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https://issues.apache.org/jira/browse/HBASE-24440?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17350726#comment-17350726
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Bharath Vissapragada commented on HBASE-24440:
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> 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.
Curious how you plan to achieve this, is the plan to sleep for a clock tick
between two consecutive invocations (if current_ts == last_read_ts) or
something more fancy?
Is the plan to make this pluggable at a table/namespace scope or for the
service? Reason I ask this because the performance penalty with delayed clock
ticks may not be acceptable for throughput favoring applications like metrics
ingestion that are fine with approximate timestamps. Also per your design, it
appears the scope of uniqueness is RS so we have a total order of all mutations
for a given RS (across regions). In most cases we are ok with a partial order
(within a region) since that is where the order of mutations matters, so we can
have optimizations like per region clock instances that guarantee this partial
order and also achieves the pluggability part (thinking out loud). WDYT.
> 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
> Fix For: 3.0.0-alpha-1, 2.5.0
>
>
> 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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