Status

Current state: "Under Discussion"

Discussion thread: here

JIRA: here

Motivation

In KIP-429: Kafka Consumer Incremental Rebalance Protocol, we proposed a new version of consumer protocol for cooperative rebalance (a.k.a incremental rebalance protocol). It adds a new field "ownedPartition" into the "subscription" and "assignment" data, and one of the purpose of the new "ownedPartition" field is for the assignors and to do "sticky" assignment dependent on the "ownedPartitions". Like in the "CooperativeStickyAssignor and custom COOPERATIVE Assignors" section of KIP-429, it said:

The assignor can leverage the new ownedPartitions field that the Subscription has been augmented with in order to determine the previous assignment. Note that "stickiness" is important for the cooperative protocol to be effective, as in the limit that the new assignment is totally different than the previous one then the cooperative protocol just reduces to the old eager protocol as each member will have to completely revoke all partitions and get a whole new assignment.

However, recently, we've encountered some rebalance stuck issues, and the root cause of them are due to the out-of-date "ownedPartition". Because there are chances that the "ownedPartitions" are out-of-date, the assignors will blindly "trust" the "ownedPartitions", and do assignment depend on them, and cause unexpected results. ex: KAFKA-12984, KAFKA-13406. Currently, we tried to workaround this issue by adding "generation" field into subscription "userData" field in cooperative sticky assignor, and deserialize them when doing assignment, to identify if the "ownedPartitions" are out-of-date or not. However, this workaround only works for cooperative sticky assignor, if users have their own custom cooperative assignor, they also need to workaround it manually. Otherwise, the same issues also happen to them.

Therefore, we should add the "generation" field into "Subscription" data of the consumer protocol. (i.e. ConsumerProtocolSubscription)

Public Interfaces

Consumer protocol for Subscription:

Proposed Changes

Describe the new thing you want to do in appropriate detail. This may be fairly extensive and have large subsections of its own. Or it may be a few sentences. Use judgement based on the scope of the change.

Compatibility, Deprecation, and Migration Plan

• What impact (if any) will there be on existing users?
• If we are changing behavior how will we phase out the older behavior?
• If we need special migration tools, describe them here.
• When will we remove the existing behavior?

Rejected Alternatives

If there are alternative ways of accomplishing the same thing, what were they? The purpose of this section is to motivate why the design is the way it is and not some other way.