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https://issues.apache.org/jira/browse/LUCENE-4658?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13604683#comment-13604683
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David Smiley commented on LUCENE-4658:
--------------------------------------
You raise a good point there Rob; BinaryDocValues is pretty close and might be
sufficient as-is. But do we need segment based tracking hooks? Perhaps it's
useful for parallel / overlay indexes that maintain docid consistency
(LUCENE-4258 ?), but I don't think that needs to be centered around any
particular special field. Shai's issue description points to a comment I made
but it was in turn a quote of Rob. Rob & I didn't call out a need for segment
level tracking; it was commit level tracking. A couple use-cases I had in mind
when I made the comment are:
* Storing per-document data that changes often like the number of
clicks/accesses to the search result -- ultimately used to influence scoring.
The application's backing store would probably be an in-memory cache with
occasional syncs to disk.
* Storing a large per-document body text in an external data source (e.g. a DB
or file system). Lucene needlessly merges stored fields which I think is quite
wasteful, not to mention putting it in Lucene is redundant if you already
manage it somewhere else. It's ultimately needed via Lucene's API for
highlighting.
Is per-segment tracking needed for this? Or is this really about hooks to
enable a parallel segment level index? I dunno.
> Per-segment tracking of external/side-car data
> ----------------------------------------------
>
> Key: LUCENE-4658
> URL: https://issues.apache.org/jira/browse/LUCENE-4658
> Project: Lucene - Core
> Issue Type: Improvement
> Reporter: Michael McCandless
> Assignee: Michael McCandless
> Attachments: LUCENE-4658.patch, LUCENE-4658.patch
>
>
> Spinoff from David's idea on LUCENE-4258
> (https://issues.apache.org/jira/browse/LUCENE-4258?focusedCommentId=13534352&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-13534352
> )
> I made a prototype patch that allows custom per-segment "side-car
> data". It adds an abstract ExternalSegmentData class. The idea is
> the app implements this, and IndexWriter will pass each Document
> through to it, and call on it to do flushing/merging. I added a
> setter to IndexWriterConfig to enable it, but I think this would
> really belong in Codec ...
> I haven't tackled the read-side yet, though this is already usable
> without that (ie, the app can just open its own files, read them,
> etc.).
> The random test case passes.
> I think for example this might make it easier for Solr/ElasticSearch
> to implement things like ExternalFileField.
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