Consumer groups concept in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a consumer group named 'mygroup' for the stream 'mystream'.

Redis

XGROUP CREATE mystream [1] $ MKSTREAM

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

Agroup1

Bmygroup

Cstreamgroup

DconsumerA

Attempts:

3 left

💡 Hint

Common Mistakes

Using a consumer name instead of a group name.

Confusing the stream name with the group name.

The XGROUP CREATE command creates a consumer group. The group name here is 'mygroup'.

2fill in blank

medium

Complete the code to read messages from the stream 'mystream' using consumer group 'mygroup' and consumer 'consumer1'.

Redis

XREADGROUP GROUP mygroup [1] COUNT 10 STREAMS mystream >

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

AconsumerA

Bgroup1

Cconsumer1

Dmystream

Attempts:

3 left

💡 Hint

Common Mistakes

Using the group name instead of the consumer name.

Using the stream name instead of the consumer name.

The XREADGROUP command requires the consumer name after the group name. Here, 'consumer1' is the consumer reading messages.

3fill in blank

hard

Fix the error in the command to acknowledge a message with ID '1526985058136-0' in group 'mygroup' on stream 'mystream'.

Redis

XACK mystream mygroup [1]

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

A1526985058136-0

Bconsumer1

Cmystream

Dmygroup

Attempts:

3 left

💡 Hint

Common Mistakes

Using the consumer or group name instead of the message ID.

Using the stream name as the message ID.

The XACK command acknowledges a message by its ID. The message ID '1526985058136-0' must be provided.

4fill in blank

hard

Fill both blanks to claim pending messages for consumer 'consumer2' in group 'mygroup' on stream 'mystream' that have been idle for more than 60000 milliseconds.

Redis

XCLAIM mystream mygroup [1] 60000 [2]

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

Aconsumer2

B1526985058136-0

Cconsumer1

D1526985058137-0

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping consumer names or message IDs.

Using a wrong message ID that does not exist.

The XCLAIM command requires the consumer name and the message ID to claim pending messages. Here, 'consumer2' claims the message '1526985058136-0'.

5fill in blank

hard

Fill all three blanks to get the list of consumers in group 'mygroup' for stream 'mystream' and count their pending messages.

Redis

XINFO CONSUMERS [1] [2] | awk '{{print $1, $[3]'

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

Amystream

Bmygroup

C7

D8

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up stream and group names.

Using the wrong field number in awk.

The XINFO CONSUMERS command lists consumers for a group. The stream is 'mystream', the group is 'mygroup'. The awk command prints the first and eighth fields, where the eighth field is the pending message count.

Practice

1. What is the main purpose of Redis consumer groups in streams?

easy

A. Allow multiple clients to read the same stream without duplicating work

B. Store data permanently in Redis

C. Encrypt messages in the stream

D. Automatically delete old messages from the stream

Solution

1. Step 1: Understand consumer groups role

   Consumer groups let many clients read from the same stream safely and efficiently.

2. Step 2: Identify main benefit

   They avoid duplicate processing by tracking which messages each consumer has read.

3. Final Answer:

   Allow multiple clients to read the same stream without duplicating work -> Option A

4. Quick Check:

   Consumer groups = safe multi-client reading [OK]

Hint: Consumer groups = shared reading without duplicates [OK]

Common Mistakes:

• Thinking consumer groups store data permanently
• Confusing consumer groups with encryption
• Assuming consumer groups delete old messages automatically

2. Which Redis command is used to create a consumer group for a stream named mystream?

easy

A. XGROUP NEW mystream mygroup $

B. XGROUP CREATE mystream mygroup $

C. XGROUP ADD mystream mygroup $

D. XREADGROUP CREATE mystream mygroup $

Solution

1. Step 1: Recall command to create consumer group

   The correct command is XGROUP CREATE followed by stream name, group name, and ID.

2. Step 2: Match command syntax

   XGROUP CREATE mystream mygroup $ correctly creates group mygroup on mystream.

3. Final Answer:

   XGROUP CREATE mystream mygroup $ -> Option B

4. Quick Check:

   Create group = XGROUP CREATE [OK]

Hint: Use XGROUP CREATE to make groups [OK]

Common Mistakes:

• Using XREADGROUP instead of XGROUP CREATE
• Using non-existent commands like XGROUP ADD or XGROUP NEW
• Mixing command order or missing parameters

3. Given a consumer group mygroup on stream mystream, what does this command return?

XREADGROUP GROUP mygroup consumer1 COUNT 2 STREAMS mystream > 

medium

A. Returns up to 2 new messages never delivered to any consumer

B. Returns all messages in the stream from the beginning

C. Returns the last 2 messages already acknowledged

D. Returns an error because of wrong syntax

Solution

1. Step 1: Understand XREADGROUP with '>' ID

   Using '>' means read only new messages never delivered to any consumer in the group.

2. Step 2: COUNT 2 limits messages returned

   The command returns up to 2 such new messages for consumer1.

3. Final Answer:

   Returns up to 2 new messages never delivered to any consumer -> Option A

4. Quick Check:

   XREADGROUP with '>' = new messages only [OK]

Hint: '>' ID in XREADGROUP means new messages only [OK]

Common Mistakes:

• Thinking it returns all messages from start
• Assuming it returns acknowledged messages
• Believing the command syntax is incorrect

4. You run this command but get an error:

XREADGROUP GROUP mygroup consumer1 STREAMS mystream >

What is the likely cause?

medium

A. Missing COUNT argument causes syntax error

B. The '>' symbol is invalid here

C. The command is missing the ID after STREAMS

D. Consumer group 'mygroup' does not exist

Solution

1. Step 1: Verify command syntax

   The syntax is correct: XREADGROUP GROUP mygroup consumer1 STREAMS mystream > uses '>' ID for new messages. COUNT is optional.

2. Step 2: Identify prerequisite error

   XREADGROUP requires an existing consumer group created with XGROUP CREATE. If missing, it returns a NOGROUP error.

3. Final Answer:

   Consumer group 'mygroup' does not exist -> Option D

4. Quick Check:

   XREADGROUP needs existing group [OK]

Hint: XREADGROUP requires existing consumer group [OK]

Common Mistakes:

• Thinking COUNT is mandatory (it's optional)
• Believing '>' is invalid
• Mistakenly thinking ID is missing after STREAMS

5. You have a stream orders and consumer group ordergroup. You want to ensure that if a consumer crashes before acknowledging messages, another consumer can claim and process those pending messages. Which Redis command helps you do this?

hard

A. XREADGROUP GROUP ordergroup consumer2 STREAMS orders >

B. XACK orders ordergroup 1526985058136

C. XCLAIM orders ordergroup consumer2 60000 1526985058136

D. XGROUP CREATE orders ordergroup $

Solution

1. Step 1: Understand the problem of crashed consumers

   If a consumer crashes, its pending messages remain unacknowledged and need to be claimed by others.

2. Step 2: Identify command to claim pending messages

   XCLAIM lets another consumer take ownership of pending messages after a timeout.

3. Step 3: Match command usage

   XCLAIM orders ordergroup consumer2 60000 1526985058136 claims message ID 1526985058136 for consumer2.

4. Final Answer:

   XCLAIM orders ordergroup consumer2 60000 1526985058136 -> Option C

5. Quick Check:

   Claim pending messages = XCLAIM [OK]

Hint: Use XCLAIM to recover unacknowledged messages [OK]

Common Mistakes:

• Using XACK which only acknowledges, not claims
• Using XREADGROUP with '>' which reads new messages only
• Trying to create group again instead of claiming