SUBSCRIBE to channels in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to subscribe to a Redis channel named 'news'.

Redis

SUBSCRIBE [1]

Drag options to blanks, or click blank then click option'

Anews

B'news'

Cchannel_news

Dnews_channel

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the channel name like 'news'.

Using a different channel name than 'news'.

The SUBSCRIBE command takes the channel name without quotes. So, SUBSCRIBE news is correct.

2fill in blank

medium

Complete the code to subscribe to two Redis channels named 'sports' and 'weather'.

Redis

SUBSCRIBE [1] [2]

Drag options to blanks, or click blank then click option'

Asports

B'sports'

Cweather

D'weather'

Attempts:

3 left

💡 Hint

Common Mistakes

Adding quotes around channel names.

Using commas between channel names.

SUBSCRIBE command accepts multiple channel names without quotes. So, SUBSCRIBE sports weather is correct.

3fill in blank

hard

Fix the error in the code to subscribe to the channel 'alerts'.

Redis

SUBSCRIBE [1]

Drag options to blanks, or click blank then click option'

A'alerts'

BALERTS

Calert

Dalerts

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the channel name.

Using wrong case or misspelled channel name.

Channel names are case-sensitive and should not be quoted. SUBSCRIBE alerts is correct.

4fill in blank

hard

Fill both blanks to subscribe to channels 'news' and 'updates'.

Redis

SUBSCRIBE [1] [2]

Drag options to blanks, or click blank then click option'

Anews

B'news'

Cupdates

D'updates'

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around channel names.

Separating channel names with commas.

SUBSCRIBE command requires channel names without quotes separated by space.

5fill in blank

hard

Fill all three blanks to subscribe to channels 'chat', 'alerts', and 'news'.

Redis

SUBSCRIBE [1] [2] [3]

Drag options to blanks, or click blank then click option'

Achat

Balerts

Cnews

D'chat'

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around channel names.

Using wrong channel names or misspellings.

SUBSCRIBE command takes multiple channel names without quotes separated by spaces.

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