Kafka consumer offset is a critical concept in Apache Kafka, a popular distributed streaming platform. In simple terms, an offset is a unique identifier that denotes a position in a Kafka topic partition. When a consumer group consumes messages from a topic partition, it stores the offset of the last message it has read from that partition. This is known as the consumer group's current offset.
The current offset is important because it allows the consumer group to track its progress in the topic partition. It also enables the consumer group to resume reading from the partition at the last recorded offset in case of failure or rebalancing.
Imagine a Kafka topic partition as a book. The offset is like a bookmark that keeps track of the last page you have read in the book. When you resume reading the book, you can pick it up from the page marked by the bookmark. This is similar to how a consumer group uses the offset to resume consuming messages from a topic partition.
However, it is important to note that the offset is only stored by the consumer group, not by Kafka itself. This means that if the consumer group fails or is deleted, the offset information is also lost. This can result in the consumer group starting from the beginning of the topic partition when it is restarted, potentially leading to duplicate messages being processed.
To prevent this, Kafka provides a feature called offset commit. This allows the consumer group to periodically commit its current offset to persistent storage, such as Apache Zookeeper or Kafka's built-in offset storage. This ensures that the consumer group can resume from the last committed offset in case of failure or rebalancing.
In summary, Kafka consumer offset is a unique identifier that denotes the position of a consumer group in a Kafka topic partition. It allows the consumer group to track its progress and resume consuming messages from the partition in case of failure or rebalancing. By committing the offset to persistent storage, the consumer group can ensure that its progress is not lost.