[Lucene-java Wiki] Update of "NearRealtimeSearch" by JasonRutherglen

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The following page has been changed by JasonRutherglen:
http://wiki.apache.org/lucene-java/NearRealtimeSearch

New page:
= Near Realtime Search =

Near realtime search in Lucene refers to features added to IndexWriter in Lucene 2.9/trunk that enable updates to be efficiently searched on soon after the updates are completed.  

* Minimize IO overhead
* Transparent to the user

One goal was to make the system as transparent as possible for users, so the API is fairly simple.  

The goal of the near realtime search design is to make NRT as transparent as possible for the user and minimize IO.  The side benefit was we removed the IndexReader/IndexWriter update index dichotomy where in some cases one would need to open an IndexReader for norm and delete updates, and an IndexWriter for document updates.  Now one simply calls IndexWriter.getReader after updates and the IndexWriter returns an IndexReader representing all of the previous updates. 

IndexWriter manages the subreaders internally so there is no need to call IndexReader.reopen.  The benefit of this design is the efficiency of deletes.  Where before NRT, deletes needed to be materialized to disk before being available for searching, they are now carried over in the segmentReaders until an explicit IndexWriter.commit is made.  This is useful for on disk segments where writing potentially many .del files would become IO costly over time.  With the deletes carried over in RAM by making use of the IndexReader.clone method internally, there is no need to call IndexReader.flush after every update to the index.

NRT adds an internal RAMDirectory to IndexWriter where documents are flushed to before being merged to disk.  This technique decreases the turnaround time required for updating the index when calling getReader to search the index including those updates.  

Sample code:

{{{
IndexWriter writer;
writer.addDocument(doc); // update
IndexReader reader = writer.getReader(); // get a reader with the new doc
}}}

== Internals == 

* IndexWriter pools segmentreaders
* IndexWriter.getReader flushes changes without calling commit or flushing deletes to disk
*