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Subject Re: Open an IndexWriter in parallel with an IndexReader on the same index.
Date Tue, 21 Feb 2006 16:26:23 GMT

I have a set of classes similar in function to IndexModifier but a little
more advanced. The idea is  to keep the IndexReaders and IndexWriters open
as long as possible only closing them when absolutely necessary. Using the
concurrency package allows for me to have multiple readers and a single
writer. I use a watchdog timer to flush the index if the index is idle for
a while (closes the index flushes unwritten writes on a writer and
unwritten deletes on a reader). The downside to this approach is you lose
all the benefits of any caching that Lucene does of sorted results and of
any filters that are weakly cached on IndexReaders which can have a massive
impact on searching (I sometimes take a several seconds hit after the index
is reopened on the first sorted query).

In addition to this index management code I have a queue onto which I place
new documents/updates/deletes. Every time a document is added the tasks on
the queue are reordered to batch up similar actions. The downside here is
that this is effectively single threaded which arguably affects the
performance. In addition for this to work well (and to prevent to
equivalent of a databases "dirty read") I also have to queue up the
querying of the index until all previous updates have been carried out -
not ideal but not causing too significant a problem in my situation.

My experience is that producing something to manage the indexes swapping
between readers and writers is relatively straightforward.  The task of
batching up updates/deletes and the like may be too application specific -
my code relies on my own unique document ids being in the index and so is
quite specific.


Paul I.

             "Nadav Har'El"                                                
             21/02/2006 15:35         
             Please respond to                                     Subject 
             java-user@lucene.         Re: Open an IndexWriter in parallel 
                   with an IndexReader on the same     

"Yonik Seeley" <> wrote on 21/02/2006 05:13:52 PM:
> On 2/21/06, Pierre Luc Dupont <> wrote:
> >     is it possible to open an IndexWriter and an IndexReader on the
> > index, at the same time,
> > to do deleteTerm and addDocument?
> No, it's not possible.  You should batch things: do all your
> deletions, close the IndexReader, then open an IndexWriter and do all
> the addDocument calls.

For some applications, the seperation of indexWriter (which can add a
document) and indexReader (which can delete a document) is very
For example, consider a case where documents are often updated, and we
often need to find and remove the old document and add the new version
of the document. the "indexModifier" class nicely hides the complexity
from us and allows both addition and deletion, but the documentation
says its performance sucks (when used in the way I just outlined):
imagine 1000 documents being modified, and now we start deleting and
adding each one, one after another.

It would have been nice if someone wrote something like indexModifier,
but with a cache, similar to what Yonik suggested above: deletions will
not be done immediately, but rather cached and later done in batches.
Of course, batched deletions should not remember the term to delete,
but rather the matching document numbers at the time of the deletion -
because after the addition of the modified document if we search for
the term again we'll find two documents.

What about this idea? Does an implementation of something similar
already exist?


Nadav Har'El

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