Consumer groups concept in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this XREADGROUP command?

Given a Redis stream 'mystream' with messages and a consumer group 'mygroup', what will this command return?

XREADGROUP GROUP mygroup consumer1 COUNT 2 STREAMS mystream >

Assume the stream has 5 pending messages for this group and consumer1 has not read any yet.

Redis

XREADGROUP GROUP mygroup consumer1 COUNT 2 STREAMS mystream >

A Returns an empty array because consumer1 has no pending messages.

B Returns the first 2 messages in the stream regardless of delivery status.

C Returns the 2 oldest pending messages assigned to consumer1.

D Returns the next 2 new messages that were never delivered to any consumer in 'mygroup'.

💡 Hint

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

Using '>' in XREADGROUP returns only new messages that have never been delivered to any consumer in the group. Pending messages are not returned with '>'.

🧠 Conceptual

intermediate

What happens when a consumer acknowledges a message in a consumer group?

In Redis streams, when a consumer calls XACK for a message ID in its consumer group, what is the effect?

A The message is removed from the pending entries list for that consumer group.

B The message is deleted from all consumer groups.

C The message is removed from the stream permanently.

D The message is marked as delivered but remains pending.

💡 Hint

Think about what 'acknowledging' means in message queue systems.

XACK removes the message ID from the pending entries list of the consumer group, marking it as processed by that consumer.

📝 Syntax

advanced

Which XGROUP command correctly creates a consumer group starting at the beginning of the stream?

You want to create a consumer group named 'group1' on stream 'stream1' starting from the very first message. Which command is correct?

A XGROUP CREATE stream1 group1 $

B XGROUP CREATE stream1 group1 0

C XGROUP CREATE stream1 group1 >

D XGROUP CREATE stream1 group1 -

💡 Hint

The starting ID '0' means from the beginning of the stream.

Using '0' as the ID in XGROUP CREATE means the group will start reading from the first message in the stream.

🔧 Debug

advanced

Why does this XREADGROUP command return no messages?

A consumer runs:

XREADGROUP GROUP mygroup consumer1 COUNT 5 STREAMS mystream 0

But it returns an empty array even though the stream has messages. Why?

Redis

XREADGROUP GROUP mygroup consumer1 COUNT 5 STREAMS mystream 0

A Because XREADGROUP with a specific ID reads only new messages with IDs greater than the given ID, but '0' is not valid here.

B Because XREADGROUP with a specific ID reads only pending messages with IDs greater than the given ID, and '0' excludes all.

C Because XREADGROUP expects '>' to read new messages; using '0' reads only pending messages, but none are pending for this consumer.

D Because '0' means read messages with ID greater than 0, but all messages have IDs less than or equal to 0.

💡 Hint

Consider the difference between '>' and a specific ID in XREADGROUP.

Using '0' as the ID in XREADGROUP reads only pending messages with IDs greater than 0 assigned to the consumer. If none are pending, it returns empty. To read new messages, use '>'.

❓ optimization

expert

How to efficiently claim pending messages from a failed consumer?

In a Redis consumer group, consumer 'A' crashed leaving 100 pending messages. You want consumer 'B' to claim all messages pending for 'A' that are idle for more than 5 minutes. Which command achieves this efficiently?

A XPENDING mystream mygroup IDLE 300000 - + 100 A | then XCLAIM mystream mygroup B 300000 <list_of_ids>

B XCLAIM mystream mygroup B 300000 - COUNT 100 IDLE 300000

C XCLAIM mystream mygroup B 300000 - IDLE 300000

D XPENDING mystream mygroup - + 100 B | then XCLAIM mystream mygroup B 300000 <list_of_ids>

💡 Hint

You need to find the IDs first, then claim them.

XPENDING with range and consumer filters returns the IDs of pending messages for consumer 'A' idle over 5 minutes (300000 ms). Then XCLAIM claims those IDs for consumer 'B'.