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https://issues.apache.org/jira/browse/LUCENE-5125?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13713872#comment-13713872
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Hoss Man commented on LUCENE-5125:
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bq. They just have to merge to the default implementation before upgrading
Understood ... but w/o more explicit docs drawing attention to this in more
places, it won't be obvious to some users to even think of doing this when they
get a weird error on trying to upgrade.
Ideally it should be more obvious to all users (not just solr users) in advance
of even using these codecs that "if you use this class, then X is what you will
likeley have to do to upgrade lucene later"
bq. I don't see how they could have configured what they did without
intentionally choosing to ignore the documentation.
Perhaps by following some advice they got from a blog or mailing list that
suggested they try {{docValuesFormat="Disk"}} w/o repeating the caveat?
I'm not trying to re-hash LUCENE-5121 -- i'm just trying to ensure that we
cover all our bases in better documenting what codecs will and won't
automatically work on upgrade.
> Codec classes/packages that do not provide file format back compat need to be
> more explicit about this in javadocs
> ------------------------------------------------------------------------------------------------------------------
>
> Key: LUCENE-5125
> URL: https://issues.apache.org/jira/browse/LUCENE-5125
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Hoss Man
>
> rmuir noted in LUCENE-5121...
> bq. Currently (as documented), we don't provide index back compat for
> experimental codecs in lucene-codecs.jar.
> ...but except for a solr wiki page and solrconfig.xml comment, it's extremely
> non-obvious that any of these codec classes don't provide index backcompat.
> * the codec module overview.html page describes the module as "Collection of
> useful codec, postings format and terms dictionary implementations" -- with
> no indication that by using these "useful" implementations, the user gives up
> index backcompat.
> * the package.html files in the individual packages of the codec module
> (appending, blockterms, bbloom, diskdv, etc...) also say nothing about index
> backcompat
> * the individual classes in these codecs are mostly labeled with
> {{@lucene.experimental}} but in the resulting javadoc that merely says that
> "WARNING: This _API_ is experimental and might change in incompatible ways in
> the next release". Lots of classes in Lucene have this warning on them about
> their API (including the abstract codec apis themselves in lucene-core:
> DocValuesFormat, PostingsFormat, etc...) and that annotation (as far back as
> i can remember) has always only refered to the java API of the labeled class
> -- never to whether using that class ment you were giving up on index format
> back compat.
> Given how much effort and work is put into ensuring good index backcompat for
> default codec, we should be extremely explicit when/if alternative codecs do
> not support backcompat, so we don't frustrate/confuse users and leave them
> with the impression that they can never count on index backcompat just
> because they may not realize they were using an "unsupported" format option
> because of a blog post they read or advice they got on the mailing list about
> how to make something faster or use less ram.
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