SUBSCRIBE to channels in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis SUBSCRIBE command do?

The SUBSCRIBE command listens to messages published on one or more channels. It allows a client to receive messages sent to those channels in real time.

beginner

How do you subscribe to multiple channels in Redis?

You list all channel names after the SUBSCRIBE command, separated by spaces. For example: SUBSCRIBE channel1 channel2 subscribes to both channels.

intermediate

What happens to a Redis client after it issues a SUBSCRIBE command?

The client enters a listening mode and can only receive messages from subscribed channels. It cannot run other commands until it unsubscribes.

beginner

How does Redis deliver messages to subscribed clients?

When a message is published to a channel, Redis sends the message to all clients subscribed to that channel immediately.

beginner

What command do you use to stop listening to channels in Redis?

You use the UNSUBSCRIBE command to stop receiving messages from channels you subscribed to.

What does the Redis SUBSCRIBE command do?

A Listens for messages on specified channels

B Deletes a channel

C Publishes a message to a channel

D Creates a new database

The SUBSCRIBE command listens for messages published on the specified channels.

After subscribing to channels, what can a Redis client do?

A Delete channels

B Run any Redis command freely

C Publish messages to channels

D Only receive messages from subscribed channels

Once subscribed, the client can only receive messages and cannot run other commands until it unsubscribes.

How do you subscribe to multiple channels at once?

A Use multiple <code>SUBSCRIBE</code> commands

B List all channel names after one <code>SUBSCRIBE</code> command

C Use <code>PUBLISH</code> command

D Use <code>UNSUBSCRIBE</code> command

You list all channel names separated by spaces after one SUBSCRIBE command.

Which command stops a client from listening to channels?

A PUBLISH

B STOP

C UNSUBSCRIBE

D DISCONNECT

The UNSUBSCRIBE command stops the client from receiving messages.

What happens when a message is published to a channel?

A Redis sends the message to all subscribed clients immediately

B Redis deletes the channel

C Redis stores the message permanently

D Redis ignores the message

Redis immediately delivers the message to all clients subscribed to that channel.

Explain how the Redis SUBSCRIBE command works and what happens to the client after subscribing.

Think about how a radio listens to stations and cannot do other things at the same time.

Describe how to subscribe to multiple channels and how messages are delivered to subscribed clients.

Imagine tuning into several radio stations and hearing all broadcasts live.

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