Re: Non-SQL Persistence for PL

Kamiz,

we should probably take this off-line from the forum, since it is
getting too much into internal details.  The persistence scheme uses a
few tables for different objects such as constants, definitions,
propositions, etc., but it is similar to a "one generic table" approach
as used, for example, by Jena.  That has some performance implications
that we still want to address down the road, but haven't yet.  The
system for loading objects dynamically is quite hairy, so I don't
advise you do try to mess with that.  Anyway, we can continue this
conversation in private email if you want.

Hans

>>>>> darabi  <[email protected]> writes:

> Hi,
> if there is a way of efficiently serialising and deserialising
> only those objects which represent definitions and propositions
> as you do it with your 'schema-less' RDBMS, that would surely
> be a better approach than serialising the complete object graph.

> When you say you use the RDBMS to store serialised objects in it,
> does that mean that you have one single table with rows for each
> proposition or definition? Or different tables for different
> types of objects? And if I understand correctly, the caching/
> paging is implemented in Stella on top of the RDBMS?

> I would like to try either AllegroCache and/or other object 
> persistence tools, so I looked into the code of the rdbms 
> system and saw how you use demons and db-relation-goes-xxx 
> functions and have a vague idea of what would be necessary.

> The experimental code you mention, would be very interesting
> as the rdbms code is still very close to the notion of
> tables (deftable ...). I would appreciate even small code 
> snippets or other information about the mechanism you use
> to store propositions in a generic way.


> Thank you.


> Kambiz

> ----- Original Message -----
> From: "Hans Chalupsky" <[email protected]>
> To: [email protected]
> Cc: "PowerLoom Forum" <[email protected]>
> Sent: Donnerstag, 30. August 2007 0.00 Uhr (GMT+0100) Europe/Berlin
> Subject: [PowerLoom Forum] Non-SQL Persistence for PL

> Kambiz,

> we do have an experimental version of persistence that doesn't require
> you to have a DB schema, but that basically persists PowerLoom
> definitions and propositions in a special-purpose "schema-less" RDBMS
> database.  The system then pages those objects in on demand and uses
> extensive caching to make things fast.  In a sense this is
> implementing a type of persistent object system by serializing the
> objects to an RDBMS.  The problem with this is that it isn't quite
> complete.  We've loaded, persisted and queried large ontologies with
> that, but there are a variety of loose ends that would make this
> difficult for you to use at this point.

> Several years ago we've played with some persistent object systems
> (such as Versant and PLOB - a free version of persistent Lisp
> objects), but the performance was never very satisfying.  That might
> have changed by now with Allegro Cache, but I don't have any real data
> on that.  The other issue is that a loaded PowerLoom module has a host
> of PowerLoom and STELLA data structures associated with it that all
> would need to be persisted, which means that you basically have to
> make all object classes of STELLA and PowerLoom persistent to make things
> work (without some major re-architecting).  I don't know how difficult
> that would be with Allegro Cache, and I'd be interested to hear some
> experiences from you if you venture to go that route.

> Hans

>>>>> darabi  <[email protected]> writes:

>> Hi, I sent the below message using the wrong mail address.
>> I hope, it doesn't show up twice in the list.:

>> ----

>> Hello,

>> I have a need for persistent knowledge bases and thus
>> have been experimenting with the rdbms (and sdbc) system
>> which you kindly released some time ago.

>> My (maybe short-sighted) conclusion is that rdbms is very
>> well suited for accessing large amount of data which
>> is already stored in an SQL DB and for making that data
>> available to PL's reasoning mechanisms.

>> But, if one has no existing SQL data and wants to persist
>> a KB of concepts and relations which are subject to frequent
>> change, then the limitations of the DB in terms of 'high
>> cost' of schema evolution also limit the possibilities
>> of rdbms/sdbc.

>> Have you or others taken another path: to use object level
>> persistence as provided by Franz' AllegroCache or other
>> 'object databases' to store the relevant parts of the
>> knowledge base?

>> Of course, this would only work in the lisp version of PL,
>> so it might be out of scope. OTOH, there is also db4o for
>> Java and C++, so maybe it wouldn't be impossible to implement
>> it in a cross-platform way.

>> But, I would be interested in any thoughts you might have
>> on this topic. Would it be possible to use the CLOS version
>> and identify some classes (e.g. named-description,
>> native-relation, context, module) which make up a PL module,
>> and persist them to an object database?

>> If yes, could you give me some hints as where to look into
>> the code?

>> Thank you very much.


>> Kambiz



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