Acknowledgment modes (acks=0, 1, all) in Kafka - Commands & Configuration

Introduction

When sending messages to Kafka, the producer needs to know if the message was received successfully. Acknowledgment modes control how many Kafka servers must confirm the message before the producer considers it sent. This helps balance speed and safety.

When you want the fastest message sending without waiting for confirmation.

When you want a balance between speed and message delivery guarantee.

When you want the highest safety ensuring all servers have the message.

When you want to avoid losing messages even if some servers fail.

When you want to tune Kafka producer behavior for your application's needs.

Commands

Start a Kafka producer that sends messages without waiting for any acknowledgment from the server. This is the fastest mode but may lose messages if the server fails.

Terminal

kafka-console-producer --broker-list localhost:9092 --topic example-topic --producer-property acks=0

Expected Output

>

→

acks=0 - No acknowledgment required from Kafka brokers.

Start a Kafka producer that waits for acknowledgment from the leader broker only. This balances speed and reliability.

Terminal

kafka-console-producer --broker-list localhost:9092 --topic example-topic --producer-property acks=1

Expected Output

>

→

acks=1 - Wait for leader broker acknowledgment.

Start a Kafka producer that waits for acknowledgment from all in-sync replicas. This is the safest mode but slower.

Terminal

kafka-console-producer --broker-list localhost:9092 --topic example-topic --producer-property acks=all

Expected Output

>

→

acks=all - Wait for all in-sync replicas to acknowledge.

Consume the first 3 messages from the topic to verify messages were sent successfully.

Terminal

kafka-console-consumer --bootstrap-server localhost:9092 --topic example-topic --from-beginning --max-messages 3

Expected Output

message1

message2

message3

→

--from-beginning - Read messages from the start of the topic.

→

--max-messages 3 - Stop after reading 3 messages.

Key Concept

If you remember nothing else, remember: acks control how many Kafka servers confirm a message before the producer moves on, balancing speed and safety.

Common Mistakes

Setting acks=0 when message loss is unacceptable.

Messages may be lost if the broker fails before storing them.

Use acks=1 or acks=all to ensure message delivery.

Using acks=all without ensuring enough in-sync replicas.

Producer may wait indefinitely if replicas are not available.

Ensure your Kafka cluster has enough in-sync replicas before using acks=all.

Not verifying messages after sending with different acks settings.

You may not notice message loss or delays.

Use kafka-console-consumer to check messages were received.

Summary

Use acks=0 for fastest sending but no delivery guarantee.

Use acks=1 to wait for leader broker confirmation balancing speed and safety.

Use acks=all to wait for all in-sync replicas for highest safety.

Verify messages with kafka-console-consumer after producing.

Practice

1. Which acks mode in Kafka provides the fastest message sending but with the least safety?

easy

A. acks=0

B. acks=1

C. acks=all

D. None of the above

Solution

1. Step 1: Understand acks=0 behavior

   This mode means the producer does not wait for any acknowledgment from the broker, so it sends messages fastest but with no guarantee of delivery.

2. Step 2: Compare with other modes

   acks=1 waits for leader confirmation, and acks=all waits for all replicas, both slower but safer.

3. Final Answer:

   acks=0 -> Option A

4. Quick Check:

   Fastest but least safe = acks=0 [OK]

Hint: No wait for confirmation means fastest mode is acks=0 [OK]

Common Mistakes:

• Confusing acks=0 with acks=1
• Thinking acks=all is fastest
• Assuming acks=0 waits for leader

2. Which of the following is the correct way to set the acknowledgment mode to wait for all replicas in Kafka producer configuration?

easy

A. "acks=1"

B. "acks=0"

C. "acks=all"

D. "ack=all"

Solution

1. Step 1: Recall correct property name and value

   The property is acks (plural), and to wait for all replicas, the value is all.

2. Step 2: Check options for syntax correctness

   "acks=all" uses correct key and value: "acks=all". "ack=all" has a typo "ack" instead of "acks".

3. Final Answer:

   "acks=all" -> Option C

4. Quick Check:

   Wait for all replicas = acks=all [OK]

Hint: Remember plural 'acks' and value 'all' for all replicas [OK]

Common Mistakes:

• Using 'ack' instead of 'acks'
• Confusing 'acks=1' with 'acks=all'
• Setting 'acks=0' for all replicas

3. Consider this Kafka producer code snippet:

props.put("acks", "1");
producer.send(record);
System.out.println("Message sent");

What does acks=1 guarantee before printing "Message sent"?

medium

A. Message is stored only in producer buffer

B. No guarantee; message may be lost

C. All replicas have confirmed the message

D. Leader broker has confirmed the message

Solution

1. Step 1: Understand acks=1 meaning

   This mode waits for the leader broker to confirm the message is received and written to its log.

2. Step 2: Compare with other options

   acks=0 gives no guarantee, acks=all waits for all replicas, and producer buffer storage is unrelated.

3. Final Answer:

   Leader broker has confirmed the message -> Option D

4. Quick Check:

   acks=1 means leader confirmation [OK]

Hint: acks=1 waits for leader confirmation only [OK]

Common Mistakes:

• Thinking acks=1 waits for all replicas
• Assuming no guarantee with acks=1
• Confusing producer buffer with broker confirmation

4. You have this Kafka producer configuration:

props.put("acks", "alll");

What is the problem with this configuration?

medium

A. Typo in the value; should be "all"

B. Typo in the key; should be "ack"

C. No problem; "alll" is valid

D. Should be an integer, not a string

Solution

1. Step 1: Check the value spelling

   The correct value for waiting all replicas is "all", but here it is misspelled as "alll" with an extra 'l'.

2. Step 2: Verify key and value types

   The key "acks" is correct and the value should be a string. So the problem is only the typo in the value.

3. Final Answer:

   Typo in the value; should be "all" -> Option A

4. Quick Check:

   Value must be "all", not "alll" [OK]

Hint: Check spelling of 'all' carefully in acks value [OK]

Common Mistakes:

• Assuming 'alll' is valid
• Changing key to 'ack' instead of 'acks'
• Using integer instead of string for acks

5. You want to ensure maximum data safety in Kafka producer but can tolerate some delay. Which acks setting should you use and why?

hard

A. acks=1 because leader confirmation is enough

B. acks=all because it waits for all replicas

C. acks=0 because it is fastest

D. acks=-1 because it disables acknowledgments

Solution

1. Step 1: Identify safety vs speed tradeoff

   acks=all waits for all replicas to confirm, ensuring maximum data safety but slower performance.

2. Step 2: Evaluate other options

   acks=0 is fastest but unsafe, acks=1 waits only for leader, and acks=-1 is invalid or equivalent to acks=all but less clear.

3. Final Answer:

   acks=all because it waits for all replicas -> Option B

4. Quick Check:

   Max safety = acks=all [OK]

Hint: Max safety means waiting for all replicas: acks=all [OK]

Common Mistakes:

• Choosing acks=0 for safety
• Confusing acks=-1 as disabling acknowledgments
• Thinking leader confirmation is max safety