Pub/sub limitations (no persistence) 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'

Amessages

Bupdates

Cchannel1

Dnews

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different channel name than 'news'.

Forgetting to specify the channel name.

The SUBSCRIBE command listens to the specified channel. Here, 'news' is the correct channel name.

2fill in blank

medium

Complete the code to publish the message 'Hello' to the 'news' channel.

Redis

PUBLISH [1] 'Hello'

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

Aupdates

Bchannel1

Cnews

Dmessages

Attempts:

3 left

💡 Hint

Common Mistakes

Publishing to the wrong channel.

Omitting the channel name.

The PUBLISH command sends a message to the specified channel. Here, 'news' is the correct channel.

3fill in blank

hard

Fix the error in the code to subscribe to multiple channels: 'news' and 'updates'.

Redis

SUBSCRIBE [1] [2]

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

Anews

Bupdates

Cnews,updates

D'news' 'updates'

Attempts:

3 left

💡 Hint

Common Mistakes

Using commas to separate channel names.

Adding quotes around channel names.

The SUBSCRIBE command accepts multiple channel names separated by spaces, not commas or quotes.

4fill in blank

hard

Fill both blanks to explain why messages sent before subscription are missed.

Redis

Redis Pub/Sub [1] messages and [2] them for later delivery.

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Adoes not store

Bstores

Cdoes not queue

Dqueues

Attempts:

3 left

💡 Hint

Common Mistakes

Thinking Redis queues messages for later.

Assuming messages are stored for offline clients.

Redis Pub/Sub does not store or queue messages. If a client is not subscribed when a message is sent, it misses that message.

5fill in blank

hard

Fill all three blanks to describe a limitation of Redis Pub/Sub.

Redis

If a subscriber [1] before a message is published, it [2] the message because Redis Pub/Sub [3] persistence.

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

Aconnects

Bmisses

Clacks

Ddisconnects

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'connects' instead of 'disconnects'.

Assuming Redis Pub/Sub has message persistence.

If a subscriber disconnects before a message is published, it misses the message because Redis Pub/Sub lacks persistence.

Practice

1. What happens to messages sent via Redis Pub/sub if no clients are subscribed at that moment?

easy

A. Messages are lost and not stored anywhere.

B. Messages are saved and delivered later when clients subscribe.

C. Messages are queued until a client connects.

D. Messages are stored in a Redis list automatically.

Solution

1. Step 1: Understand Redis Pub/sub behavior

   Redis Pub/sub delivers messages only to clients currently subscribed.

2. Step 2: Analyze message persistence

   Since Pub/sub does not save messages, if no client listens, messages are lost.

3. Final Answer:

   Messages are lost and not stored anywhere. -> Option A

4. Quick Check:

   Pub/sub = no persistence [OK]

Hint: Pub/sub delivers live only, no message storage [OK]

Common Mistakes:

• Assuming messages queue up for later delivery
• Thinking messages are saved automatically
• Confusing Pub/sub with Redis streams

2. Which of the following is the correct Redis command to subscribe to a channel named news?

easy

A. SUBSCRIBE TO news

B. PUBLISH news

C. SUBSCRIBE news

D. LISTEN news

Solution

1. Step 1: Recall Redis Pub/sub subscribe syntax

   The correct command to subscribe is SUBSCRIBE <channel>.

2. Step 2: Match command to channel name

   Using SUBSCRIBE news subscribes to the channel named 'news'.

3. Final Answer:

   SUBSCRIBE news -> Option C

4. Quick Check:

   Subscribe syntax = SUBSCRIBE channel [OK]

Hint: Subscribe uses 'SUBSCRIBE channel' command [OK]

Common Mistakes:

• Using PUBLISH instead of SUBSCRIBE
• Adding extra words like TO
• Using LISTEN which is not a Redis command

3. Consider this Redis Pub/sub scenario:

Client A subscribes to channel 'chat'.
Client B publishes message 'Hello' to 'chat'.
Client C subscribes to 'chat' after the message is published.

Which clients receive the message 'Hello'?

medium

A. Only Client A

B. Clients A and C

C. Only Client C

D. Clients A, B, and C

Solution

1. Step 1: Identify subscribers at publish time

   Only Client A was subscribed when Client B published the message.

2. Step 2: Understand Pub/sub delivery

   Messages are sent only to clients subscribed at publish time; Client C subscribed later and misses the message.

3. Final Answer:

   Only Client A -> Option A

4. Quick Check:

   Subscribers at publish = receivers [OK]

Hint: Only current subscribers get messages [OK]

Common Mistakes:

• Assuming late subscribers get past messages
• Including publisher as receiver
• Thinking all clients get all messages

4. You wrote this Redis Pub/sub code snippet:

SUBSCRIBE news
PUBLISH news "Update 1"

But your subscriber never receives the message. What is the likely problem?

medium

A. The PUBLISH command syntax is incorrect.

B. The subscriber must be subscribed before the message is published.

C. Redis does not support Pub/sub messaging.

D. The channel name 'news' is reserved and cannot be used.

Solution

1. Step 1: Check Pub/sub message delivery timing

   Subscribers must be listening before messages are published to receive them.

2. Step 2: Analyze the code order

   If PUBLISH runs before SUBSCRIBE completes, subscriber misses the message.

3. Final Answer:

   The subscriber must be subscribed before the message is published. -> Option B

4. Quick Check:

   Subscribe first, then publish [OK]

Hint: Subscribe before publish to receive messages [OK]

Common Mistakes:

• Thinking PUBLISH syntax is wrong
• Believing Redis lacks Pub/sub support
• Assuming channel names are restricted

5. You want to ensure messages are not lost if no clients are currently subscribed. Which Redis feature should you use instead of Pub/sub?

hard

A. Use Redis transactions to save messages.

B. Use Pub/sub with multiple channels.

C. Increase Redis memory to save messages.

D. Redis Streams to store and consume messages later.

Solution

1. Step 1: Understand Pub/sub limitation

   Pub/sub does not store messages; they are lost if no subscriber is present.

2. Step 2: Identify Redis feature for persistence

   Redis Streams allow storing messages so clients can read them later.

3. Step 3: Evaluate other options

   Using multiple channels or transactions does not provide message persistence; increasing memory does not save messages automatically.

4. Final Answer:

   Redis Streams to store and consume messages later. -> Option D

5. Quick Check:

   Use Streams for persistence, not Pub/sub [OK]

Hint: Use Redis Streams for message persistence [OK]

Common Mistakes:

• Trying to fix Pub/sub by adding channels
• Thinking transactions save Pub/sub messages
• Assuming more memory stores messages