XACK for acknowledging messages in Redis - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does the Redis command XACK do?

It acknowledges one or more messages in a Redis stream, marking them as processed so they won't be delivered again to the same consumer group.

beginner

Why is acknowledging messages important in Redis streams?

Acknowledging messages confirms they were processed successfully, preventing them from being redelivered and helping manage message flow reliably.

beginner

What parameters does the XACK command require?

It requires the stream name, the consumer group name, and one or more message IDs to acknowledge.

beginner

If you run XACK mystream mygroup 1526985058136-0, what happens?

The message with ID 1526985058136-0 in the stream mystream is marked as acknowledged for the consumer group mygroup.

intermediate

Can XACK acknowledge multiple messages at once?

Yes, you can list multiple message IDs after the stream and group names to acknowledge several messages in one command.

What is the main purpose of the Redis XACK command?

A To add new messages to a stream

B To delete a stream

C To acknowledge processed messages in a stream

D To create a consumer group

XACK marks messages as processed so they won't be delivered again to the same consumer group.

Which of these is NOT a required parameter for XACK?

A Consumer name

B Consumer group name

C Message ID(s)

D Stream name

XACK needs the stream, group, and message IDs, but not the consumer name.

What happens if you acknowledge a message that was never delivered to your consumer group?

A The command returns an error

B The message is acknowledged anyway

C The message is deleted from the stream

D Nothing happens, no acknowledgment is recorded

Only messages pending for the group can be acknowledged; others are ignored.

Can XACK be used to acknowledge messages from multiple streams at once?

A Yes, you can list multiple streams

B No, it works on one stream at a time

C Only if streams belong to the same group

D Only if streams have the same name

XACK acknowledges messages only from one stream per command.

What does XACK return after successful acknowledgment?

A The number of messages acknowledged

B The list of acknowledged message IDs

C The total number of messages in the stream

D A confirmation string 'OK'

XACK returns how many messages were acknowledged successfully.

Explain how the XACK command helps manage message processing in Redis streams.

Think about how you tell Redis you finished handling messages.

Describe the parameters needed for the XACK command and what each represents.

Consider what you need to identify the messages and the group.

Practice

1. What is the primary purpose of the XACK command in Redis streams?

easy

A. To delete a stream

B. To add a new message to a stream

C. To acknowledge that a message has been processed by a consumer

D. To create a new consumer group

Solution

1. Step 1: Understand the role of XACK

   XACK is used in Redis streams to mark messages as processed by a consumer in a consumer group.

2. Step 2: Differentiate from other commands

   Adding messages uses XADD, deleting streams uses DEL, and creating groups uses XGROUP. XACK specifically acknowledges messages.

3. Final Answer:

   To acknowledge that a message has been processed by a consumer -> Option C

4. Quick Check:

   XACK purpose = acknowledge processed message [OK]

Hint: XACK always marks messages as processed in consumer groups [OK]

Common Mistakes:

• Confusing XACK with XADD or XGROUP
• Thinking XACK deletes messages
• Assuming XACK creates consumer groups

2. Which of the following is the correct syntax to acknowledge a message with ID 1526985058136-0 in consumer group group1 on stream mystream?

easy

A. XACK 1526985058136-0 mystream group1

B. XACK mystream group1 1526985058136-0

C. XACK mystream 1526985058136-0 group1

D. XACK group1 mystream 1526985058136-0

Solution

1. Step 1: Recall XACK command syntax

   The syntax is: XACK <stream> <group> <ID> [ID ...].

2. Step 2: Match the given IDs and names

   Stream is mystream, group is group1, and message ID is 1526985058136-0, so the correct order is XACK mystream group1 1526985058136-0.

3. Final Answer:

   XACK mystream group1 1526985058136-0 -> Option B

4. Quick Check:

   XACK syntax = XACK stream group ID [OK]

Hint: XACK order: stream, group, then message IDs [OK]

Common Mistakes:

• Swapping group and stream names
• Placing message ID before stream or group
• Using incorrect command order

3. Given the following Redis commands executed in order:

XGROUP CREATE mystream group1 $ MKSTREAM
XADD mystream 1609459200000-0 field1 value1
XREADGROUP GROUP group1 consumer1 COUNT 1 STREAMS mystream >
XACK mystream group1 1609459200000-0

What will be the result of the XACK command?

medium

A. 1

B. 0

C. An error because the message ID does not exist

D. An error because the consumer group does not exist

Solution

1. Step 1: Analyze the commands and message ID

   The group 'group1' is created on 'mystream'. A message is added with XADD, then read by consumer1. The XACK acknowledges message ID 1609459200000-0.

2. Step 2: Confirm message ID existence and acknowledgment

   XADD adds the message with the specified ID 1609459200000-0. The XREADGROUP delivers it to consumer1's pending entries. XACK then acknowledges it and returns 1, the count of acknowledged messages.

3. Final Answer:

   1 -> Option A

4. Quick Check:

   XACK returns count of acknowledged messages = 1 [OK]

Hint: XACK returns number of messages acknowledged [OK]

Common Mistakes:

• Assuming XACK returns message content
• Thinking XACK errors if message ID is valid
• Confusing XACK with XREADGROUP output

4. You run the command XACK mystream group1 1609459200000-0 but get an error: (error) NOGROUP No such consumer group 'group1' for stream 'mystream'. What is the most likely cause?

medium

A. The stream 'mystream' does not exist

B. The message ID is invalid

C. The XACK command syntax is incorrect

D. The consumer group 'group1' was not created on 'mystream'

Solution

1. Step 1: Interpret the error message

   The error says no such consumer group 'group1' for stream 'mystream', meaning the group does not exist on that stream.

2. Step 2: Identify the cause

   Either the group was never created or was created on a different stream. The stream existing or message ID validity does not cause this error.

3. Final Answer:

   The consumer group 'group1' was not created on 'mystream' -> Option D

4. Quick Check:

   Error NOGROUP means missing consumer group [OK]

Hint: NOGROUP error means consumer group missing on stream [OK]

Common Mistakes:

• Assuming stream absence causes NOGROUP error
• Blaming message ID format
• Misreading error as syntax issue

5. You have a Redis stream orders with a consumer group processors. You want to acknowledge multiple messages with IDs 1609459200000-0, 1609459200001-0, and 1609459200002-0 in one command. Which command correctly does this?

hard

A. XACK orders processors 1609459200000-0 1609459200001-0 1609459200002-0

B. XACK orders processors [1609459200000-0, 1609459200001-0, 1609459200002-0]

C. XACK orders processors "1609459200000-0 1609459200001-0 1609459200002-0"

D. XACK orders processors {1609459200000-0,1609459200001-0,1609459200002-0}

Solution

1. Step 1: Recall XACK syntax for multiple IDs

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

2. Step 2: Evaluate options

   XACK orders processors 1609459200000-0 1609459200001-0 1609459200002-0 lists IDs separated by spaces correctly. Options B, C, and D use brackets, quotes, or braces which are invalid syntax.

3. Final Answer:

   XACK orders processors 1609459200000-0 1609459200001-0 1609459200002-0 -> Option A

4. Quick Check:

   Multiple IDs in XACK = space-separated list [OK]

Hint: List multiple IDs space-separated in XACK command [OK]

Common Mistakes:

• Using brackets or quotes around IDs
• Separating IDs with commas
• Passing IDs as a single string