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.