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From Marvin Humphrey <mar...@rectangular.com>
Subject Re: Per-Thread DW and IW
Date Tue, 20 Apr 2010 15:03:52 GMT
On Tue, Apr 20, 2010 at 07:27:57AM -0400, Michael McCandless wrote:
> There are elements of IW that still must be "centralized" -- managing
> the merge policy/schedulers, deletion policy, writing/committing the
> segments files, managing ongoing addIndexes, tracking pending
> deletions, the reader pool, etc.

I've got a prototype BackgroundMerger working for KS/Lucy which can work
concurrently with an Indexer.  It drops a "merge.lock" file which blocks
Indexer from merging any segments that existed at the moment of the lockfile's
creation.  When it's done merging, it acquires the write.lock, carries forward
any deletions that Indexer has written against the segments it's merging away,
then commits and releases both locks.

Based on the success of this prototype, I believe merging policy is a
theoretically solvable problem using mutexes to lay claim to the mergable
segments while doing the heavy lifting, and the write lock to coordinate
deletions and committing.  

However, The management of individual deletions seems like a daunting problem
every time I consider how to expand this model out to multiple indexing
processes operating against the same index when those processes must be
allowed to create new deletions.  I think there are insoluble race conditions
until you get into document-level locking.

I imagine NRT readers make this problem even harder.

Marvin Humphrey


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