Transactions can be dropped because leader election uses last committed zxid
instead of last acknowledged/received zxid
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Key: ZOOKEEPER-1131
URL: https://issues.apache.org/jira/browse/ZOOKEEPER-1131
Project: ZooKeeper
Issue Type: Bug
Components: leaderElection, server
Affects Versions: 3.4.0
Reporter: Alexander Shraer
Suppose we have 3 servers - A, B, C.
- A is the leader and it sends 2 proposals.
- Everyone ack the first proposal, only A and C receive and ack the second
proposal.
- A commits the first proposal, and A and B see the commit.
- A commits the second proposal, but fails before anyone else sees the commit.
- B and C start leader election.
- Since servers propose their last committed zxid in leader election, and not
their last received / acked zxid (this is not being tracked, AFAIK), B will be
elected as leader since it has seen more commits than C.
- The last transaction is lost forever even though it has been acked by a
majority.
Notice that if C reboots before starting the leader election, this problem
doesn't happen since it "locally commits" the second proposal updating its last
committed zxid.
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