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https://issues.apache.org/jira/browse/HBASE-3842?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13075985#comment-13075985
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Gary Helmling commented on HBASE-3842:
--------------------------------------
I think the stacking issue is key here: are we expecting the common case to be
loading a single "CompactionObserver" that overrides the compaction
implementation, or loading multiple that each override/customize compaction
policy but not necessarily behavior?
I agree on the one hand that having a {{KeyValue}} oriented interface for
{{preCompactWrite()}} and {{postCompactWrite()}} may not be sufficient. At the
same time, I don't think we want to force the implementations to write their
own {{StoreFiles}} though, as that will be massively inefficient -- for N
loaded coprocessors this becomes N compactions being written (assuming we
bypass the core compaction code at the end of chaining).
One alternative would be to have {{preCompact}} take the scanner to be used as
a parameter, as suggested, and return a scanner instance that would allow
overriding policy and mutating KVs, while still relying on the core writer
functionality. This would allow wrapping the store scanner with a custom
scanner that inspects and emits KVs as needed on the fly. In this case,
{{preCompact}} would look like:
{{code}}
StoreScanner preCompact(ObserverContext<~> context, Store store, StoreScanner
scanner);
{{code}}
Wrapping the scanner seems much easier for chaining multiple observers. On the
other hand we lose the clean {{boolean}} return to indicate that core
compaction processing should be skipped. Are there cases that would still want
to handling the store file writing portion of the implementation entirely in
the coprocessor? If so, can we still emit a flag to skip normal processing
another way? We could skip normal processing if {{null}} is returned. Seems a
little clunky, but it could work with appropriate documentation.
> Refactor Coprocessor Compaction API
> -----------------------------------
>
> Key: HBASE-3842
> URL: https://issues.apache.org/jira/browse/HBASE-3842
> Project: HBase
> Issue Type: Improvement
> Components: coprocessors, regionserver
> Affects Versions: 0.92.0
> Reporter: Nicolas Spiegelberg
> Assignee: Nicolas Spiegelberg
> Labels: compaction
> Fix For: 0.92.0
>
>
> After HBASE-3797, the compaction logic flow has been significantly altered.
> Because of this, the current compaction coprocessor API is insufficient for
> gaining full insight into compaction requests/results. Refactor coprocessor
> API after HBASE-3797 is committed to be more extensible and increase
> visibility.
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