tison created FLINK-14149:
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Summary: Introduce ZooKeeperLeaderElectionServiceNG
Key: FLINK-14149
URL: https://issues.apache.org/jira/browse/FLINK-14149
Project: Flink
Issue Type: Sub-task
Components: Runtime / Coordination
Reporter: tison
Assignee: tison
Subsequent to the discussion in FLINK-10333, we reach a consensus that refactor
ZK based storage with a transaction store mechanism. The overall design can be
found in the design document linked below.
This subtask is aimed at introducing the prerequisite to adopt transaction
store, i.e., a new leader election service for ZK scenario. The necessity is
that we have to retrieve the corresponding latch path per contender following
the algorithm describe in FLINK-10333.
Here is the (descriptive) details about the implementation.
We adopt the optimized version of [this
recipe|https://zookeeper.apache.org/doc/current/recipes.html#sc_leaderElection][1].
Code details can be found in [this
branch|https://github.com/TisonKun/flink/tree/election-service] and the state
machine can be found in the design document attached. Here is only the most
important two differences from the former implementation:
(1) *Leader election is an one-shot service.*
Specifically, we only create one latch for a specific contender. We tolerate
{{SUSPENDED}} a.k.a. {{CONNECTIONLOSS}} so that the only situation we lost
leadership is session expired, which infers the ephemeral latch znode is
deleted. We don't re-participant as contender so after {{revokeLeadership}} a
contender will never be granted any more. This is not a problem but we can do
further refactor in contender side for better behavior.
(2) *Leader info znode is {{PERSISTENT}}.*
It is because we now regard create/setData to leader info znode a leader-only
operation and thus do it in a transaction. If we keep using ephemeral znode it
is hard to test. Because we share ZK client so the ephemeral znode is not
deleted so that we should deal with complex znode stat that transaction cannot
simply deal with. And since znode is {{PERSISTENT}} we introduce a
{{concealLeaderInfo}} method called back on contender stop to clean up.
Another topic is about interface. Back to the big picture of FLINK-10333 we
eventually use a transaction store for persisting job graph and checkpoint and
so on. So there will be a {{getLeaderStore}} method added on
{{LeaderElectionServices}}. Because we don't use it at all it is an open
question that whether we add the method to the interface in this subtask. And
if so, whether we implement it for other election services implementation.
{{concealLeaderInfo}} is another method appeared in the document that aimed at
clean up leader info node on stop. So the same problem as {{getLeaderStore}}.
**For what we gain**
1. Basics for the overall goal under FLINK-10333
2. Leader info node must be modified by the current leader. Thus we can reduce
a lot of concurrency handling logic in currently ZLES, including using
{{NodeCache}} as well as dealing with complex stat of ephemeral leader info
node.
[1] For other implementation, I start [a
thread|https://lists.apache.org/x/thread.html/594b66ecb1d60b560a5c4c08ed1b2a67bc29143cb4e8d368da8c39b2@%3Cuser.zookeeper.apache.org%3E]
in ZK and Curator to discuss. Anyway, it will be implementation details only,
and interfaces and semantics should not be affected.
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