Pub/sub vs streams comparison in Redis - Quick Revision & Key Differences

Recall & Review

beginner

What is Redis Pub/Sub?

Redis Pub/Sub is a messaging pattern where messages are sent to channels and all subscribers to those channels receive the messages instantly. It is a simple, real-time broadcast system.

beginner

What is Redis Streams?

Redis Streams is a data structure that stores messages in an append-only log. Consumers can read messages at their own pace and keep track of what they have processed.

intermediate

How does message delivery differ between Pub/Sub and Streams?

In Pub/Sub, messages are delivered only to active subscribers at the time of publishing and are not stored. In Streams, messages are stored and can be read later by consumers, even if they were offline when the message was published.

intermediate

Which Redis messaging system supports message persistence and consumer groups?

Redis Streams supports message persistence and consumer groups, allowing multiple consumers to share the workload and track message processing.

beginner

When should you choose Redis Pub/Sub over Streams?

Choose Redis Pub/Sub for simple, real-time messaging where you don't need to store messages or track consumers. It's best for live notifications or chat where messages are transient.

Which Redis feature stores messages for later consumption?

A Pub/Sub

B Streams

C Hashes

D Sets

Redis Streams stores messages in an append-only log for later consumption, unlike Pub/Sub which does not store messages.

In Redis Pub/Sub, what happens if a subscriber is offline when a message is published?

A The message is stored and delivered later

B The message is lost for that subscriber

C The subscriber receives the message when back online

D The message is queued automatically

Pub/Sub does not store messages, so offline subscribers miss messages published while they were disconnected.

Which Redis messaging system supports consumer groups for load balancing?

A Pub/Sub

B Streams

C Lists

D Sorted Sets

Redis Streams supports consumer groups, allowing multiple consumers to share message processing.

What is a key advantage of Redis Pub/Sub?

A Message persistence

B Real-time message delivery

C Message replay

D Consumer tracking

Pub/Sub provides instant, real-time message delivery to all active subscribers.

Which Redis feature is better for building a chat application with message history?

A Pub/Sub

B Streams

C Sets

D Hashes

Streams allow storing messages and reading them later, which is useful for chat history.

Explain the main differences between Redis Pub/Sub and Redis Streams.

Think about message storage, delivery timing, and consumer management.

When would you choose Redis Streams over Pub/Sub in a project?

Consider use cases involving message durability and consumer coordination.

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