Pub/sub limitations (no persistence) in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a key limitation of Redis Pub/Sub regarding message delivery?

Redis Pub/Sub does not store messages. If a subscriber is offline when a message is published, it will miss that message.

beginner

Why can't Redis Pub/Sub guarantee message delivery to all subscribers?

Because Redis Pub/Sub sends messages only to currently connected subscribers and does not save messages for later delivery.

intermediate

How does Redis Pub/Sub handle message persistence?

Redis Pub/Sub does not support message persistence; messages are transient and lost if no subscriber is listening at publish time.

intermediate

What happens if a subscriber disconnects and then reconnects in Redis Pub/Sub?

The subscriber will not receive any messages published while it was disconnected because Redis Pub/Sub does not store messages.

advanced

Name one alternative to Redis Pub/Sub if message persistence is required.

Redis Streams or message brokers like RabbitMQ or Kafka, which support message persistence and replay.

What happens to messages in Redis Pub/Sub if no subscriber is connected at publish time?

A Messages are stored and delivered later

B Messages are lost immediately

C Messages are queued until a subscriber connects

D Messages are saved to disk

Redis Pub/Sub does not store messages; if no subscriber is connected, messages are lost.

Which feature is NOT supported by Redis Pub/Sub?

A Real-time message delivery

B Message persistence

C Multiple subscribers

D Topic-based messaging

Redis Pub/Sub does not support message persistence; messages are transient.

If a subscriber disconnects, what happens to messages published during that time in Redis Pub/Sub?

A They are delivered when the subscriber reconnects

B They are stored in a queue

C They are lost

D They are sent to other subscribers only

Messages published while a subscriber is disconnected are lost because Redis Pub/Sub has no persistence.

Which Redis feature can be used if you need message persistence instead of Pub/Sub?

A Redis Lists

B Redis Streams

C Redis Sets

D Redis Hashes

Redis Streams support message persistence and replay, unlike Pub/Sub.

Why might Redis Pub/Sub be unsuitable for critical message delivery?

A It is too slow

B It requires complex setup

C It does not guarantee message delivery

D It only supports one subscriber

Redis Pub/Sub does not guarantee message delivery because it lacks persistence.

Explain the main limitation of Redis Pub/Sub related to message persistence and delivery.

Think about what happens if a subscriber is offline.

Describe alternatives to Redis Pub/Sub when message persistence is needed and why.

Consider systems designed for guaranteed delivery.

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