SUBSCRIBE to channels in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of subscribing to a single channel?

You run the command SUBSCRIBE news in Redis CLI. What will be the first message you receive?

Redis

SUBSCRIBE news

A ["unsubscribe", "news", 0]

B ["message", "news", "Hello World"]

C ["error", "unknown command"]

D ["subscribe", "news", 1]

💡 Hint

When you subscribe, Redis confirms the subscription first.

The first message after subscribing is a confirmation message showing the subscription to the channel and the total number of subscriptions.

❓ query_result

intermediate

What happens when you subscribe to multiple channels?

You run SUBSCRIBE sports weather. What is the output after subscribing?

Redis

SUBSCRIBE sports weather

A ["subscribe", "sports", 1]

B ["subscribe", "sports", 2]

C ["subscribe", "weather", 2]

D ["message", "sports", "Game tonight"]

💡 Hint

Redis sends a confirmation for each channel subscribed, increasing the count.

Redis sends a confirmation message for each channel. The first confirmation shows 1 total subscription, the second shows 2 total subscriptions.

📝 Syntax

advanced

Which command syntax correctly subscribes to channels with patterns?

You want to subscribe to all channels starting with 'news.'. Which command is correct?

A PSUBSCRIBE news.*

B SUBSCRIBE news.*

C SUBSCRIBE 'news.*'

D PSUBSCRIBE 'news'

💡 Hint

Pattern subscriptions use a special command starting with P.

PSUBSCRIBE is used to subscribe to channels matching a pattern like 'news.*'.

🔧 Debug

advanced

Why does this subscription command fail?

You run SUBSCRIBE without any channel names. What error do you get?

Redis

SUBSCRIBE

A (error) ERR syntax error

B (error) ERR no channels specified

C (error) ERR wrong number of arguments for 'subscribe' command

D (error) ERR unknown command 'SUBSCRIBE'

💡 Hint

Redis commands require at least one channel name for SUBSCRIBE.

SUBSCRIBE requires at least one channel argument; otherwise, it returns a wrong number of arguments error.

🧠 Conceptual

expert

What is the behavior of Redis SUBSCRIBE command regarding message delivery?

Which statement correctly describes how Redis delivers messages to subscribers?

A Messages published to a channel are delivered only to clients subscribed to that exact channel.

B Messages published to a channel are delivered to all clients, regardless of subscription.

C Messages published to a channel are delivered only to clients subscribed to matching patterns, not exact channels.

D Messages published to a channel are stored and delivered later when clients subscribe.

💡 Hint

Think about how pub/sub works in real time.

Redis delivers messages only to clients subscribed to the exact channel or matching pattern at the time of publishing. Messages are not stored for later delivery.