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From kiril.z...@epiphany.com
Subject Concurency in Lucene
Date Wed, 16 Oct 2002 22:44:42 GMT
My company, Epiphany, has decided to integrate our products with Lucene.
I'm leading this effort, and for this I have developed a solution around
Lucene that allows concurrent processes to search, insert, update and delete
documents. 
This solution solves the following:
	- concurrent writing (insert, update, delete) to the Index (see
http://nagoya.apache.org/bugzilla/show_bug.cgi?id=12588 and
http://www.mail-archive.com/lucene-user@jakarta.apache.org/msg01795.html
	- not-transactional nature of Lucene. Solution puts transaction
around every insert, update and delete. All writes are guaranteed to be in
the index eventually.
	- running out of file handles.
	- solution does all of the book-keeping, clients do not worry about
when to open and close  IndexReader/Writer. Technically one can do this
after every operation, but creating/deleting of .lock file slows things
down.


In summary, every write (update, delete, insert) is made to log file first.
There is a worker thread that wakes up every so often, examines the logs,
and makes a decision on whether to propagate changes or not (this is
configurable). If decision is to propagate changes, thread creates new log
files, locks current log files,  makes a copy of the new index, merges
changes from logs to the index, and then hot-swaps the newly created index
and deletes the old logs and index. At any given time, result from search
will not contain deleted documents, but newly created/updated documents will
not be in search result until merge is finished. Worker thread also keeps
state of the logs/index in case of crash. 

Here is what were the driven factors to create this solution. 
	Need for concurrent non-blocking writes (insert/update/delete)
	Need for deleted documents not to show up in the query result (Hits)
once deleted
	Lucene does not handle crashes well. The mentality is "if in doubt,
redo index" which does not work in some cases. Rebuilding of the index is
fast, but in our case a) it takes too many non-Lucene related recourses
(documents can be stored in database), b) high availability of search is a
requirement
		- Lucene can leave .lock files.
		- Lucene keeps state (documents) in memory


I wanted to see how much interest is out there for such a solution and
whether Lucene developers feel that this should be part of Lucene. If there
is enough interest I would like to donate this code to Lucene.

Thanks,

Kiril Zack

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