Pub/sub vs streams comparison in Redis - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to subscribe to a Redis channel named 'news'.

Redis

SUBSCRIBE [1]

Drag options to blanks, or click blank then click option'

Anews

Bstream

Cchannel1

Dmessages

Attempts:

3 left

💡 Hint

Common Mistakes

Using a stream name instead of a channel name

Misspelling the channel name

The SUBSCRIBE command listens to a specific channel. Here, 'news' is the channel name.

2fill in blank

medium

Complete the code to add a message to a Redis stream named 'mystream'.

Redis

XADD mystream * [1] message

Drag options to blanks, or click blank then click option'

Atopic

Bchannel

Cfield1

Ddata

Attempts:

3 left

💡 Hint

Common Mistakes

Using channel or topic which are for pub/sub

Omitting the field name

The XADD command adds an entry to a stream. 'field1' is the field name for the message.

3fill in blank

hard

Fix the error in the command to read messages from a stream 'mystream' starting from the last ID.

Redis

XREAD COUNT 10 STREAMS mystream [1]

Drag options to blanks, or click blank then click option'

A0

B-

C*

D$

Attempts:

3 left

💡 Hint

Common Mistakes

Using '0' to read only new messages

Using '*' which is invalid here

The XREAD command uses '$' to read new messages added after the command runs.

4fill in blank

hard

Fill both blanks to publish a message 'Hello' to channel 'chatroom' and add the same message to stream 'chatstream'.

Redis

PUBLISH [1] 'Hello'
XADD [2] * message 'Hello'

Drag options to blanks, or click blank then click option'

Achatroom

Bchatstream

Cnews

Dmystream

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up channel and stream names

Using the wrong command for each data type

PUBLISH sends a message to a channel, and XADD adds a message to a stream. Here, 'chatroom' is the channel and 'chatstream' is the stream.

5fill in blank

hard

Fill all three blanks to create a consumer group 'group1' for stream 'orders', read new messages, and acknowledge message ID '1526985058136-0'.

Redis

XGROUP CREATE [1] [2] $ MKSTREAM
XREADGROUP GROUP [1] consumer1 COUNT 10 STREAMS [2] >
XACK [2] [1] 1526985058136-0

Drag options to blanks, or click blank then click option'

Agroup1

Borders

Cgroup2

Dmystream

Attempts:

3 left

💡 Hint

Common Mistakes

Using different group or stream names in commands

Not acknowledging messages properly

XGROUP CREATE creates a consumer group named 'group1' for the stream 'orders'. XREADGROUP reads messages for that group, and XACK acknowledges a message ID for the same group and stream.

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