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.

Practice

1. Which Redis feature is best suited for instant message delivery without storing messages?

easy

A. Hashes

B. Streams

C. Pub/Sub

D. Sorted Sets

Solution

1. Step 1: Understand Pub/Sub purpose

   Pub/Sub delivers messages instantly to subscribers without saving them.

2. Step 2: Compare with Streams

   Streams store messages for later processing, unlike Pub/Sub.

3. Final Answer:

   Pub/Sub -> Option C

4. Quick Check:

   Instant delivery without storage = Pub/Sub [OK]

Hint: Instant delivery without storage means Pub/Sub [OK]

Common Mistakes:

• Confusing Streams with Pub/Sub for instant delivery
• Thinking Streams do not store messages
• Choosing data structures unrelated to messaging

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

easy

A. PUBLISH news "Hello"

B. XADD news * message "Hello"

C. SUBSCRIBE news

D. GET news

Solution

1. Step 1: Identify publish command syntax

   The PUBLISH command sends a message to a channel immediately.

2. Step 2: Check other commands

   XADD adds to streams, SUBSCRIBE listens, GET retrieves keys, so only PUBLISH fits.

3. Final Answer:

   PUBLISH news "Hello" -> Option A

4. Quick Check:

   Publish message = PUBLISH command [OK]

Hint: Use PUBLISH to send messages to channels [OK]

Common Mistakes:

• Using XADD instead of PUBLISH for Pub/Sub
• Confusing SUBSCRIBE with PUBLISH
• Using GET which is for keys, not messages

3. Given the following Redis commands, what will be the output of XREAD COUNT 2 STREAMS mystream 0 after adding two entries with XADD mystream * field1 val1 and XADD mystream * field2 val2?

medium

A. An error because COUNT is invalid here

B. Two entries with their IDs and fields returned

C. No entries returned because 0 means no messages

D. Only the first entry returned ignoring COUNT

Solution

1. Step 1: Understand XADD and XREAD

   XADD adds entries to a stream with unique IDs. XREAD reads entries from streams.

2. Step 2: Analyze XREAD COUNT 2 STREAMS mystream 0

   This reads up to 2 entries from 'mystream' starting from ID 0 (the beginning).

3. Final Answer:

   Two entries with their IDs and fields returned -> Option B

4. Quick Check:

   XREAD with COUNT 2 returns up to 2 entries [OK]

Hint: XREAD with COUNT returns that many entries from stream start [OK]

Common Mistakes:

• Thinking COUNT is invalid in XREAD
• Assuming 0 means no messages returned
• Expecting only one entry despite COUNT 2

4. You wrote this Redis command to subscribe to a channel: SUBSCRIBE mychannel. But your client never receives messages published with PUBLISH mychannel "Hello". What is the likely problem?

medium

A. The client must use XREAD to receive Pub/Sub messages

B. PUBLISH command syntax is wrong

C. Streams must be used instead of Pub/Sub for messages

D. The client is not listening because it did not call SUBSCRIBE correctly

Solution

1. Step 1: Verify SUBSCRIBE usage

   SUBSCRIBE must be called on the client to listen to messages; if not, no messages arrive.

2. Step 2: Check other options

   PUBLISH syntax is correct; Streams and XREAD are unrelated to Pub/Sub listening.

3. Final Answer:

   The client is not listening because it did not call SUBSCRIBE correctly -> Option D

4. Quick Check:

   Client must SUBSCRIBE to receive messages [OK]

Hint: Client must SUBSCRIBE to receive published messages [OK]

Common Mistakes:

• Thinking PUBLISH syntax is wrong
• Confusing Streams with Pub/Sub for message delivery
• Using XREAD to get Pub/Sub messages

5. You want to build a task queue in Redis where tasks must be stored reliably and processed even if the worker restarts. Which Redis feature should you use and why?

hard

A. Use Streams because they store messages and support replaying tasks

B. Use Sorted Sets because they keep tasks ordered by score

C. Use Pub/Sub because it delivers messages instantly without storage

D. Use Hashes because they store key-value pairs efficiently

Solution

1. Step 1: Identify requirements for task queue

   Tasks must be stored reliably and processed even after worker restarts.

2. Step 2: Compare Redis features

   Pub/Sub does not store messages, so tasks can be lost. Streams store messages and allow replay.

3. Final Answer:

   Use Streams because they store messages and support replaying tasks -> Option A

4. Quick Check:

   Reliable storage and replay = Streams [OK]

Hint: Reliable task queues need Streams, not Pub/Sub [OK]

Common Mistakes:

• Choosing Pub/Sub for reliable storage
• Confusing Sorted Sets with message storage
• Using Hashes which are not for message queues