed to channels 'news' and 'updates', and a message is published to 'news', what will the client receive?

A The client receives the message only if published to 'updates'

B The client receives messages from all channels regardless of publish

C The client receives the message published to 'news' channel

D The client receives no messages

💡 Hint

Think about how subscriptions work per channel.

A client subscribed to multiple channels receives messages only from the channels where messages are published. Here, the client will receive messages published to 'news'.

🔧 Debug

expert

Why does this PUBLISH command fail to deliver messages?

A developer runs the following commands:

1. SUBSCRIBE updates
2. PUBLISH updates "New version released"

But the subscriber never receives the message. What is the most likely reason?

Redis

SUBSCRIBE updates
PUBLISH updates "New version released"

A The subscriber is blocked and cannot receive messages because SUBSCRIBE is a blocking command

B The channel name 'updates' is reserved and cannot be used

C The PUBLISH command syntax is incorrect

D Redis server does not support PUBLISH/SUBSCRIBE

💡 Hint

Consider how SUBSCRIBE affects the client connection.

SUBSCRIBE puts the client into a mode where it only listens for messages and blocks other commands. If the same client tries to publish, it will not work as expected.

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