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From Paul Mellor <>
Subject Concurrent searching & re-indexing
Date Wed, 16 Feb 2005 10:21:09 GMT

I've read from various sources on the Internet that it is perfectly safe to
simultaneously search a Lucene index that is being updated from another
Thread, as long as all write access to the index is synchronized.  But does
this apply only to updating the index (i.e. deleting and adding documents),
or to a complete re-indexing (i.e. create a new IndexWriter with the
'create' argument true and then re-add all the documents)?

I have a class which encapsulates all access to my index, so that writes can
be synchronized.  This class also exposes a method to obtain an
IndexSearcher for the index.  I'm running unit tests to test this which
create many threads - each thread does a complete re-indexing and then
obtains an IndexSearcher and does a query.

I'm finding that with sufficiently high numbers of threads, I'm getting the
occasional failure, with the following exception thrown when attempting to
construct a new IndexWriter (during the reindexing) - couldn't delete _a.f1
at org.apache.lucene.index.IndexWriter.<init>(

The exception occurs quite infrequently (usually for somewhere between 1-5%
of the Threads).

Does the IndexSearcher take a 'snapshot' of the index at creation?  Or does
it access the filesystem whilst searching?  I am also synchronizing creation
of the IndexSearcher with the write lock, so that the IndexSearcher is not
created whilst the index is being recreated (and vice versa).  But do I need
to ensure that the IndexSearcher cannot search whilst the index is being
recreated as well?

Note that a similar unit test where the threads update the index (rather
than recreate it from scratch) works fine, as expected.

This is running on Windows 2000.

Any help would be much appreciated!


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