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.