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From "Shai Erera (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (LUCENE-3441) Add NRT support to LuceneTaxonomyReader
Date Mon, 19 Nov 2012 15:26:59 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-3441?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13500306#comment-13500306
] 

Shai Erera commented on LUCENE-3441:
------------------------------------

The side effect of sharing the cache is for TR-1 to receive an ordinal which doesn't actually
exist. We can document that if you use openIfChanged, but continue to use the old reader,
then this sort of thing can happen. Really, this is not how people should use openIfChanged
...

Another idea I had is to wrap the cache with an object that keeps track of 'upto'. I.e. TR-1's
instance's upto will be e.g. 123. On openIfChanged we allocate a new instance for TR-2, with
upto=1035, sharing the same LRU instance. On each access to the cache, we check if the returned
ordinal is <= upto, otherwise return INVALID.

But that adds another 'if' to every cache-check. How expensive is it, not sure yet, since
the taxonomy is not accessed for so many facets.

It feels like we need to make some compromise here - either at runtime performance or possible
correctness. Since the usage of openIfChanged is well defined, and assuming that people who
interact with the taxonomy are aware Lucen's NRT, should we rely on the old taxo reader instances
to not be used after openIfChanged?
                
> Add NRT support to LuceneTaxonomyReader
> ---------------------------------------
>
>                 Key: LUCENE-3441
>                 URL: https://issues.apache.org/jira/browse/LUCENE-3441
>             Project: Lucene - Core
>          Issue Type: New Feature
>          Components: modules/facet
>            Reporter: Shai Erera
>            Assignee: Shai Erera
>            Priority: Minor
>         Attachments: LUCENE-3441.patch
>
>
> Currently LuceneTaxonomyReader does not support NRT - i.e., on changes to LuceneTaxonomyWriter,
you cannot have the reader updated, like IndexReader/Writer. In order to do that we need to
do the following:
> # Add ctor to LuceneTaxonomyReader to allow you to instantiate it with LuceneTaxonomyWriter.
> # Add API to LuceneTaxonomyWriter to expose its internal IndexReader
> # Change LTR.refresh() to return an LTR, rather than void. This is actually not strictly
related to that issue, but since we'll need to modify refresh() impl, I think it'll be good
to change its API as well. Since all of facet API is @lucene.experimental, no backwards issues
here (and the sooner we do it, the better).

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