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Kafka Consumers

Understanding Kafka Consumers

This note includes some quick summary of different practices we discovered and studied over time. It may be useful for beginner or seasoned developers who want a refresh after some time far away from Kafka…

Consumer group

Consumers belong to consumer groups. You specify the group name as part of the consumer connection parameters using the group.id configuration:

properties.put(ConsumerConfig.GROUP_ID_CONFIG, groupid);

Consumer groups are grouping consumers to cooperate to consume messages from one or more topics. Consumers can run in separate hosts and separate processes. The figure below represents 2 consumer apps belonging to one consumer group. Consumer 1 is getting data from 2 partitions, while consumer 2 is getting from one partition.

consumer group

When a consumer is unique in a group, it will get data from all partitions. There is always at least one consumer per partition.

One broker is responsible to be the consumer group coordinator which is responsible for assigning partitions to the consumers in the group. The first consumer to join the group will be the group leader. It will get the list of consumers and it is responsible for assigning a subset of partitions to each consumer

Membership in a consumer group is maintained dynamically. Consumers send hearbeats to the group coordinator broker (see configuration like heartbeat.interval.ms) and session.timeout.ms. Partition assignement is done by different strategies from range, round robin, sticky and cooperative sticky (See partition assignement strategy).

When a consumer fails, the partitions assigned to it will be reassigned to another consumer in the same group. When a new consumer joins the group, partitions will be moved from existing consumers to the new one. Group rebalancing is also used when new partitions are added to one of the subscribed topics. The group will automatically detect the new partitions through periodic metadata refreshes and assign them to members of the group. During a rebalance, depending of the strategy, consumers may not consume messages (Need Kafka 2.4+ to get cooperative balancing feature).

consumer groups

Implementing a Topic consumer is using the kafka KafkaConsumer class which the API documentation is a must read.

It is interesting to note that:

  • To support the same semantic of a queue processing like other integration messaging systems, you need to have all the consumers assigned to a single consumer group, so that each record delivery would be balanced over the group like with a queue.
  • To support pub/sub like other messaging systems, each consumer would have its own consumer group, and subscribes to all the records published to the topic.
  • With client.rack setting a consumer can consume from a local replica, which will have better latency when using a stretched cluster.

The implementation is simple for a single thread consumer, and the code structure looks like:

  • prepare the consumer properties
  • create an instance of KafkaConsumer to subscribe to at least one topic
  • loop on polling events: the consumer ensures its liveness with the broker via the poll API. It will get n records per poll.
  • process the ConsumerRecords and commit the offset by code or use the autocommit attribute of the consumer

As long as the consumer continues to call poll(), it will stay in the group and continue to receive messages from the partitions it was assigned. When the consumer does not send heartbeats for a duration of session.timeout.ms, then it is considered dead and its partitions will be reassigned.

Examples of Java consumers can be found in the order management microservice project under the order-command-ms folder.

We are proposing a deep dive study on this manual offset commit in this consumer code that persists events to cassandra. Example of Javascript implementation is in this repository/folder

But the complexity comes from the offset management and multithreading needs. So the following important considerations need to be addressed while implementing a consumer.

Assess number of consumers needed

The KafkaConsumer is not thread safe so it is recommended to run in a unique thread. But if needed you can implement a multi-threads solution, but as each thread will open a TCP connection to the Kafka broker, be sure to close the connection to avoid memory leak. The alternate is to start n processes (JVM process).

If you need multiple consumers running in parallel to scale horizontally, you have to define multiple partitions while configuring the topic and use fine-grained control over offset persistence. You’ll use one consumer per partition of a topic. This consumer-per-partition pattern maximizes throughput. When consumers run in parallel and you use multiple threads per consumer you need to be sure the total number of threads across all instances do not exceed the total number of partitions in the topic.

Also, a consumer can subscribe to multiple topics. The brokers are doing rebalancing of the assignment of topic-partition to a consumer that belong to a group. When creating a new consumer you can specify the group id in the options.

