Yonik Seeley wrote:
>>6. Index locally and synchronize changes periodically. This is an
>>interesting idea and bears looking into. Lucene can combine multiple
>>indexes into a single one, which can be written out somewhere else, and
>>then distributed back to the search nodes to replace their existing
>>index.
>
> This is a promising idea for handling a high update volume because it
> avoids all of the search nodes having to do the analysis phase.

A clever way to do this is to take advantage of Lucene's index file
structure.  Indexes are directories of files.  As the index changes
through additions and deletions most files in the index stay the same.
So you can efficiently synchronize multiple copies of an index by only
copying the files that change.

The way I did this for Technorati was to:

1. On the index master, periodically checkpoint the index.  Every minute
or so the IndexWriter is closed and a 'cp -lr index index.DATE' command
is executed from Java, where DATE is the current date and time.  This
efficiently makes a copy of the index when its in a consistent state by
constructing a tree of hard links.  If Lucene re-writes any files (e.g.,
the segments file) a new inode is created and the copy is unchanged.

2. From a crontab on each search slave, periodically poll for new
checkpoints.  When a new index.DATE is found, use 'cp -lr index
index.DATE' to prepare a copy, then use 'rsync -W --delete
master:index.DATE index.DATE' to get the incremental index changes.
Then atomically install the updated index with a symbolic link (ln -fsn
index.DATE index).

3. In Java on the slave, re-open 'index' it when its version changes.
This is best done in a separate thread that periodically checks the
index version.  When it changes, the new version is opened, a few
typical queries are performed on it to pre-load Lucene's caches.  Then,
in a synchronized block, the Searcher variable used in production is
updated.

4. In a crontab on the master, periodically remove the oldest checkpoint
indexes.

Technorati's Lucene index is updated this way every minute.  A
mergeFactor of 2 is used on the master in order to minimize the number
of segments in production.  The master has a hot spare.

Doug

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