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Re: java stored procedures

From: Laszlo Hornyak <hornyakl(at)freemail(dot)hu>
To: Barry Lind <barry(at)xythos(dot)com>
Cc: hornyakl(at)users(dot)sourceforge(dot)net, pgsql-general(at)postgresql(dot)org, pgsql-jdbc(at)postgresql(dot)org
Subject: Re: java stored procedures
Date: 2001-12-05 09:06:10
Message-ID: 3C0DE382.1050400@freemail.hu (view raw or flat)
Thread:
Lists: pgsql-generalpgsql-hackerspgsql-jdbc
Hi!

Barry Lind wrote:

> Does the mechanism you are planning support running any JVM?  In my 
> opionion Kaffe isn't good enough to be widely useful.  I think you 
> should be able to plugin whatever jvm is best on your platform, which 
> will likely be either the Sun or IBM JVMs.

Ok, I also had problems with caffe, but it may work. I like it becouse 
it is small (the source is about 6M). As much as I know Java VM`s has a 
somewhat standard native interface called JNI. I use this to start the 
VM, and communicate with it. If you think I should change I will do it, 
but it may take a long time to get the new VM. For then I have to run kaffe.

> Also, can you explain this a little bit more.  How does the jvm 
> process get started? (I would hope that the postgresql server 
> processes would start it when needed, as opposed to requiring that it 
> be started separately.)  How does the jvm access these shared memory 
> structures? Since there aren't any methods in the java API to do such 
> things that I am aware of.

JVM does not. 'the java process' does with simple posix calls. I use 
debian potatoe, on any other posix system it should work, on any other 
somewhat posix compatible system it may work, I am not sure...

>
> I don't understand how you do this in java?  I must not be 
> understanding  something correctly here.

My failure.
The 'java request_handler' is not a java function, it is the C 
call_handler in the Postgres side, that is started when a function of 
language 'pljava' is called.
I made some failure in my previous mail. At home I named the pl/java 
language pl/pizza (something that is not caffe, but well known enough 
:). The application has two running binaries:
-pizza (which was called 'java process' last time) This is a small C 
program that uses JNI to start VM and call java methods.
-plpizza.so the shared object that contains the call_handler function.


>
>
>> -when java thread receives the signal, it reads the message(s) from 
>> the queue, and starts some actions. When done it tells postgres with 
>> a signal that it is ready, and it can come for its results. This will 
>> be rewritten see below problems.
>
>
>
> Are signals the best way to accomplish this? 

I don`t know if it is the best, it is the only way I know :)
Do you know any other ways?

>
>
>> -And postgres is runing, while java is waiting for postgres to say 
>> something.
>
> But in reality if the postgres process is executing a stored function 
> it needs to wait for the result of that function call before 
> continuing doesn't it? 

Surely, this is done. How could Postgres tell the result anyway ? :)

>
>>
>> Threading on the java process side is not done yet, ok, it is not 
>> that hard, I will write it, if it will be realy neccessary.
>
> Agreed, this is important.
>
> Shouldn't this code use all or most of the logic found in the FE/BE 
> protocol?  Why invent and code another mechanism to transfer data when 
> one already exists.  (I will admit that the current FE/BE mechanism 
> isn't the ideal choice, but it seems easier to reuse what exists for 
> now and improve on it later). 

Well, I am relatively new to Postgres, and I don`t know these protocols. 
In the weekend I will start to learn it, and in Sunday or Monday I maybe 
I will understand it, if not, next weekend..

>
> You didn't mention how you plan to deal with the transaction 
> symantics.  So what happens when the pl/java function calls through 
> jdbc back to the server to insert some data?  That should happen in 
> the same transaction as the caller correct? 

I don`t think this will be a problem, I have ideas for this. Idea mean: 
I know how I will start it, it may be good, or it may be fataly stupid 
idea, it will turn out when I tried it. Simply: The same way plpizza 
tells pizza the request, pizza can talk back to plpizza. This is planed 
to work with similar mechanism I described last time (shm+signals).

Monday I will try to send a little pieces of code to make thing clear, ok?

thanks,
Laszlo Hornyak


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