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From "Michael McCandless (JIRA)" <>
Subject [jira] Commented: (LUCENE-2320) Add MergePolicy to IndexWriterConfig
Date Mon, 15 Mar 2010 15:44:27 GMT


Michael McCandless commented on LUCENE-2320:

bq. Or, maybe, we should think of MergePolicy API that doesn't require one to keep a reference
to IW?

Looks like IW is used pretty widely: for messaging (when infoStream is set), for retrieving
the merges, for getting the Directory, and for getting number of deleted docs for a given
segment.  I guess an option would be to simply pass it around everywhere.  Then we wouldn't
have to break the circular dependendy.

This is what MergeScheduler appears to do -- it's passed to .merge, and then each bg thread
in CMS holds a reference to the writer (since it needs to ask for followon merges).

> Add MergePolicy to IndexWriterConfig
> ------------------------------------
>                 Key: LUCENE-2320
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Shai Erera
>            Assignee: Michael McCandless
>             Fix For: 3.1
>         Attachments: LUCENE-2320.patch
> Now that IndexWriterConfig is in place, I'd like to move MergePolicy to it as well. The
change is not straightforward and so I've kept it for a separate issue. MergePolicy requires
in its ctor an IndexWriter, however none can be passed to it before an IndexWriter actually
exists. And today IW may create an MP just for it to be overridden by the application one
line afterwards. I don't want to make iw member of MP non-final, or settable by extending
classes, however it needs to remain protected so they can access it directly. So the proposed
changes are:
> * Add a SetOnce object (to o.a.l.util), or Immutable, which can only be set once (hence
its name). It'll have the signature SetOnce<T> w/ *synchronized set<T>* and *T
get()*. T will be declared volatile, so that get() won't be synchronized.
> * MP will define a *protected final SetOnce<IndexWriter> writer* instead of the
current writer. *NOTE: this is a bw break*. any suggestions are welcomed.
> * MP will offer a public default ctor, together with a set(IndexWriter).
> * IndexWriter will set itself on MP using set(this). Note that if set will be called
more than once, it will throw an exception (AlreadySetException - or does someone have a better
suggestion, preferably an already existing Java exception?).
> That's the core idea. I'd like to post a patch soon, so I'd appreciate your review and

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