Message key and value in Kafka - Commands & Configuration

Introduction

Kafka sends messages as pairs of keys and values. The key helps decide where the message goes, and the value is the actual data you want to send.

When you want to keep related messages together in the same order.

When you need to update or delete messages by key later.

When you want to balance load evenly across servers.

When you want to group messages by customer ID or session ID.

When you want to ensure messages with the same key go to the same place.

Commands

Start a Kafka producer that reads messages from your keyboard. It uses ':' to split the key and value in each message.

Terminal

kafka-console-producer --broker-list localhost:9092 --topic example-topic --property parse.key=true --property key.separator=:

Expected Output

>

→

--property parse.key=true - Enable parsing of message keys from input

→

--property key.separator=: - Set ':' as the separator between key and value

Send a message with key 'user1' and value 'Hello Kafka'. This keeps messages with key 'user1' together.

Terminal

user1:Hello Kafka

Expected Output

No output (command runs silently)

Start a consumer to read messages from the beginning, showing both keys and values.

Terminal

kafka-console-consumer --bootstrap-server localhost:9092 --topic example-topic --property print.key=true --from-beginning

Expected Output

user1	Hello Kafka

→

--property print.key=true - Show message keys in output

→

--from-beginning - Read all messages from the start

Key Concept

The message key groups related messages and controls where they go, while the value holds the actual data.

Common Mistakes

Not setting --property parse.key=true when producing messages with keys

Kafka treats the whole input as the value, ignoring the key part.

Always use --property parse.key=true and set a key separator when sending keyed messages.

Not using --property print.key=true when consuming messages with keys

You only see the value, missing the key information.

Add --property print.key=true to see both keys and values.

Summary

Use message keys to group related messages and control partitioning.

Set --property parse.key=true and key.separator when producing keyed messages.

Use --property print.key=true when consuming to see keys with values.

Practice

1. What is the primary role of the message key in Kafka?

easy

A. To decide which partition the message goes to

B. To store the actual data of the message

C. To encrypt the message content

D. To delete old messages automatically

Solution

1. Step 1: Understand message key purpose

   The message key in Kafka is used to determine the partition where the message will be stored.

2. Step 2: Differentiate key and value roles

   The message value holds the actual data, while the key helps with partitioning and ordering.

3. Final Answer:

   To decide which partition the message goes to -> Option A

4. Quick Check:

   Message key = Partition decision [OK]

Hint: Key decides partition, value holds data [OK]

Common Mistakes:

• Confusing key with message value
• Thinking key encrypts data
• Assuming key deletes messages

2. Which of the following is the correct way to send a Kafka message with a key and value in Java?

easy

A. producer.send(new ProducerRecord<>("topic", value));

B. producer.send(new ProducerRecord<>(key, value));

C. producer.send("topic", value);

D. producer.send(new ProducerRecord<>("topic", key, value));

Solution

1. Step 1: Review ProducerRecord constructor

   The correct constructor for sending a message with key and value includes topic, key, and value in that order.

2. Step 2: Check each option

   producer.send(new ProducerRecord<>("topic", key, value)); uses new ProducerRecord<>("topic", key, value), which is correct syntax. Others miss topic or key.

3. Final Answer:

   producer.send(new ProducerRecord<>("topic", key, value)); -> Option D

4. Quick Check:

   ProducerRecord(topic, key, value) = correct [OK]

Hint: ProducerRecord needs topic, key, then value [OK]

Common Mistakes:

• Omitting the topic parameter
• Swapping key and value order
• Using wrong constructor overload

3. Given the following Kafka producer code snippet, what will be the key of the message sent?

ProducerRecord record = new ProducerRecord<>("orders", "12345", "order details");
producer.send(record);

medium

A. "orders"

B. null

C. "12345"

D. "order details"

Solution

1. Step 1: Identify parameters in ProducerRecord

   The constructor is ProducerRecord(topic, key, value). Here, "orders" is topic, "12345" is key, and "order details" is value.

2. Step 2: Determine the key value

   The key is the second argument, which is "12345".

3. Final Answer:

   "12345" -> Option C

4. Quick Check:

   Second argument = key = "12345" [OK]

Hint: Second argument in ProducerRecord is key [OK]

Common Mistakes:

• Confusing topic with key
• Choosing message value as key
• Assuming key is null if not obvious

4. Identify the error in this Kafka producer code snippet:

ProducerRecord record = new ProducerRecord<>("topic", null, "data");
producer.send(record);

medium

A. Message key cannot be null

B. This code is valid; no error

C. ProducerRecord constructor parameters are in wrong order

D. Message value cannot be null

Solution

1. Step 1: Check if null key is allowed

   Kafka allows message keys to be null; this means messages can be sent without a key.

2. Step 2: Verify constructor usage

   The constructor ProducerRecord<>("topic", null, "data") is valid and sends a message with no key.

3. Final Answer:

   This code is valid; no error -> Option B

4. Quick Check:

   Null key allowed in Kafka messages [OK]

Hint: Null keys are allowed in Kafka messages [OK]

Common Mistakes:

• Assuming key must never be null
• Confusing parameter order
• Thinking value cannot be null without checking

5. You want to ensure all messages with the same user ID go to the same partition for ordering. Which Kafka message feature should you use and how?

hard

A. Use the user ID as the message key to ensure partitioning by key

B. Include the user ID only in the message value and leave key null

C. Set the user ID as the topic name

D. Send messages without keys and rely on default partitioning

Solution

1. Step 1: Understand partitioning by key

   Kafka uses the message key to decide the partition. Messages with the same key go to the same partition.

2. Step 2: Apply to user ID scenario

   Using the user ID as the message key ensures all messages for that user go to the same partition, preserving order.

3. Final Answer:

   Use the user ID as the message key to ensure partitioning by key -> Option A

4. Quick Check:

   Key = user ID for partitioning [OK]

Hint: Use key for grouping messages by user ID [OK]

Common Mistakes:

• Putting user ID only in value loses partition control
• Changing topic per user is inefficient
• Relying on default partitioning breaks ordering