Partition Assignor

Kafka Streams

Kafka Streams distributes work across multiple processes by using the consumer group protocol introduced in Kafka 0.9.0. Kafka elects one of the consumers in the consumer group to use its partition assignment strategy to assign partitions to the consumers in the group. The leader gets access to every client’s subscriptions and assigns partitions accordingly.

Kafka Streams uses a sticky partition assignment strategy to minimize movement in the case of rebalancing. Further, it is also redundant in its partition assignment in the sense that it assigns some standby tasks to maintain state store replicas.

The StreamPartitionAssignor used by Kafka Streams works as follows:

  1. Check all repartition source topics and use internal topic manager to make sure they have been created with the right number of partitions.

  2. Using customized partition grouper (DefaultPartitionGrouper) to generate tasks along with their assigned partitions; also make sure that the task’s corresponding changelog topics have been created with the right number of partitions.

  3. Using StickyTaskAssignor to assign tasks to consumer clients.

    • Assign a task to a client which was running it previously. If there is no such client, assign a task to a client which has its valid local state.
    • A client may have more than one stream threads. The assignor tries to assign tasks to a client proportionally to the number of threads.
    • Try not to assign the same set of tasks to two different clients

    The assignment is done in one-pass. The result may not satisfy above all.

  4. Within each client, tasks are assigned to consumer clients in round-robin manner.


Faust differs from Kafka Streams in some fundamental ways one of which is that a task in Faust differs from a task in Kafka Streams. Further, Faust doesn’t have the concept of a pre-defined topology and subscribes to streams as and when required in the application.

As a result, the PartitionAssignor in Faust can get rid of steps one and two mentioned above and rely on the primitives repartitioning streams and creating changelog topics to create topics with the correct number of partitions based on the source topics.

We can largely simplify step three above since there is no concept of task as in Kafka Streams, i.e. we do not introspect the application topology to define a task that would be assigned to the clients. We simply need to make sure that the correct partitions are assigned to the clients and the client streams and processors should handle dealing with the co-partitioning while processing the streams and forwarding data between the different processors.


This can be simplified immensely by grouping the same partition numbers onto the same clients for all topics with the same number of partitions. This way we can guarantee that co-partitioning for all topics requiring co-partitioning (ex: in the case of joins and aggregates) as long as the topics have the correct number of partitions (which we are making the processors implicitly guarantee).


With our simple PartitionGrouper we can use a StickyPartitionAssignor to assign partitions to the clients. However we need to explicitly handle standby assignments here. We use the StickyPartitionAssignor design approved in KIP-54 as the basis for our sticky assignor.


With the above design we need to be careful around the following concerns:

  • We need to assign a partition (where changelog) is involved to a client which contains a standby replica for the given topic/partition whenever possible. This can result in unbalanced assignment. We can fix this by evenly and randomly distributing standbys such that over the long term each rebalance will cause the partitions being re-assigned be evenly balanced across all clients.
  • Network Partitions and other distributed systems failure cases - We delegate this to the Kafka protocol. The Kafka Consumer Protocol handles a lot of the failure conditions involved with the Consumer group leader election such as leader failures, node failures, etc. Network Partitions in Kafka are not handled here as those would result in bigger issues than consumer partition assignment issues.