Package org.apache.kafka.clients.consumer

Class RoundRobinAssignor

java.lang.Object
org.apache.kafka.clients.consumer.internals.AbstractPartitionAssignor
org.apache.kafka.clients.consumer.RoundRobinAssignor
All Implemented Interfaces:
ConsumerPartitionAssignor
public class RoundRobinAssignor
extends org.apache.kafka.clients.consumer.internals.AbstractPartitionAssignor

The round robin assignor lays out all the available partitions and all the available consumers. It then proceeds to do a round robin assignment from partition to consumer. If the subscriptions of all consumer instances are identical, then the partitions will be uniformly distributed. (i.e., the partition ownership counts will be within a delta of exactly one across all consumers.)

For example, suppose there are two consumers C0 and C1, two topics t0 and t1, and each topic has 3 partitions, resulting in partitions t0p0, t0p1, t0p2, t1p0, t1p1, and t1p2.

The assignment will be:

  • C0: [t0p0, t0p2, t1p1]
  • C1: [t0p1, t1p0, t1p2]

When subscriptions differ across consumer instances, the assignment process still considers each consumer instance in round robin fashion but skips over an instance if it is not subscribed to the topic. Unlike the case when subscriptions are identical, this can result in imbalanced assignments. For example, we have three consumers C0, C1, C2, and three topics t0, t1, t2, with 1, 2, and 3 partitions, respectively. Therefore, the partitions are t0p0, t1p0, t1p1, t2p0, t2p1, t2p2. C0 is subscribed to t0; C1 is subscribed to t0, t1; and C2 is subscribed to t0, t1, t2.

That assignment will be:

  • C0: [t0p0]
  • C1: [t1p0]
  • C2: [t1p1, t2p0, t2p1, t2p2]
Since the introduction of static membership, we could leverage group.instance.id to make the assignment behavior more sticky. For example, we have three consumers with assigned member.id C0, C1, C2, two topics t0 and t1, and each topic has 3 partitions, resulting in partitions t0p0, t0p1, t0p2, t1p0, t1p1, and t1p2. We choose to honor the sorted order based on ephemeral member.id.

The assignment will be:

  • C0: [t0p0, t1p0]
  • C1: [t0p1, t1p1]
  • C2: [t0p2, t1p2]
After one rolling bounce, group coordinator will attempt to assign new member.id towards consumers, for example C0 -> C5 C1 -> C3, C2 -> C4.

The assignment could be completely shuffled to:

  • C3 (was C1): [t0p0, t1p0] (before was [t0p1, t1p1])
  • C4 (was C2): [t0p1, t1p1] (before was [t0p2, t1p2])
  • C5 (was C0): [t0p2, t1p2] (before was [t0p0, t1p0])
This issue could be mitigated by the introduction of static membership. Consumers will have individual instance ids I1, I2, I3. As long as 1. Number of members remain the same across generation 2. Static members' identities persist across generation 3. Subscription pattern doesn't change for any member

The assignment will always be:

  • I0: [t0p0, t1p0]
  • I1: [t0p1, t1p1]
  • I2: [t0p2, t1p2]