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From "Shai Erera (JIRA)" <>
Subject [jira] Commented: (LUCENE-2320) Add MergePolicy to IndexWriterConfig
Date Mon, 15 Mar 2010 20:05:32 GMT


Shai Erera commented on LUCENE-2320:

The thing is that we were at that position already, before I changed it so that MP requires
writer up front. The reason was, like Mike mentioned, that writer had to be passed on all
method calls, for really no good reason. A MP is usually coupled w/ an IW instance and I don't
think we should opt for decoupling them.

Most of this patch removes MP setting from IW to IWC (and hence changes test code to use the
new API). The SetOnce juggling is done only to ensure an IW is set exactly once on MP, and
allows us to resolve that circular dependency. We can do two things:
# Continue w/ SetOnce as introduced in this patch.
# Introduce a setIndexWriter on MP which anyone can call, even more than once.

With (1) I don't think we complicate anything, and SetOnce can be useful in other places as
well. (2) is really like passing writer on all method calls, so let's at least not have it
as part of all methods signature. I prefer (1) slightly over (2) but am fine w/ (2) as well.
I wouldn't want to change MP back to require IW on all its methods.

> 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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