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From "Michael McCandless (JIRA)" <>
Subject [jira] [Commented] (LUCENE-4025) ReferenceManager.maybeRefresh should allow the caller to block
Date Mon, 30 Apr 2012 16:17:47 GMT


Michael McCandless commented on LUCENE-4025:

I'm torn here...

Always blocking seems dangerous because "simple" apps, that call .maybeRefresh() from searching
threads, will suddenly see all search threads block when a reopen is happening ... I think
it's too trappy to do that.  I think when it comes to concurrent code you should try hard
to not have "blocks all threads" trappiness...

But I also agree it's a hassle now for callers that want to ensure specific changes are now
visible... though, we do have NRTManager for that use case.

Maybe we can add an optional boolean (doWait, defaulting to false) to maybeRefresh?  Or we
a separate method for it (maybeRefreshAndWaitIfItsAlreadyHappening), or, we can make decoupled
method (waitForRefreshToFinish) and change maybeRefresh to return 3 results (IS_CURRENT, DID_REFRESH,
OTHER_THREAD_IS_REFRESHING)... or, something else?
> ReferenceManager.maybeRefresh should allow the caller to block
> --------------------------------------------------------------
>                 Key: LUCENE-4025
>                 URL:
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: core/search
>            Reporter: Shai Erera
>            Priority: Minor
>             Fix For: 4.0
> ReferenceManager.maybeRefresh() returns a boolean today, specifying whether the maybeRefresh
logic was executed by the caller or not. If it's false, it means that another thread is currently
refreshing and the call returns immediately.
> I think that that's inconvenient to the caller. I.e., if you wanted to do something like:
> {code}
> writer.commit();
> searcherManager.maybeRefresh();
> searcherManager.acquire();
> {code}
> It'd be better if you could guarantee that when the maybeRefresh() call returned, the
follow on acquire() will return a refreshed IndexSearcher. Even if you omit the commit instruction,
it'd be good if you can guarantee that.
> I don't quite see the benefit of having the caller thread not block if there's another
thread currently refreshing. In, I believe, most cases, you'd anyway have just one thread
calling maybeRefresh(). Even if not, the only benefit of not blocking is if you have commit()
followed by maybeRefresh() logic done by some threads, while other threads acquire searchers
- maybe then you wouldn't care if another thread is currently doing the refresh?
> Actually, I tend to think that not blocking is buggy? I mean, what if two threads do
commit() + maybeRefresh(). The first thread finishes commit, enters maybeRefresh(), acquires
the lock and reopens the Reader. Then the second thread does its commit(), enters maybeRefresh,
fails to obtain the lock and exits. Its changes won't be exposed by SM until the next maybeRefresh()
is called.
> So it looks to me like current logic may be buggy in that sense?

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