XACK for acknowledging messages in Redis - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What does the XACK command return after acknowledging a message?

Given a Redis stream consumer group, when you run XACK to acknowledge a message ID, what is the output of the command?

Redis

XACK mystream mygroup 1526985058136-0

A 1

B 0

C An error message

D The acknowledged message ID

💡 Hint

Think about what XACK returns when it successfully acknowledges a message.

The XACK command returns the number of messages it successfully acknowledged. If the message ID exists and is acknowledged, it returns 1. If the message ID does not exist or was not pending, it returns 0.

🧠 Conceptual

intermediate

Why is acknowledging messages with XACK important in Redis streams?

In Redis streams, what is the main purpose of using the XACK command after processing a message?

A To remove the message from the stream permanently

B To reset the stream to the beginning

C To delete the consumer group

D To mark the message as processed so it won't be delivered again to the same consumer group

💡 Hint

Think about how Redis tracks which messages are pending for a consumer group.

XACK marks a message as processed in the pending entries list (PEL) of a consumer group. This prevents the message from being delivered again to the same group, ensuring reliable message processing.

📝 Syntax

advanced

Which XACK command syntax is correct to acknowledge multiple messages?

You want to acknowledge two messages with IDs 1609459200000-0 and 1609459200001-0 in the consumer group group1 on stream mystream. Which command is syntactically correct?

A XACK mystream group1 '1609459200000-0 1609459200001-0'

B XACK mystream group1 1609459200000-0 1609459200001-0

C XACK mystream group1 (1609459200000-0, 1609459200001-0)

D XACK mystream group1 [1609459200000-0, 1609459200001-0]

💡 Hint

XACK accepts multiple message IDs separated by spaces without brackets or quotes.

The correct syntax for acknowledging multiple messages is to list the message IDs separated by spaces after the stream and group names. Brackets, parentheses, or quotes are not used.

🔧 Debug

advanced

Why does this XACK command return 0 instead of 1?

You run the command XACK mystream mygroup 1609459200000-0 but it returns 0. What is the most likely reason?

A The consumer group does not exist

B The stream is empty

C The message ID is not pending in the consumer group's pending entries list

D The message ID does not exist in the stream

💡 Hint

XACK only acknowledges messages that are pending for the group.

If the message ID is not in the pending entries list (PEL) for the consumer group, XACK returns 0 because there is nothing to acknowledge.

❓ optimization

expert

How to efficiently acknowledge a large batch of messages in Redis streams?

You have processed 10,000 messages from a Redis stream consumer group and want to acknowledge them all efficiently. Which approach is best?

A Use XACK with a Lua script to batch acknowledge messages in chunks

B Run 10,000 separate XACK commands, one per message ID

C Run a single XACK command listing all 10,000 message IDs separated by spaces

D Delete the stream to remove all messages at once

💡 Hint

Consider Redis command length limits and network overhead.

Sending a single XACK with 10,000 IDs may exceed command length limits and cause performance issues. Running many separate commands causes high network overhead. Using a Lua script to batch acknowledge messages in manageable chunks balances efficiency and reliability.