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https://issues.apache.org/jira/browse/SOLR-4909?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13679951#comment-13679951
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Michael Garski commented on SOLR-4909:
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bq. It opens a new indexwriter when the index directory is completely
changed/moved - and I don't see that going away anytime soon.
That makes sense - new physical directory, new writer - I would not expect that
to change.
bq. it doesn't reopen the writer to be aware of any changes if the same index
dir is used
That's not the behavior that is occurring when index deltas from the master are
applied to the existing index directory. Here is a trace of the calls made in
that case:
SnapPuller.fetchLatestIndex(SolrCore core, forceReplication = false)
SnapPuller.openNewWriterAndSearcher(isFullCopyNeeded = false) [isFullCopyNeeded
is false as the index deltas are applied to the existing index directory]
DirectUpdateHandler2.newIndexWriter(rollback = false) [isFullCopyNeeded is
passed in as the value of the rollback parameter]
DefaultSolrCoreState.newIndexWriter(SolrCore core, rollback = false)
With the value of rollback == false the writer is now closed and a new one is
created, resulting in the loss of all segment-level caches.
It appears as if when isFullCopyNeeded == false, then the call to
DefaultSolrCoreState.newIndexWriter should not be made, however if that is
changed to not open a new writer a handful of the TestReplicationHandler tests
then fail.
> Solr and IndexReader Re-opening on Replication Slave
> ----------------------------------------------------
>
> Key: SOLR-4909
> URL: https://issues.apache.org/jira/browse/SOLR-4909
> Project: Solr
> Issue Type: Improvement
> Components: replication (java), search
> Affects Versions: 4.3
> Reporter: Michael Garski
> Fix For: 5.0, 4.4
>
> Attachments: SOLR-4909-demo.patch
>
>
> I've been experimenting with caching filter data per segment in Solr using a
> CachingWrapperFilter & FilteredQuery within a custom query parser (as
> suggested by [~yo...@apache.org] in SOLR-3763) and encountered situations
> where the value of getCoreCacheKey() on the AtomicReader for each segment can
> change for a given segment on disk when the searcher is reopened. As
> CachingWrapperFilter uses the value of the segment's getCoreCacheKey() as the
> key in the cache, there are situations where the data cached on that segment
> is not reused when the segment on disk is still part of the index. This
> affects the Lucene field cache and field value caches as well as they are
> cached per segment.
> When Solr first starts it opens the searcher's underlying DirectoryReader in
> StandardIndexReaderFactory.newReader by calling
> DirectoryReader.open(indexDir, termInfosIndexDivisor), and the reader is
> subsequently reopened in SolrCore.openNewSearcher by calling
> DirectoryReader.openIfChanged(currentReader, writer.get(), true). The act of
> reopening the reader with the writer when it was first opened without a
> writer results in the value of getCoreCacheKey() changing on each of the
> segments even though some of the segments have not changed. Depending on the
> role of the Solr server, this has different effects:
> * On a SolrCloud node or free-standing index and search server the segment
> cache is invalidated during the first DirectoryReader reopen - subsequent
> reopens use the same IndexWriter instance and as such the value of
> getCoreCacheKey() on each segment does not change so the cache is retained.
> * For a master-slave replication set up the segment cache invalidation occurs
> on the slave during every replication as the index is reopened using a new
> IndexWriter instance which results in the value of getCoreCacheKey() changing
> on each segment when the DirectoryReader is reopened using a different
> IndexWriter instance.
> I can think of a few approaches to alter the re-opening behavior to allow
> reuse of segment level caches in both cases, and I'd like to get some input
> on other ideas before digging in:
> * To change the cloud node/standalone first commit issue it might be possible
> to create the UpdateHandler and IndexWriter before the DirectoryReader, and
> use the writer to open the reader. There is a comment in the SolrCore
> constructor by [~yo...@apache.org] that the searcher should be opened before
> the update handler so that may not be an acceptable approach.
> * To change the behavior of a slave in a replication set up, one solution
> would be to not open a writer from the SnapPuller when the new index is
> retrieved if the core is enabled as a slave only. The writer is needed on a
> server configured as a master & slave that is functioning as a replication
> repeater so downstream slaves can see the changes in the index and retrieve
> them.
> I'll attach a unit test that demonstrates the behavior of reopening the
> DirectoryReader and it's effects on the value of getCoreCacheKey. My
> assumption is that the behavior of Lucene during the various reader reopen
> operations is correct and that the changes are necessary on the Solr side of
> things.
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