XACK for acknowledging messages in Redis - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to acknowledge a message in a Redis stream consumer group.

Redis

XACK mystream mygroup [1]

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

Amyconsumer

B0-0

C1526985058136-0

Dmystream

Attempts:

3 left

💡 Hint

Common Mistakes

Using the consumer name instead of the message ID.

Using the stream name as the message ID.

Using '0-0' which is not a valid message ID.

The XACK command requires the stream name, the consumer group name, and the message ID to acknowledge the message. Here, the message ID is 1526985058136-0.

2fill in blank

medium

Complete the code to acknowledge multiple messages in a Redis stream consumer group.

Redis

XACK mystream mygroup [1] [2]

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

A1526985058136-0

B1526985058137-0

Cmyconsumer

Dmystream

Attempts:

3 left

💡 Hint

Common Mistakes

Using consumer or stream names instead of message IDs.

Providing only one message ID when multiple are needed.

You can acknowledge multiple messages by listing their IDs after the stream and group names. Here, two message IDs are acknowledged.

3fill in blank

hard

Fix the error in the code to correctly acknowledge a message in Redis.

Redis

XACK [1] mygroup 1526985058136-0

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

Amyconsumer

B1526985058136-0

Cmygroup

Dmystream

Attempts:

3 left

💡 Hint

Common Mistakes

Using the consumer name instead of the stream name.

Swapping the group name and stream name.

The first argument after XACK must be the stream name. Here, mystream is the correct stream name.

4fill in blank

hard

Fill both blanks to acknowledge two messages from a Redis stream consumer group.

Redis

XACK [1] [2] 1526985058136-0 1526985058137-0

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

Amystream

Bmygroup

Cmyconsumer

Dothergroup

Attempts:

3 left

💡 Hint

Common Mistakes

Using consumer name instead of group name.

Swapping stream and group names.

The first blank is the stream name, and the second blank is the consumer group name. Both are required before listing message IDs.

5fill in blank

hard

Fill all three blanks to acknowledge a message in Redis with correct stream, group, and message ID.

Redis

XACK [1] [2] [3]

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

Amystream

Bmygroup

C1526985058136-0

Dmyconsumer

Attempts:

3 left

💡 Hint

Common Mistakes

Including the consumer name as an argument.

Swapping the order of stream and group names.

The XACK command requires the stream name, the consumer group name, and the message ID to acknowledge the message properly.

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