Offset management

Recall that offset is just a numeric identifier of a consumer position of the last record read within a partition. Consumers periodically need to commit the offsets they have received, to present a recovery point in case of failure. To commit offset (via API or automatically) the consumer sends a message to kafka broker to the special topic named __consumer_offsets to keep the committed offset for each partition.

Consumers do a read commit for the last processed record:

of-1

When a consumer starts and is assigned a partition to consume, it will start at its group’s committed offset or latest or ealiest as auto.offset.reset (When there is a committed offset, the auto.offset.reset property is not used).

As shown in the figure below, it is possible to get duplicates if the last message processed by the consumer before crashing and committing its offset, is bigger than the last commited offset.

kafka commit offset Source: Kafka definitive guide book from Todd Palino, Gwen Shapira

In the opposite, if the last committed offset is after the last processed messages and there were multiple messages returned in the poll, then those messages may be lost.

kafka commit offset 2

It is possible to commit by calling API or by setting some properties at the consumer creation level to enable autocommit offset enable.auto.commit.

When doing manual offset commit, there are two types of approaches:

  • offsets—synchronous
  • asynchronous

As soon as you are coding manual commit, it is strongly recommended to implement the ConsumerRebalanceListener interface to be able to do the state modifications when the topic is rebalanced.

If a consumer fails after processing a message but before committing its offset, the committed offset information will not reflect the processing of the message.

of-2

This means that the message will be processed again by the next consumer in that group to be assigned the partition.

Assess if it is acceptable to loose messages from topic. If so, when a consumer restarts it will start consuming the topic from the latest committed offset within the partition allocated to itself.

As storing a message to an external system and storing the offsets are two separate operations, and in case of failure between them, it is possible to have stale offsets, which will introduce duplicate messages when consumers restart to process from last known committed offset. In this case, consumer’s idempotence is needed to support updating the same row in the table, or use the event timestamp as update timestamp in the database record or use other clever solution.

As presented in the producer coding practice, using transaction to support “exactly-once”, also means the consumers should read committed data only. This can be achieved by setting the isolation.level=read_committed in the consumer’s configuration. The last offset will be the first message in the partition belonging to an open not yet committed transaction. This offset is known as the ‘Last Stable Offset’(LSO).

Finally in the case where consumers are set to auto commit, it means the offset if committed at the poll() level and if the service crashed while processing of this record as:

of-3

then the record (partition 0 - offset 4) will never be processed.

Producer transaction

When consuming from a Kafka topic and producing to another topic, like in Kafka Stream, but also in CQRS implementation, we can use the producer’s transaction feature to send the committed offset message and the new records in the second topic in the same transaction. This can be seen as a consume-transform-produce loop pattern so that every input event is processed exactly once.

An example of such pattern in done in the order management microservice - command part.

Consumer lag

The consumer lag for a partition is the difference between the offset of the most recently published message and the consumer’s committed offset.

If the lag starts to grow, it means the consumer is not able to keep up with the producer’s pace.

The risk, is that slow consumer may fall behind, and when partition management may remove old log segments, leading the consumer to jump forward to continnue on the next log segment. Consumer may have lost messages.

offsets 4

You can use the kafka-consumer-groups tool to see the consumer lag.

Kafka useful Consumer APIs

  • KafkaConsumer a topic consumer which support:
    • transparently handles brokers failure
    • transparently adapt to partition migration within the cluster
    • support grouping for load balancing among consumers
    • maintains TCP connections to the necessary brokers to fetch data
    • subscribe to multiple topics and being part of consumer groups
    • each partition is assigned to exactly one consumer in the group
    • if a process fails, the partitions assigned to it will be reassigned to other consumers in the same group
  • ConsumerRecords holds the list ConsumerRecord per partition for a particular topic.
  • ConsumerRecord A key/value pair to be received from Kafka. This also consists of a topic name and a partition number from which the record is being received, an offset that points to the record in a Kafka partition, and a timestamp

Repositories with consumer code

References