[jira] [Commented] (HBASE-24440) Prevent temporal misordering on timescales smaller than one clock tick

["Anoop Sam John (Jira)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/HBASE-24440?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17120737#comment-17120737 ] 

Anoop Sam John commented on HBASE-24440:
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When 2 such mutate reqs comes and applied at same TS, need to keep both of these as 2 versions. That is the need right?  Right now it will be treated as duplicate entry and the one with higher seqNo will come out of the scan.

> 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
>            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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