Pub/sub vs streams comparison in Redis - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Difference in message delivery guarantees

Which statement correctly describes the message delivery guarantee difference between Redis Pub/Sub and Redis Streams?

A Pub/Sub guarantees message delivery to all subscribers even if they are offline; Streams do not.

B Both Pub/Sub and Streams guarantee message delivery only if consumers are online.

C Streams guarantee message delivery to consumers even if they are offline; Pub/Sub does not.

D Neither Pub/Sub nor Streams guarantee message delivery under any circumstances.

💡 Hint

Think about what happens if a subscriber is offline when a message is published.

Redis Streams store messages so consumers can read them later, ensuring delivery even if offline. Pub/Sub sends messages only to currently connected subscribers.

❓ query_result

intermediate

Output of a Pub/Sub message reception

Given a Redis Pub/Sub subscriber listening on channel 'news', what will be the output if a publisher sends the message 'Hello' to 'news' while the subscriber is connected?

Redis

SUBSCRIBE news
PUBLISH news Hello

A The subscriber receives and prints: 'Hello'

B The subscriber receives and prints: 'message news Hello'

C The subscriber receives no message because Pub/Sub does not deliver messages

D The subscriber receives an error message

💡 Hint

Consider the format Redis uses to notify subscribers.

Redis Pub/Sub sends messages with a pattern like 'message channel message'. Here, the subscriber receives 'message news Hello'.

📝 Syntax

advanced

Correct command to add an entry to a Redis Stream

Which Redis command correctly adds a new entry with field 'user' and value 'alice' to a stream named 'mystream'?

A APPEND mystream user alice

B ADD mystream user alice

C XADD mystream user alice *

D XADD mystream * user alice

💡 Hint

The XADD command uses '*' to auto-generate the entry ID.

The correct syntax is XADD mystream * field value. Option D follows this format.

❓ optimization

advanced

Choosing between Pub/Sub and Streams for offline consumers

You want to build a chat application where users can receive messages sent while they were offline. Which Redis feature should you choose for message delivery and why?

A Use Streams because it allows consumers to read messages they missed while offline.

B Use Pub/Sub because it stores messages for offline users automatically.

C Use Pub/Sub because it supports message persistence by default.

D Use neither because Redis does not support offline message delivery.

💡 Hint

Think about which feature stores messages for later reading.

Redis Streams store messages so consumers can read them later, supporting offline message delivery. Pub/Sub does not store messages.

🔧 Debug

expert

Why does a Redis Pub/Sub subscriber miss messages?

A Redis Pub/Sub subscriber sometimes misses messages published to a channel. What is the most likely reason?

A The subscriber was disconnected or offline when the messages were published.

B Redis Pub/Sub queues messages for offline subscribers but the queue is full.

C The subscriber did not acknowledge the messages, so Redis dropped them.

D Redis Pub/Sub requires explicit message commits to deliver messages.

💡 Hint

Consider how Pub/Sub handles offline subscribers.

Redis Pub/Sub delivers messages only to connected subscribers. If a subscriber is offline, it misses messages.