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https://issues.apache.org/jira/browse/LUCENE-4339?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13444143#comment-13444143
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Robert Muir commented on LUCENE-4339:
-------------------------------------
{quote}
I would like to applay the current patch, as developed by Robert that allows
Lucene 4.0 to open 3.x indexes and do the following:
add documents to it (this was always possible)
delete/update documents from the 3.x segments only. This is a small change
because it was always working and was only willingful disabled in some cleanup
commit.
{quote}
This last statement is totally false. it was not always working. I had to
disable this so that we could move all portions of the index
under codec control (as i already said on the mailing list). The problem was at
the time, Codec provided its own segment infos writer,
so it could customize commits. This was necessary to support e.g.
AppendableCodec. Because of that it was not really possible to
allow 3.x deletes, because we had no real way to write a "3.x compatible commit
that also contains newer 4.x information/4.x segments".
The way SegmentInfosReader/Writer worked at the time presented no opportunity
to make a "frankenstein" 3.x/4.x specialized mixed commit
point to do this either.
Its just now only easy to re-enable because we did a ton of work to detangle
this in LUCENE-4050/LUCENE-4055, split the per-segment metadata
(which is owned by Codec) from the commit information (which is just a list of
segments+deletes gens). So we can easily write a 4.x commit,
and at the same time preserve the 3.x per-segment metadata.
So this was no cleanup commit!
And again thats why I am against changing the backwards policy, because if we
had enforced this earlier we would have likely never been able to
iterate to this point: it would have presented a massive roadblock.
> Allow deletions on Lucene3x codecs again
> ----------------------------------------
>
> Key: LUCENE-4339
> URL: https://issues.apache.org/jira/browse/LUCENE-4339
> Project: Lucene - Core
> Issue Type: Bug
> Components: core/index
> Affects Versions: 4.0-BETA
> Reporter: Uwe Schindler
> Priority: Blocker
> Fix For: 4.0
>
> Attachments: LUCENE-4339.patch
>
>
> On dev@lao Hoss reported that a user in Solr was not able to update or delete
> documents in his 3.x index with Solr 4:
> {quote}
> On the solr-user list, Dirk Högemann recently mentioned a problem he was
> seeing when he tried upgrading his existing solr setup from 3.x to 4.0-BETA.
> Specifically this exception getting logged...
> http://find.searchhub.org/document/cdb30099bfea30c6
> auto commit error...:java.lang.UnsupportedOperationException: this codec can
> only be used for reading
> at
> org.apache.lucene.codecs.lucene3x.Lucene3xCodec$1.writeLiveDocs(Lucene3xCodec.java:74)
> at
> org.apache.lucene.index.ReadersAndLiveDocs.writeLiveDocs(ReadersAndLiveDocs.java:278)
> at
> org.apache.lucene.index.IndexWriter$ReaderPool.release(IndexWriter.java:435)
> at
> org.apache.lucene.index.BufferedDeletesStream.applyDeletes(BufferedDeletesStream.java:278)
> at
> org.apache.lucene.index.IndexWriter.applyAllDeletes(IndexWriter.java:2928)
> at
> org.apache.lucene.index.IndexWriter.maybeApplyDeletes(IndexWriter.java:2919)
> at
> org.apache.lucene.index.IndexWriter.prepareCommit(IndexWriter.java:2666)
> at
> org.apache.lucene.index.IndexWriter.commitInternal(IndexWriter.java:2793)
> at org.apache.lucene.index.IndexWriter.commit(IndexWriter.java:2773)
> at
> org.apache.solr.update.DirectUpdateHandler2.commit(DirectUpdateHandler2.java:531)
> at org.apache.solr.update.CommitTracker.run(CommitTracker.java:214)
> Dirk was able to work arround this by completely re-indexing, but it seemed
> strange to me that this would happen.
> My understanding is that even though an IndexUpgrader tool was now available,
> it wasn't going to be required for users to use it when upgrading from 3.x to
> 4.x. Explicitly upgrading the index format might be a good idea, and might
> make hte index more performant, but as I understood it, the way things had
> been implemented with codecs explicitly upgrading the index format wasn't
> strictly neccessary, and that users should be able to upgrade their lucene
> apps same way that was supported with other index format upgrades in the
> past: the old index can be read, and as changes are made new segments will be
> re-written in the new format. (Note in
> particular: at the moment we don't mention IndexUpgrader in MIGRATE.txt at
> all.)
> It appears however, based on this stack trace and some other experiements i
> tried, that any attempts to "delete" documents in a segment that is using the
> Lucene3xCodec will fail.
> This seems like a really scary time bomb sitaution, because if you upgrade,
> things will seem to be working -- you can even add documents, and depending
> on the order that you do things, some "old" segments may get merged and use
> the new format, so *some* deletes of "old" documents (in those
> merged/upgraded) segments may work, but then somewhere down the road, you may
> try to a delete that affects docs in a still un-merge/upgraded segment, and
> that delete will fail -- 5 minutes later, if another merge has happened,
> attempting to do the exact same delete may succeed.
> All of which begs the question: is this a known/intended limitation of the
> Lucene3xCodec, or an oversight in the Lucene3xCodec?
> if it's expected, then it seems like we should definitely spell out this
> limitation in MIGRATE.txt and advocate either full rebuilds, or the use of
> IndexUpgrader for anyone who's indexes are non-static.
> On the Solr side of things, i think we should even want to consider
> automaticly running IndexUpgrader on startup if we detect that the
> Lucene3xCodec is in use to simplify things -- we can't even suggest running
> "optimize" as a quick/easy way to force and index format upgrade because if
> the 3x index as already optimized then it's a no-op and the index stays in
> the 3x format.
> {quote}
> Robert said, that this is a wanted limitation (in fact its explicitely added
> to the code, without that UOE it "simply works"), but I disagree here and
> lots of other people:
> {quote}
> In the early days (I mean in the time when it was already read only until we
> refactored the IndexReader.delete()/Codec stuff), this was working, because
> the LiveDocs were always handled in a special way. Making it now 100%
> read-only is in my opinion very bad, as it does not allow to update documents
> in a 3.x index anymore, so you have no chance, you must run IndexUpgrader.
> The usual step like opening old Index and adding documents works (because the
> new documents are added always to new segment), but the much more usual
> IW.updateDocument() which is commonly used also to add documents fails on old
> Indexes. This is a no-go, we have to fix this. If we allow the trick with
> updating LiveDocs on 3.x codec, for the end-user the "read-only" stuff in
> Lucene3x codec would be completely invisible, as he can do everything
> IndexWriter provides. The other horrible things like changing norms is no
> longer possible, so deletes are the only thing that affects here. The
> read-only ness of Lucene3x codec would only be visible to the user when
> someone tries to explicitly create an index with Lucene3x codec. And I
> understood the CHANGES/MIGRATE.txt exactly as that.
> {quote}
> On the list, Robert added a simple patch, reverting the UOE in Lucene3xCodec,
> so the LiveDocs format is RW again.
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