Using JPL (Java + Prolog) in a Java EE web application

Question

I would like to develop a Java EE web application that requires Prolog, via JPL, for certain search related tasks. The web application will be deployed in the JBoss application server. The Prolog engine can be either YAP or SWI (afaik the only Prolog engines compatible with JPL at the moment). The Prolog queries depend on information stored in a (potentially large) database.

If someone has tried this or something similar, could you please give me feedback about the following questions?:

• What is the best way to manage concurrent http sessions that need access to the Prolog engine?. Is it possible -desirable?- to assign to each separate session its own Prolog engine ?. If this solution works, is it possible to implement something similar to a 'Prolog engine pooling' to quickly assign prolog engines to new sessions ? . Or the best solution is to have a single Prolog engine that will manage all the query requests synchronously ? (and slowly).
• How could be managed the interaction of Prolog with the database ?. If the data is changing often in the database and Prolog needs this data to solve its queries, what is the best strategy to keep the facts in the Prolog engine synchronized with the data in the database ?. The navy option of starting from scratch at each new session (e.g., reloading all the data from the database as Prolog facts) does not seem to be a good idea if the database grows large.
• Any other expected issues/difficulties related to the java-prolog-database interaction during the implementation ?

Thanks in advance!

Answer 1

What is the best way to manage concurrent http sessions that need access to the Prolog engine?.

If I look at the source of JPL, it looks like it uses an engine pool. The query data type implements the enumerator pattern plus a close() operation. I guess an engine is automatically assigned to a query as long as it is active.

So each http request can independently access the Prolog system via new query objects. If you don't want to close your query object during a http request, I guess you can also attach it to a http session. And reuse it an another request.

How could be managed the interaction of Prolog with the database ?

This depends on the usage pattern of the data in the database and the available access paths. It could be that you can quickly access very large databases during a request and refetch the data during each request. For example if the needed matching data set is small and if the database has good indexes, so that the matching data can be quickly accessed.

Otherwise you would need to implement some intelligent caching. I am currently working at a solution where I use a kind of a check-in/check-out pattern. But this is not suitable for a web server, where you have multiple users. I am using this pattern for a standalone solution where there is one user and one checked out data junk in memory. For a web server with varying multiple users the junks could overflow the webserver memory.

So caching only works if you can limit and throttle the junks or if you have a very large webserver memory. Maybe you can find such an invariant for your application. Otherwise the conclusion could be that you cannot go Java EE independent of whether you use Prolog or not.