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Subject: [Pljava-dev] Newbie Questions
Date: Fri, 29 Sep 2006 21:12:00 +0200
Message-ID: <451D7000.70406@tada.se> (raw)
In-Reply-To: <451D6A12.7020604@softhome.net>
References: <451D6A12.7020604@softhome.net>

Scott Petersen wrote:
> Folks,
>
> YES, I am a newbie.  But I have been tasked to verify several questions 
> specific to the PLJava implementation (version 1.3. 0, Postgresql 
> 8.1+).  I have read the manual at 
> http://wiki.tada.se/wiki/display/pljava/The+choice+of+JNI which has been 
> most helpful and informative.  But I need to verify the following:
>
>     1)   When I create a connection "Connection db = 
> DriverManager.getConnection("jdbc:default:connection");", does this 
> reuse/copy/duplicate the existing connection or does it create a new 
> (second) connection to the database?
>
>   
Well, that depends on how you look at it. Let me elaborate:

You start with a client application. If you use Java, you will probably 
use JDBC but the same is true if you use the psql client or something 
else. This application will utilize some kind of IPC mechanism (often a 
socket) and connect your client process to the backend server process. 
The first time the backend server process executes a Java function, it 
will instantiate a JVM. If the Java method in turn uses the 
getConnection() method that you give in your example, it will establish 
an *internal* connection within the backend. Unlike the the socket 
connection, this connection is in-process, has very low overhead, and 
executes within the same transaction as the caller.


>     2)    In reading the above referenced web page it appears that each 
> connection has its own JVM.  Is this true, or is there a single JVM per 
> schema, or a single JVM per database?
>
>   
It's a single JVM per client connection. I recommend reading 
http://wiki.tada.se/wiki/display/pljava/The+choice+of+JNI for a deeper 
understanding of the choice of technology.

Kind Regards,
Thomas Hallgren

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