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From Adrien Grand <jpou...@gmail.com>
Subject Re: SOLR-12259: online index schema modification - adding docValues to existing indexed data?
Date Tue, 18 Dec 2018 19:36:35 GMT
UninvertingReader#wrap seems to skip uninverting if the field to
uninvert already has doc values of the expected type?

On Tue, Dec 18, 2018 at 8:24 PM Andrzej Białecki
<andrzej.bialecki@lucidworks.com> wrote:
>
> The unexpected part is that I would have expected the code to handle franken-segments
as well, because at some point we finally resorted to always forcing the wrapping even for
segments that don’t need it (ie. they claim the field contains DVs) but the test is still
failing.
>
> On 18 Dec 2018, at 19:05, Adrien Grand <jpountz@gmail.com> wrote:
>
> I had a quick look and couldn't find anything to prevent what you
> called “franken-segments” in the Lucene test?
>
> On Tue, Dec 18, 2018 at 5:59 PM Erick Erickson <erickerickson@gmail.com> wrote:
>
>
> A couple of additions:
>
> AddDVMPLuceneTest2 does not use Solr constructs at all, so is the test
> we think is most interesting at this point, it won't lead anyone down
> the path of "what's all this Solr stuff and is it right" kinds of
> questions (believe me, we've spent some time on that path!). Please
> feel free to look at all the rest of it of course, but the place we're
> stuck is why this test fails.
>
> AddDvStress is intended as an integration-level test, it requires some
> special setup (in particular providing a particular configset), we put
> that together to reliably make the problem visible. We thought the new
> code was the issue at first and needed something to narrow down the
> possibilities...
>
> The reason we're obsessing about this is that it calls into question
> how segments are merged when "things change". We don't understand why
> this is happening at the Lucene level so don't know how to insure that
> things like the schema API in Solr aren't affected.
>
> Andrzej isn't the only one running out of ideas ;).
>
> On Tue, Dec 18, 2018 at 4:46 AM Andrzej Białecki <ab@getopt.org> wrote:
>
>
> Hi,
>
> I'm working on a use case where an existing Solr setup needs to migrate to a schema that
uses docValues for faceting, instead of uninversion. This case fits into a broader subject
of SOLR-12259 (Robustly upgrade indexes). However, in this case there are two major requirements
for this migration process:
>
> * data cannot be reindexed from scratch - I need to work with the already indexed documents
(which do contain the values needed for faceting, but these values are simply indexed and
not stored as doc values)
>
> * indexing can’t be stopped while the schema is being changed (the conversion process
needs to work on-the-fly while the collection is online, both for searching and for updates).
Collection reloads / reopening is ok but it’s not ok to take the collection offline for
several minutes (or hours).
>
> Together with Erick Erickson we implemented a solution that uses MergePolicy (actually
MergePolicyFactory in Solr) to enforce re-writing of segments that no longer match the schema,
ie. don’t contain docValues in a field where the new schema requires it. This merge policy
determines what segments need this conversion and then forces the “merging” (actually
re-writing) of these segments by first wrapping them into UninvertingReader to supply docValues
where they are required by the new schema but actually are missing in the segment’s data.
This “AddDocValuesMergePolicy” (ADVMP for short) is supposed to deal with the following
types of segments:
>
> * old segments created before the schema change - these don’t contain any docValues
in the target fields and so they are wrapped in UninvertingReader for merging (and for searching)
according to the new schema.
>
> * new segments created after the schema change - if FieldInfo-s for these fields claim
that the segment already contains docValues where it should then the segment is passed as-is
to merging, otherwise it’s wrapped again. An optimisation was also put here to “mark”
the already converted segments using a marker in SegmentInfo diagnostics map so that we can
avoid re-checking and re-converting already converted data.
>
> So, long story short, this process works very well when there’s no concurrent indexing
activity - all old segments are properly wrapped and re-written and merging with new segments
works as intended. However, in a situation with concurrent indexing it works well but only
for a short while. At some point this conversion process seems to lose large percentage of
the docValues, even though it seems that at all points the source segments are properly wrapped
- the ADVMP merge policy adds a lot of debugging information to track the source and type
of segments across many levels of merging and whether they were wrapped or not.
>
> My working theory is that somehow this schema change produces “franken-segments”
(while they still haven’t been flushed) where only some of the most recent docs have the
docValues and earlier ones don’t. As I understand it, this should not happen in Solr because
a schema change results in a core reload. The tracking information from ADVMP  seems to indicate
that all generations of segments, both those that were flushed and merged earlier, have been
properly wrapped.
>
> My alternate theory is that there’s some bug in the doc values merging process when
UninvertingReader is involved, because this problem occurs also when we modify ADVMP to always
force the wrapping of all segments in UninvertingReader-s. The percentage of lost doc values
is sometimes quite large, up to 50%, perhaps it’s a bug somewhere where the code accounts
for the presence of doc values in FieldCacheImpl?
>
> Together with Erick we implemented a bunch of tests that illustrate this issue - both
the tests and the code can be found on branch "jira/solr-12259":
>
> * code.tests.AddDVMPLuceneTest2 - this is a pure Lucene test that shows how doc values
are lost after several rounds of merging while concurrent indexing is going on. This failure
is reproducible 100%.
>
> * code.tests.AddDvStress - this is a Solr test that repeatedly creates a collection without
doc values, starts the indexing, changes the config to use ADVMP, makes the schema change
to turn doc values on, and verifies the number of facets on the target field. This test also
fails after a while with the same symptoms as the Lucene one, so I think that solving the
Lucene test failure should solve this failure too.
>
> Any suggestions or insights are very much appreciated - I'm running out of ideas to try...
>
> —
>
> Andrzej Białecki
>
>
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>
>
> --
> Adrien
>
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>


-- 
Adrien

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