UNSUBSCRIBE behavior in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What happens after UNSUBSCRIBE without arguments?

Consider a Redis client subscribed to channels 'news' and 'sports'. What is the output after executing UNSUBSCRIBE with no arguments?

Redis

SUBSCRIBE news sports
UNSUBSCRIBE

A 1) unsubscribed from 'news' only, subscription count is 1

B 1) unsubscribed from 'news', 2) unsubscribed from 'sports', 3) subscription count is 0

C Error: UNSUBSCRIBE requires at least one channel

D No output; client remains subscribed to all channels

💡 Hint

UNSUBSCRIBE with no arguments unsubscribes from all channels.

When UNSUBSCRIBE is called without arguments, Redis unsubscribes the client from all channels it is currently subscribed to, sending an unsubscribe message for each channel and updating the subscription count to zero.

❓ query_result

intermediate

What is the output of UNSUBSCRIBE for a non-subscribed channel?

A Redis client is subscribed to channel 'chat'. What is the output after executing UNSUBSCRIBE news where 'news' is not subscribed?

Redis

SUBSCRIBE chat
UNSUBSCRIBE news

A Error: Cannot unsubscribe from a channel not subscribed

B No unsubscribe message; subscription count remains 1

C Unsubscribe message for 'news' with subscription count 1

D Unsubscribe message for 'chat' with subscription count 0

💡 Hint

UNSUBSCRIBE sends an unsubscribe message even if the client was not subscribed to the channel.

Redis sends an unsubscribe message for each channel requested, even if the client was not subscribed to it. The subscription count remains unchanged if the channel was not subscribed.

🧠 Conceptual

advanced

How does UNSUBSCRIBE affect pattern subscriptions?

If a client is subscribed to channels 'a' and 'b' and also to pattern 'p*', what happens when UNSUBSCRIBE b is executed?

A Unsubscribes only from channel 'b'; pattern subscription remains active

B Unsubscribes from all channels and patterns

C Unsubscribes from channel 'b' and all pattern subscriptions

D No effect; pattern subscriptions block UNSUBSCRIBE

💡 Hint

UNSUBSCRIBE affects only channel subscriptions, not pattern subscriptions.

UNSUBSCRIBE commands affect only channel subscriptions. Pattern subscriptions remain active unless explicitly unsubscribed with PUNSUBSCRIBE.

📝 Syntax

advanced

What error occurs with incorrect UNSUBSCRIBE syntax?

What error does Redis return when executing UNSUBSCRIBE 123 where 123 is a number instead of a string channel name?

A Syntax error: wrong number of arguments for 'UNSUBSCRIBE' command

B Runtime error: channel not found

C Error: invalid channel name type

D No error; Redis converts 123 to string and unsubscribes

💡 Hint

Redis treats all channel names as strings internally.

Redis accepts numeric arguments for channel names by converting them to strings internally, so no error occurs.

🔧 Debug

expert

Why does UNSUBSCRIBE not reduce subscription count to zero?

A client subscribed to channels 'x', 'y' and pattern 'z*' executes UNSUBSCRIBE x y. Why does the subscription count reported remain 1 instead of 0?

A Because pattern subscriptions are not affected by UNSUBSCRIBE and remain active

B Because UNSUBSCRIBE only marks channels for removal but does not remove them immediately

C Because the client must call PUNSUBSCRIBE to remove all subscriptions

D Because UNSUBSCRIBE requires a delay before updating subscription count

💡 Hint

Pattern subscriptions are managed separately from channel subscriptions.

UNSUBSCRIBE only removes channel subscriptions. Pattern subscriptions remain active and count towards the subscription count, so the count stays at 1.

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