My application database can be updated outside the application also. Whenever
there is a change in database by some other source, I want to update my index.
Is there any way to do so?
I am using Java and the database is DB2. I saw the DB2 UDF. But I have to put the jar inside the DB2 installation directory. I dont know how to communicate with my application/update index.
Yes, this certainly is possible.
The simplest approach would be to rebuild the entire index periodically,
however, this is not very efficient.
A better approach is to reindex just the rows that have changed since
you last indexed.
To do this, first, you need to implement your own method of tracking
which rows of which tables have changed inside DB2. Perhaps you could
use a timestamp column and then issue a SELECT WHERE timestamp >
last-index-time. Make sure last-index-time is DB2's timestamp to avoid
clock skew issues.
Then, for each changed document, you need to add the new doc and remove
the old one (Lucene doesn't have a "replace document" ). Typically you
would index the primary key into Lucene, and then use IndexReader's or
IndexModifier's 'deleteDocuments(Term term)' to delete the document by
its primary key. Then add the new document. If you have a group of
documents that need re-indexing, it's best to first delete all of them,
and then re-add all of them (ie, bunch the deletes & adds). This gives
better performance.
The third (and likely best, depending on tradeoffs) approach is to have
some sort of "push" or "triggers" coming out of DB2 that notifies you
whenever a row is changed. If UDF inside DB2 allows for this that would
be great (I don't know anything about UDF -- likely you'd need to roll
your own such communication coming out of DB2). Then, you re-index each
document when it's changed instead of polling periodically.
In any case, make sure you close/re-open your IndexSearchers
periodically so that they see the newly deleted/added documents. And
make sure the lock directory is the same directory across all of your
processes, and, it is a directory in a locally mounted file system (ie
not NFS or Samba) as there known issues for those.
Mike
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