[ZOOKEEPER-3707] Leadership Election gets stuck in 5 node ensemble

• Type: Bug
• Status: Open
• Priority: Major
• Resolution: Unresolved
• Affects Version/s: 3.5.5
• Component/s: leaderElection
• Assignee: Unassigned
• Reporter: Suhas Dantkale
• Created: 28/Jan/20 01:48
• Updated: 30/Jan/20 19:05

Description

Scenario:
1. 5 node ensemble-(SID 1,2,3,4,5). 5 is the current Leader.
2. Test brings down 5's ZK process.
3. Leadership election begins. First each SID votes itself to be the leader as expected.
4. SID 1 and SID 2 gets notification from SID 3 before they get Notification from SID 4. They update their vote to propose 3 as the Leader as expected and send notifications.
5. SID 3 receives the notification from 1, 2 and itself and its Election predicate is successfully terminated and it goes to LEADING state, comes out of FLE and goes to the next phase.
6. SID 2 meantime goes to FOLLOWING state , comes out of FLE and goes to the next phase(NEWLEADER sending etc).

so far so good.
7. Meantime (somewhere after step 4) SID 1 receives notification from SID 4 and since SID 4 > SID 3(and zxid is same), SID 1 changes its mind and updates its proposal - now to elect 4 as leader and sends notification.
8. SID 4 is trying to elect itself as leader. And even though SID 2 and SID 3 are out of election, the SID 4 can not get out of election because - not enough number of nodes are following 3(Only 1 is following 3).
9. SID 2 is also stuck in FLE like SID 4.

So, in summary SID 1 and 4 are stuck in FLE (in lookForLeader()) and SID 2 and SID 3 are stuck in the next phase because SID 3's NEWLEADER is not responded by the quorum.