[KAFKA-12158] Consider better return type of RaftClient.scheduleAppend

• Type: Sub-task
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Fix Version/s: 3.0.0
• Assignee: Deng Ziming
• Reporter: Jason Gustafson
• Created: 07/Jan/21 22:53
• Updated: 02/Aug/21 22:06
• Resolved: 02/Aug/21 22:06

Description

Currently `RaftClient` has the following Append API:

    Long scheduleAppend(int epoch, List<T> records);

There are a few possible cases that the single return value is trying to handle:

1. The epoch doesn't match or we are not the current leader => return Long.MaxValue
2. We failed to allocate memory to write the the batch (backpressure case) => return null
3. We successfully scheduled the append => return the expected offset

It might be better to define a richer type so that the cases that must be handled are clearer. At a minimum, it would be better to return `OptionalLong` and get rid of the null case.

Issue Links

• is related to: KAFKA-13148 Kraft Controller doesn't handle scheduleAppend returning Long.MAX_VALUE
• links to: GitHub Pull Request #10909