Pub/sub limitations (no persistence) in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What happens to messages sent before a subscriber connects?

In Redis Pub/Sub, if a message is published to a channel before a client subscribes, what happens to that message?

A The subscriber receives the message once it connects.

B The message is lost and the subscriber does not receive it.

C The message is stored and delivered when the subscriber connects.

D The message is queued and delivered in order after subscription.

💡 Hint

Think about whether Redis Pub/Sub stores messages for future subscribers.

Redis Pub/Sub does not store messages. If a subscriber is not connected when a message is published, it misses that message.

🧠 Conceptual

intermediate

Why does Redis Pub/Sub not guarantee message delivery?

Which reason best explains why Redis Pub/Sub does not guarantee message delivery to subscribers?

A Because Redis stores messages only for a limited time.

B Because subscribers must acknowledge each message to receive the next.

C Because messages are sent only to currently connected subscribers without persistence.

D Because Redis queues messages for offline subscribers but drops them if full.

💡 Hint

Consider how Redis handles message storage and subscriber connection status.

Redis Pub/Sub sends messages only to subscribers connected at the time of publishing. It does not store or queue messages for later delivery.

📝 Syntax

advanced

Identify the incorrect Redis Pub/Sub command usage

Which of the following Redis commands is NOT valid for subscribing to a channel?

A SUBSCRIBER news

B PUBLISH news "Hello"

C SUBSCRIBE news sports

D SUBSCRIBE news

💡 Hint

Check the exact spelling of Redis Pub/Sub commands.

The correct command to subscribe is SUBSCRIBE. 'SUBSCRIBER' is not a valid Redis command.

🔧 Debug

advanced

Why does a subscriber miss messages after reconnecting?

A subscriber disconnects and then reconnects to a Redis Pub/Sub channel. It notices it missed messages sent during the disconnection. Why?

A Redis Pub/Sub queues messages but the queue was full.

B The subscriber was subscribed to the wrong channel after reconnecting.

C The subscriber did not send an ACK to receive missed messages.

D Redis Pub/Sub does not store messages for disconnected clients.

💡 Hint

Think about Redis Pub/Sub message persistence and client connection status.

Redis Pub/Sub does not store messages for clients that are disconnected. Messages sent during disconnection are lost to that client.

❓ optimization

expert

How to ensure message persistence with Redis Pub/Sub limitations?

Given Redis Pub/Sub does not persist messages, which approach best ensures message persistence and delivery to clients that may disconnect?

A Use Redis Streams instead of Pub/Sub for message persistence.

B Increase Redis memory to store more Pub/Sub messages.

C Use multiple Pub/Sub channels to duplicate messages.

D Subscribe clients with a delay to catch missed messages.

💡 Hint

Consider Redis features designed for message persistence and replay.

Redis Streams provide message persistence and allow clients to read messages even after disconnecting, unlike Pub/Sub.