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.

Practice

1. What does the Redis SUBSCRIBE command do?

easy

A. Connects the client to one or more channels to receive messages

B. Deletes a channel and all its messages

C. Saves the current database state to disk

D. Lists all keys matching a pattern

Solution

1. Step 1: Understand the purpose of SUBSCRIBE

   The SUBSCRIBE command is used to listen to messages published on channels in Redis.

2. Step 2: Identify the correct function

   Only Connects the client to one or more channels to receive messages describes connecting to channels to receive live messages, which matches SUBSCRIBE's purpose.

3. Final Answer:

   Connects the client to one or more channels to receive messages -> Option A

4. Quick Check:

   SUBSCRIBE = Connect to channels [OK]

Hint: SUBSCRIBE means listen to channels for messages [OK]

Common Mistakes:

• Confusing SUBSCRIBE with deleting or saving data
• Thinking SUBSCRIBE lists keys
• Assuming SUBSCRIBE modifies data

2. Which of the following is the correct syntax to subscribe to channels 'news' and 'sports' in Redis CLI?

easy

A. SUBSCRIBE('news', 'sports')

B. SUBSCRIBE news sports

C. subscribe news, sports

D. SUBSCRIBE TO news AND sports

Solution

1. Step 1: Recall Redis CLI command format

   Redis commands are typed in uppercase followed by arguments separated by spaces.

2. Step 2: Match syntax with options

   SUBSCRIBE news sports matches the correct Redis CLI syntax: SUBSCRIBE news sports. Other options use incorrect syntax or punctuation.

3. Final Answer:

   SUBSCRIBE news sports -> Option B

4. Quick Check:

   Correct Redis CLI syntax = SUBSCRIBE news sports [OK]

Hint: Use uppercase command and space-separated channels [OK]

Common Mistakes:

• Using parentheses like a function call
• Adding commas between channels
• Using extra keywords like TO or AND

3. You want to listen to messages from channels 'chat', 'alerts', and 'updates' simultaneously. Which Redis command should you use to subscribe to all three channels at once?

easy

A. SUBSCRIBE [chat alerts updates]

B. SUBSCRIBE chat; SUBSCRIBE alerts; SUBSCRIBE updates

C. SUBSCRIBE TO chat, alerts, updates

D. SUBSCRIBE chat alerts updates

Solution

1. Step 1: Recall how to subscribe to multiple channels

   Redis SUBSCRIBE accepts multiple channel names separated by spaces in one command.

2. Step 2: Evaluate options for correctness

   SUBSCRIBE chat alerts updates uses correct syntax: SUBSCRIBE chat alerts updates. Other options use invalid syntax or multiple commands unnecessarily.

3. Final Answer:

   SUBSCRIBE chat alerts updates -> Option D

4. Quick Check:

   One SUBSCRIBE with multiple channels = SUBSCRIBE chat alerts updates [OK]

Hint: List all channels after SUBSCRIBE separated by spaces [OK]

Common Mistakes:

• Using commas or brackets in channel list
• Trying multiple SUBSCRIBE commands instead of one
• Adding extra keywords like TO

4. What will happen if a client executes the following Redis commands?

SUBSCRIBE sports
PUBLISH sports "Game tonight!"

medium

A. An error occurs because PUBLISH cannot be used after SUBSCRIBE

B. The client publishes the message but does not receive it

C. The client receives the message "Game tonight!" on the 'sports' channel

D. The client unsubscribes from the 'sports' channel

Solution

1. Step 1: Understand SUBSCRIBE effect

   After SUBSCRIBE sports, the client enters pub/sub mode where it can receive messages published to subscribed channels.

2. Step 2: Check PUBLISH behavior

   PUBLISH can be executed by any client, including the one subscribed. The client will receive the message "Game tonight!" on the 'sports' channel.

3. Final Answer:

   The client receives the message "Game tonight!" on the 'sports' channel -> Option C

4. Quick Check:

   SUBSCRIBE then PUBLISH = client receives message [OK]

Hint: After SUBSCRIBE, client receives published messages on subscribed channels [OK]

Common Mistakes:

• Thinking PUBLISH works and client receives own message
• Assuming PUBLISH succeeds but client doesn't receive
• Believing PUBLISH causes unsubscribe

5. A developer runs this Redis command: SUBSCRIBE without any channel names. What is the result?

medium

A. Syntax error: SUBSCRIBE requires at least one channel

B. The client unsubscribes from all channels

C. The client subscribes to all channels

D. The command is ignored and no subscription happens

Solution

1. Step 1: Check SUBSCRIBE command requirements

   SUBSCRIBE must be followed by at least one channel name to subscribe to.

2. Step 2: Identify error from missing arguments

   Running SUBSCRIBE alone causes a syntax error because no channels are specified.

3. Final Answer:

   Syntax error: SUBSCRIBE requires at least one channel -> Option A

4. Quick Check:

   SUBSCRIBE needs channels = syntax error if missing [OK]

Hint: SUBSCRIBE needs channels; missing channels cause syntax error [OK]

Common Mistakes:

• Assuming SUBSCRIBE with no args subscribes to all channels
• Thinking it unsubscribes instead
• Believing command is ignored silently