RATIS-624. RaftServer should support pause/ unpause in its LifeCycle state

[amaliujia]

What changes were proposed in this pull request?

An effort to suppor pause in Raft, which can be used to pause a server (most likely follower).

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/RATIS-624

How was this patch tested?

N/A

[amaliujia]

@szetszwo @hanishakoneru

I am working on adding pause to Ratis. As RATIS-624 describes, it is useful when a follower is lagging behind. When that happens, we can pause that follower and then do a snapshot install to help that follower catches up.

This PR is still WIP. As I kept hacking a solution, I realized it is a fairly complicated change (at least to me).

Can I ask for some help: am I working on the right track? Am I adding the API in the right classes (e.g. RaftClient, RaftServerProxy, etc.) Any suggestions that I can improve my current thought?

[amaliujia]

Or, can I tackle this problem by starting from server implementation without considering client/rpc call? (not sure as this is the first time I touch Ratis core).

[szetszwo]

I have briefly checked the code change. Let's discuss the design in the JIRA.

[amaliujia]

Per the discussion in JIRA, this work will break into several JIRA so will close this draft now.