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https://issues.apache.org/jira/browse/LUCENE-7671?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15852346#comment-15852346
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Keith Laban commented on LUCENE-7671:
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Yes, in a nutshell that is my goal. Ultimately I'd like to build a request
handler in solr that can enable a core to upgrade it's segments without first
taking it offline or reconfiguring the index writer. When not engaged it should
have no effect, and when it is engaged it should do minimal work at a time.
The additional bells and whistles of this PR are for backwards compatibility.
Previous the behavior was:
1) Determine all old segments
2) Ask wrapping merge policy what to do with the old segments
3) Merge segments specified by wrapped merge policy
4) Merge the remaining old segments into a single new segment
Meaning, if you were to upgrade an old index using TierdMergePolicy or similar
as the wrapped MP and said {{w.forceMerge(Integer.MAX_INT)}}, the TMP would say
nothing to do, but the UpgradeIndexMergePolicy would then take it upon itself
to merge everything down into a single segment.
Ideally if {{max number of segments > number of segments}} and the wrapped MP
is happy, the UIMP should not take it upon itself make any merge decisions and
only upgrade segments needing upgrade.
An additional decision to rely on cascading calls from the IW was chosen so
that if this was being used as the default MP and an upgrade was in progress,
old segments could still be candidates for merges issued during a commit.
The idea is loosely based on elasticsearch's ElasticsearchMergePolicy.
There should probably also be support for a Predicate to be passed for
determining whether the segment should be upgraded (rewritten), then this MP
can be used for things such as deciding to rewrite segments with a different
codec.
> Enhance UpgradeIndexMergePolicy with additional options
> -------------------------------------------------------
>
> Key: LUCENE-7671
> URL: https://issues.apache.org/jira/browse/LUCENE-7671
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Keith Laban
>
> Enhance UpgradeIndexMergePolicy to be a MergePolicy that can be used outside
> the scope the IndexUpgrader.
> The enhancement aims to allow the UpgradeIndexMergePolicy to:
> 1) Delegate normal force merges to the underlying merge policy
> 2) Enable a flag that will explicitly tell UpgradeIndexMergePolicy when it
> should start looking for upgrades.
> 3) Allow new segments to be considered to be merged with old segments,
> depending on underlying MergePolicy.
> 4) Be configurable for backwards compatibility such that only segments
> needing an upgrade would be considered when merging, no explicit upgrades.
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