How to use the command "kcat" (with examples)

The “kcat” command is a tool used for working with Apache Kafka as both a producer and a consumer. It allows users to consume and publish messages, manage consumer groups, retrieve metadata, and get offset information for specific topics and partitions. In this article, we will explore various use cases of the “kcat” command along with their corresponding examples.

Use case 1: Consume messages starting with the newest offset

Code:

kcat -C -t topic -b brokers

Motivation: This use case is useful when you want to consume messages from a specific topic starting from the most recent offset. It allows you to process only the latest messages without consuming any older messages.

Explanation:

• -C represents the consumer mode.
• -t topic specifies the Kafka topic from which you want to consume messages.
• -b brokers indicates the list of brokers from which to consume the messages.

Example output:

Message 1
Message 2
Message 3

Use case 2: Consume messages starting with the oldest offset and exit after the last message is received

Code:

kcat -C -t topic -b brokers -o beginning -e

Motivation: This use case is helpful when you need to consume all messages from the beginning of a topic and stop consuming after the last message is received. It allows you to process all the messages in a topic without continuously consuming new messages.

Explanation:

• -o beginning specifies the starting offset as the oldest available offset.
• -e indicates that the consumer should exit after consuming the last message.

Example output:

Message 1
Message 2
Message 3

Use case 3: Consume messages as a Kafka consumer group

Code:

kcat -G group_id topic -b brokers

Motivation: This use case is useful when you want to consume messages as part of a Kafka consumer group. A consumer group allows multiple consumers to work together to process messages from one or more topics, providing fault tolerance and scalability.

Explanation:

• -G group_id specifies the Kafka consumer group ID.
• topic is the topic from which you want to consume messages.
• -b brokers indicates the list of brokers from which to consume the messages.

Example output:

Message 1 (consumed by consumer 1)
Message 2 (consumed by consumer 2)
Message 3 (consumed by consumer 1)

Use case 4: Publish message by reading from stdin

Code:

echo message | kcat -P -t topic -b brokers

Motivation: This use case is helpful when you want to publish a single message by providing it through stdin. It allows for easy integration with other tools or scripts by piping the message into the “kcat” command.

Explanation:

• -P represents the producer mode.
• -t topic specifies the Kafka topic to which you want to publish the message.
• -b brokers indicates the list of brokers to which the message should be published.

Example output: No output will be displayed if the message is successfully published.

Use case 5: Publish messages by reading from a file

Code:

kcat -P -t topic -b brokers path/to/file

Motivation: This use case is useful when you have a file containing multiple messages that you want to publish to a Kafka topic. It provides an efficient way to publish bulk messages without the need to manually input each message.

Explanation:

• path/to/file is the path to the file from which the messages will be read and published.

Example output: No output will be displayed if the messages are successfully published.

Use case 6: List metadata for all topics and brokers

Code:

kcat -L -b brokers

Motivation: This use case is helpful when you need to retrieve metadata about all topics and brokers in a Kafka cluster. It provides information such as the topic and partition details, leader brokers, and replicas.

Explanation:

• -L represents the command to list metadata.
• -b brokers indicates the list of brokers from which to retrieve the metadata.

Example output:

Topic: topic1	Partition: 0	Leader: 1	Replicas: 1
Topic: topic1	Partition: 1	Leader: 2	Replicas: 2
Topic: topic2	Partition: 0	Leader: 0	Replicas: 0

Use case 7: List metadata for a specific topic

Code:

kcat -L -t topic -b brokers

Motivation: This use case is useful when you want to retrieve metadata for a specific topic in a Kafka cluster. It allows you to view information about the topic, such as the partition details, leader brokers, and replicas.

Explanation:

• -t topic specifies the Kafka topic for which you want to retrieve metadata.

Example output:

Topic: topic1	Partition: 0	Leader: 1	Replicas: 1
Topic: topic1	Partition: 1	Leader: 2	Replicas: 2

Use case 8: Get offset for a topic/partition for a specific point in time

Code:

kcat -Q -t topic:partition:unix_timestamp -b brokers

Motivation: This use case is helpful when you want to retrieve the offset of a specific topic and partition at a given point in time. It allows you to track the exact position of the messages within a topic.

Explanation:

• -Q represents the command to query the offset.
• topic:partition:unix_timestamp identifies the topic, partition, and the Unix timestamp for which you want to get the offset.

Example output:

Offset: 1234

Conclusion:

The “kcat” command provides a versatile and efficient way to work with Apache Kafka as both a producer and a consumer. By understanding each of the provided use cases, you can effectively consume and publish messages, manage consumer groups, retrieve metadata, and get offset information for specific topics and partitions.