Details
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Bug
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Status: Resolved
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Major
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Resolution: Abandoned
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Description
Running HBase for only a few weeks we found dozen(s?) of bugs with assignment that all seem to have the same nature - split brain between 2 procedures; or between procedure and master startup (meta replica bugs); or procedure and master shutdown (HBASE-21742); or procedure and something else (when SCP had incorrect region list persisted, don't recall the bug#).
To me, it starts to look like a pattern where, like in AMv1 where concurrent interactions were unclear and hard to reason about, despite the cleaner individual pieces in AMv2 the problem of unclear concurrent interactions has been preserved and in fact increased because of the operation state persistence and isolation.
Procedures are great for multi-step operations that need rollback and stuff like that, e.g. creating a table or snapshot, or even region splitting. However I'm not so sure about assignment.
We have the persisted information - region state in meta (incl transition states like opening, or closing), server list as WAL directory list. Procedure state is not any more reliable then those (we can argue that meta update can fail, but so can procv2 WAL flush, so we have to handle cases of out of date information regardless). So, we don't need any extra state to decide on assignment, whether for recovery and balancing. In fact, as mentioned in some bugs, deleting procv2 WAL is often the best way to recover the cluster, because master can already figure out what to do without additional state.
I think there should be an option for stateless assignment that does that.
It can either be as a separate pluggable assignment procedure; or an option that will not recover SCP, RITs etc from WAL but always derive recovery procedures from the existing cluster state.