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Date: Fri, 08 Oct 2004 10:33:19 -0700
From: Doug Cutting <cutting@apache.org>
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To: Lucene Users List <lucene-user@jakarta.apache.org>
Subject: Re: locking problems
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Aad Nales wrote:
> 1. can I have one or multiple searchers open when I open a writer?
> 2. can I have one or multiple readers open when I open a writer?

Yes, with one caveat: if you've called the IndexReader methods delete(), 
undelete() or setNorm() then you may not open an IndexWriter until 
you've closed that IndexReader instance.

In general, only a single object may modify an index at once, but many 
may access it simultaneously in a read-only manner, including while it 
is modified.  Indexes are modified by either an IndexWriter or by the 
IndexReader methods delete(), undelete() and setNorm().

Typically an application which modifies and searches simultaneously 
should keep the following open:

   1. A single IndexReader instance used for all searches, perhaps 
opened via an IndexSearcher.  Periodically, as the index changes, this 
is discarded, and replaced with a new instance.

   2. Either:
      a. An IndexReader to delete documents.
      b. An IndexWriter to add documents; or

So an updating thread might open (2a), delete old documents, close it, 
then open (2b) add new documents, perhaps optimize, then close.  At this 
point, when the index has been updated (1) can be discarded and replaced 
with a new instance.  Typically the old instance of (1) is not 
explicitly closed, rather the garbage collector closes it when the last 
thread searching it completes.

Doug

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