Offset topic and consumer group coordinator of Kafka

This article is to discuss two subjects that are not frequently or clearly covered by official document or online sources.

Offset topic (the __consumer_offsets topic)

It is the only mysterious topic in Kafka log and it cannot be deleted by using TopicCommand. Unfortunately there is no dedicated official documentation to explain this internal topic. The closest source I can find is this set of slides. I strongly recommend reading it if you wish to understand how this internal topic works.

In short, __consumer_offsets is used to store offsets of consumers which was previously stored only in ZooKeeper before version At the latest version of 0.8.X serious, i.e., the storage location of offsets can be configured by whose value can be either kafka or zookeeper. If it is kafka, consumers are still able to commit offsets to ZooKeeper by enabling dual.commit.enabled. However since version 0.9, consumer offsets have been designed to be stored on brokers only.

The partition key of the messages in __consumer_offsets was handled in OffsetManager at version 0.8.X and has been named as OffsetKey in GroupMetadataManager since version 0.9.0. It contains three pieces of information: groupId, topic and partition number, and the key is serialized/de-serialized according to a schema called OFFSET_COMMIT_KEY_SCHEMA. The usage of schema is primarily for backward compatibility. Unlike the behavior of DefaultPartitioner, the partition number inside __consumer_offsets is not determined by the hash value of partition key but only determined by the hash value of consumer group, which is as simple as

Utils.abs(groupId.hashCode) % numPartitions

Here the numPartitions is configured by the value of offsets.topic.num.partitions in broker configs and is 50 by default. This algorithm will be re-introduced later in the part of consumer group coordination.

The values of offset messages which is named OffsetMetadata have two versions. At version 0.8.X, the value contains offset, metadata (often empty) and a timestamp while the timestamp had been split into commit and expire timestamps since version 0.9.0. Console consumers are able to consume messages from internal topics and print them out nicely. A sample command has been shown below:

./ --topic __consumer_offsets \
--partition 49 \
--broker-list localhost:9092 \
--formatter "kafka.server.OffsetManager\$OffsetsMessageFormatter"

By the way, absolute("testGroup".hashCode() % 50) = 49 which is the reason why partition 49 was specified. It prints out:


However this is not the end of story, because __consumer_offsets is also used by group coordinator to store group metadata! The following section discusses another new feature introduced since version 0.9.0.

Group coordinator (coordinated rebalance)

This section is my humble and shallow understanding about broker coordinator of consumer groups. Correct me if I ever miss something or make any mistake.

The introduction of coordinator, according to the official wiki of Kafka, is to solve the split brain problem which is a well known problem in a distributed system.