[ZOOKEEPER-4883] Rollover leader epoch when counter part of zxid reach limit

• Type: Improvement
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Component/s: server
• Labels: pull-request-available
• Assignee: Kezhu Wang
• Reporter: Kezhu Wang
• Created: 28/Oct/24 04:39
• Updated: 28/Oct/24 05:30

Description

Currently, zxid rollover will cause re-election(ZOOKEEPER-1277) which is time consuming.

ZOOKEEPER-2789 proposes to use 24 bits for epoch and 40 bits for counter. I do think it is promising as it promotes rollover rate from 49.7 days to 34.9 years assuming 1k/s ops.

But I think it is a one-way ticket. And the change of data format may require community wide spread to upgrade third party libraries/tools if they are ever tied to this. Inside ZooKeeper, `accepetedEpoch` and `currentEpoch` are tied to `zxid`. Given a snapshot and a txn log, we need probably deduced those two epoch values to join quorum.

So, I presents alternative solution to rollover leader epoch when counter part of zxid reach limit.

1. Treats last proposal of an epoch as rollover proposal.
2. Requests from next epoch are proposed normally.
3. Fences next epoch once rollover proposal persisted.
4. Proposals from next epoch will not be written to disk before rollover committed.
5. Leader commits rollover proposal once it get quorum ACKs.
6. Blocked new epoch proposals are logged once rollover proposal is committed in corresponding nodes.
    
   This results in:

1. No other lead cloud lead using next epoch number once rollover proposal is considered committed.
2. No proposals from next epoch will be written to disk before rollover proposal is considered committed.

Here is the branch, I will draft a pr later.

https://github.com/kezhuw/zookeeper/tree/zxid-rollover

Issue Links

• relates to: ZOOKEEPER-2789 Reassign `ZXID` for solving 32bit overflow problem
• relates to: ZOOKEEPER-1277 servers stop serving when lower 32bits of zxid roll over
• relates to: ZOOKEEPER-4571 Admin server API for restoring database from a snapshot
• relates to: ZOOKEEPER-4570 Admin server API for taking snapshot and stream out the data
• links to: GitHub Pull Request #2208