UNSUBSCRIBE behavior in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the UNSUBSCRIBE command do in Redis?

It stops the client from receiving messages from the specified channels or all channels if none are specified.

beginner

If a client is subscribed to multiple channels, what happens when it sends UNSUBSCRIBE without any channel names?

The client unsubscribes from all channels it is currently subscribed to.

intermediate

What message does Redis send back to the client after an UNSUBSCRIBE command?

Redis sends a message showing the channel unsubscribed from and the number of channels the client is still subscribed to.

intermediate

Can a client unsubscribe from channels it is not subscribed to using UNSUBSCRIBE?

No, Redis ignores unsubscribe requests for channels the client is not subscribed to and does not send an error.

beginner

What happens if a client unsubscribes from all channels and has no subscriptions left?

The client stops receiving any published messages until it subscribes again.

What does the Redis UNSUBSCRIBE command do?

A Stops receiving messages from specified channels

B Deletes the channels permanently

C Publishes a message to all subscribers

D Creates new channels

UNSUBSCRIBE stops the client from receiving messages from the specified channels.

If you call UNSUBSCRIBE with no arguments, what happens?

A Subscribes to all channels

B Returns an error

C Unsubscribes from all channels

D Does nothing

Calling UNSUBSCRIBE without arguments unsubscribes the client from all channels.

What does Redis send back after an UNSUBSCRIBE command?

A A confirmation with the total number of channels in Redis

B A message with the channel name and remaining subscriptions count

C An error message

D No response

Redis replies with the channel unsubscribed from and how many subscriptions remain.

Can you unsubscribe from a channel you are not subscribed to?

A No, Redis ignores it silently

B Yes, and it returns an error

C Yes, and it unsubscribes anyway

D No, and it disconnects the client

Redis ignores unsubscribe requests for channels the client is not subscribed to without error.

What happens after a client unsubscribes from all channels?

A It receives messages from all channels

B It automatically subscribes to a default channel

C It disconnects from the server

D It stops receiving published messages

After unsubscribing from all channels, the client stops receiving published messages.

Explain how the UNSUBSCRIBE command works in Redis Pub/Sub.

Think about what happens when you want to stop listening to messages.

Describe the client state after unsubscribing from all channels in Redis.

Consider what it means to stop listening completely.

Practice

1. What does the UNSUBSCRIBE command do in Redis Pub/Sub?

easy

A. Creates a new subscription to a channel

B. Deletes the specified channels permanently

C. Sends a message to all subscribers of a channel

D. Stops receiving messages from specified channels

Solution

1. Step 1: Understand the purpose of UNSUBSCRIBE

   The UNSUBSCRIBE command is used to stop receiving messages from channels you previously subscribed to.

2. Step 2: Differentiate from other commands

   It does not delete channels or send messages; it only stops message delivery to the client.

3. Final Answer:

   Stops receiving messages from specified channels -> Option D

4. Quick Check:

   UNSUBSCRIBE stops message delivery [OK]

Hint: UNSUBSCRIBE stops messages, it does not delete channels [OK]

Common Mistakes:

• Thinking UNSUBSCRIBE deletes channels
• Confusing UNSUBSCRIBE with PUBLISH
• Assuming UNSUBSCRIBE creates subscriptions

2. Which of the following is the correct syntax to unsubscribe from all channels in Redis?

easy

A. UNSUBSCRIBE *

B. UNSUBSCRIBE ALL

C. UNSUBSCRIBE

D. UNSUBSCRIBE ()

Solution

1. Step 1: Recall UNSUBSCRIBE syntax

   To unsubscribe from all channels, you call UNSUBSCRIBE without any arguments.

2. Step 2: Check each option

   UNSUBSCRIBE * uses '*', which is invalid syntax. UNSUBSCRIBE ALL and D are not valid Redis commands.

3. Final Answer:

   UNSUBSCRIBE -> Option C

4. Quick Check:

   UNSUBSCRIBE with no args unsubscribes all [OK]

Hint: No arguments means unsubscribe from all channels [OK]

Common Mistakes:

• Using wildcard '*' with UNSUBSCRIBE
• Adding extra keywords like ALL
• Using parentheses in command

3. Given the following Redis Pub/Sub commands executed by a client:

SUBSCRIBE news sports
UNSUBSCRIBE news

What channels will the client still receive messages from?

medium

A. news only

B. sports only

C. news and sports

D. none

Solution

1. Step 1: Analyze the subscription commands

   The client subscribes to 'news' and 'sports' channels initially.

2. Step 2: Apply UNSUBSCRIBE command

   UNSUBSCRIBE news removes subscription to 'news' only, so 'sports' remains subscribed.

3. Final Answer:

   sports only -> Option B

4. Quick Check:

   UNSUBSCRIBE removes specified channels only [OK]

Hint: UNSUBSCRIBE removes only listed channels, others stay subscribed [OK]

Common Mistakes:

• Assuming UNSUBSCRIBE removes all subscriptions
• Confusing subscribed channels after UNSUBSCRIBE
• Thinking UNSUBSCRIBE adds subscriptions

4. You run the command UNSUBSCRIBE sports but still receive messages from the sports channel. What is the most likely cause?

medium

A. You have multiple subscriptions to sports and only one was removed

B. UNSUBSCRIBE command has a syntax error

C. You never subscribed to the sports channel

D. Redis server does not support UNSUBSCRIBE

Solution

1. Step 1: Understand multiple subscriptions

   Redis allows multiple subscriptions to the same channel from the same client connection.

2. Step 2: Effect of UNSUBSCRIBE on multiple subscriptions

   UNSUBSCRIBE removes one subscription per channel per call; if subscribed multiple times, some remain.

3. Final Answer:

   You have multiple subscriptions to sports and only one was removed -> Option A

4. Quick Check:

   Multiple subscriptions require multiple UNSUBSCRIBE calls [OK]

Hint: Multiple subscriptions need multiple UNSUBSCRIBE calls [OK]

Common Mistakes:

• Assuming one UNSUBSCRIBE removes all duplicates
• Thinking UNSUBSCRIBE syntax is wrong
• Believing Redis lacks UNSUBSCRIBE support

5. A client subscribed to channels news, sports, and weather. It wants to stop receiving messages only from news and weather but keep sports. Which command should it use?

hard

A. UNSUBSCRIBE news weather

B. UNSUBSCRIBE sports

C. UNSUBSCRIBE

D. UNSUBSCRIBE all

Solution

1. Step 1: Identify channels to unsubscribe

   The client wants to stop messages from 'news' and 'weather' only.

2. Step 2: Use UNSUBSCRIBE with specific channels

   UNSUBSCRIBE followed by channel names unsubscribes only those channels, keeping others subscribed.

3. Final Answer:

   UNSUBSCRIBE news weather -> Option A

4. Quick Check:

   UNSUBSCRIBE with channels removes only those [OK]

Hint: List channels in UNSUBSCRIBE to remove only those [OK]

Common Mistakes:

• Using UNSUBSCRIBE with no args removes all
• Trying UNSUBSCRIBE all which is invalid
• Unsubscribing wrong channels