[Hibernate] Plan for a full text search facility built on top of Hibernate Lucene annotations

[Sylvain V. <svi...@ap...>]

After chatting with Emmanuel, here is a draft plan for a closer
integration between Hibernate and Lucene for performing full text
queries.
Hibernate annotations for Lucene helps keeping the lucene indexes up to
date, but doesn't provide a query facility.
It also lacks converters that would for example help store a Date with
the proper format in Lucene, so that the alphabetic order matches the
Object's natural order.

A framework like Compass ( http://www.opensymphony.com/compass ) is
meant to fix this problem, by implementing it's own OSEM (Object, Search
Engine Mapping), and having a query facility that mimics what hibernate
is doing with database side.
Compass can even reuse Hibernate's mapping thus minimizing the
configuration effort.

One short coming I've found with Compass though is that the objects that
you get when you query the full text engine aren't connected to the ones
in the database.
So if you manipulate them, the changes aren't persisted or can actually
erase some of the information in the database.

The best way to have a simple and risk free integration is to build a
Full Text query facility that would be closely integrated with Hibernate
& Hibernate Lucene annotations.

So, querying the Full Text indexes would return objects, like Compass
does, but those objects would be fetched from the database.
Actually, for performance reasons, they could be initialized with the
information from the FT index, and, through byte code enhancement, if an
uninitialized property is read, or  a property is set, the real object
could be fetched from the database and read/set accordingly.
Here are a few examples :

        1) Just make a full text search :
        query "toto" would fetch all the object with an indexed field
        containing toto from the Lucene index.
        If the objects are initialized from the Lucene index, just one
        query to the Lucene index is done, and the search results can be
        displayed.
        => Best performance.
        Loading the objects from the database is useless here, and would
        only lead to poorer performances.
        
        2) Make a full text search AND manipulate the objects :
        You want to query all the objects with "toto", and increment
        their "searchHits" property.
        You do the query, with a Load.EAGER parameter.
        Only the objects' ids are retrieved from Lucene, and the real
        objects are retrieved from Hibernate
        
        3) Mix both approaches
        Requires byte code enhancements.
        Can be useful for cases where for some types of objects you
        don't want to store all the properties required to display the
        search view results in the index.
        Only those objects will be loaded from Hibernate.
        
        All 3 modes should work, but we can always begin by implementing
        mode 2 only (retrieving only the id's from Lucene, and
        initializing the objects from Hibernate).
        Everything will still work, but performance will not be optimal.
        Later on we can implement mode 3 (which would also solve
        situation 1), and the changes will be transparent to the user.
        Only the performance will be better.
        

Another advantage of integrating the Full Text query closely with
Hibernate is that in some cases where a field isn't indexed but the
query is still simple (fiels x like toto%), the Lucene index would not
be needed, and some queries can be performed directly via Hibernate in a
transparent way for the user.

To summarize this, the biggest changes would be :

        - Add converters to Hibernate Lucene annotations, like what
        Compass is doing :
        http://www.opensymphony.com/compass/versions/0.9.1/html/core-settings.html#config-converter
        
        - Build a Full Text query facility similar to Hql / Criteria,
        but focussed on full text search (also like Compass's one :
        http://www.opensymphony.com/compass/versions/0.9.1/html/core-workingwithobjects.html#Searching ) but that would make sure the objects retrieved from the Lucene index behave as if they were retrieved from the database.
        

I would be glad to ear from your feedback on this.

Thanks,

Sylvain.