Re: [HACKERS] PL/Java issues

Jan,

In Oracle a call from sql into java (be it trigger, stored procedure or
function), is required to be a call to a static method.  Thus in Oracle
all the work is left for the programmer to manage object instances and
operate on the correct ones.  While I don't like this limitation in
Oracle, I can't see a better way of implementing things.

Therefore if you want to operate on object instances you (in Oracle)
need to code up object caches that can hold the instances across
function calls so that two or more functions can operate on the same
instance as necessary.  What this implies to the implementation is that
in order to be possible the multiple function calls need to run inside
the same jvm (so you can access the static caches across the different
calls).  If every call created a new jvm instance in Oracle you couldn't
do very much.  The Oracle jvm essentially gives you one jvm per
connection (although technically it is somewhere between one jvm for the
whole server and one per connection - i.e. it has the memory and process
footprint of a single jvm for the entire server, but appears to the user
as a jvm per connection).  Having one jvm per connection is important to
limit multiple connections ability to stomp on each others data.
Something similar could probably a done for postgres by having one jvm
running, by having each postgres connection having a unique thread in
that jvm and having each connection thread run with its own class loader
instance so that separate classes (and thus static members) are loaded
for each connection.

thanks,
--Barry


Jan Wieck wrote:
> I have included the JDBC mailing list since I guess most Java developers
> are around here, but not necessarily on Hackers.
>
> Dave Cramer and I where discussing a few issues about the PL/Java
> implementation last night and would like to get more input and
> suggestions on the matter.
>
> The basic question is the definition of the lifetime of an object and
> it's identificaition when doing nested calls in this context. In the OO
> world, ideally a real world object is translated into one instance of a
> class. And complex structures are trees of instances, possibly of
> different classes. As an example, a sales order consists of the order
> header and a variable number of order lines. Therefore, per order we
> have one OH instance and several OL's. So far so good. Naturally, one
> Java object instance would correspond to one row in a database.
>
> If we now implement a stored procedure in PL/Java, that means that a
> pg_proc entry corresponds to a specific method of a specific class (its
> signature). But there is no obvious relationship between functions and
> tables or other objects. Because of that it is not implicitly clear if
> an incoming call to a method is meant for an existing instance or if a
> new one should be created.
>
> As an example, if a PL/Java trigger on the order header executes an SPI
> query on the order lines, a trigger on the order line (also in PL/Java)
> might now want to call a method on it's parent object (the order header
> that is waiting for the SPI result set). This should NOT result in
> another OH instance being created for the same logical OH.
>
> Probably it is not possible to map these things automatically while
> keeping the system flexible enough to be usefull. But is it feasable to
> require the programmer to provide glue code for every procedure that
> does all these things? How does Oracle attack this problem?
>
>
> Jan
>