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https://issues.apache.org/jira/browse/HBASE-24440?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17350711#comment-17350711
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Andrew Kyle Purtell commented on HBASE-24440:
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I'm considering something simple:
* Ensure our discipline in using {{EnvironmentEdge#currentTime}} instead of
{{System#currentTimeMillis}} (HBASE-25912)
* Fix current issues (HBASE-25911)
* Introduce new {{EnvironmentEdge#currentTimeAdvancing}} which ensures that
when the current time is returned, it is the current time in a different clock
tick from the last time the {{EnvironmentEdge}} was used to get the current
time.
* Use {{EnvironmentEdge#currentTimeAdvancing}} wherever we go to substitute a
{{Long.MAX_VALUE}} timestamp placeholder with a real placeholder. When
processing a batch mutation we will call {{currentTimeAdvancing}} only once.
This means the client cannot bundle cells with wildcard timestamps into a batch
where those cells must be committed with different timestamps. Clients must
simply not submit mutations that must be committed with guaranteed distinct
timestamps in the same batch. Easy to understand, easy to document, and it
aligns with our design philosophy of the client knows best. It will be fine to
continue to use {{EnvironmentEdge#currentTime}} everywhere else. In this way we
will only potentially spin wait where it matters, and won't suffer serious
overheads during batch processing.
> Prevent temporal misordering on timescales smaller than one clock tick
> ----------------------------------------------------------------------
>
> Key: HBASE-24440
> URL: https://issues.apache.org/jira/browse/HBASE-24440
> Project: HBase
> Issue Type: Brainstorming
> Reporter: Andrew Kyle Purtell
> Assignee: Andrew Kyle Purtell
> Priority: Major
>
> When mutations are sent to the servers without a timestamp explicitly
> assigned by the client the server will substitute the current wall clock
> time. There are edge cases where it is at least theoretically possible for
> more than one mutation to be committed to a given row within the same clock
> tick. When this happens we have to track and preserve the ordering of these
> mutations in some other way besides the timestamp component of the key. Let
> me bypass most discussion here by noting that whether we do this or not, we
> do not pass such ordering information in the cross cluster replication
> protocol. We also have interesting edge cases regarding key type precedence
> when mutations arrive "simultaneously": we sort deletes ahead of puts. This,
> especially in the presence of replication, can lead to visible anomalies for
> clients able to interact with both source and sink.
> There is a simple solution that removes the possibility that these edge cases
> can occur:
> We can detect, when we are about to commit a mutation to a row, if we have
> already committed a mutation to this same row in the current clock tick.
> Occurrences of this condition will be rare. We are already tracking current
> time. We have to know this in order to assign the timestamp. Where this
> becomes interesting is how we might track the last commit time per row.
> Making the detection of this case efficient for the normal code path is the
> bulk of the challenge. One option is to keep track of the last locked time
> for row locks. (Todo: How would we track and garbage collect this efficiently
> and correctly. Not the ideal option.) We might also do this tracking somehow
> via the memstore. (At least in this case the lifetime and distribution of in
> memory row state, including the proposed timestamps, would align.) Assuming
> we can efficiently know if we are about to commit twice to the same row
> within a single clock tick, we would simply sleep/yield the current thread
> until the clock ticks over, and then proceed.
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