Patch to control DBConnections not used and wasting resources

[João Paulo Ribeiro <jp@xxxxxxxxxxxx>]

Hi!

We are using Enydra-5.0 at the office for our services and products.
We find a "problem" with the way the DBpool work.
I will explain it as i have understood it.

First the DBPool work based in the asumption that an open DBConnection 
should only be closed if something wrong happened using it 
(SQLException). If nothing wrong happened when using the connection when 
you call the release() mehtod on the DBConnection it will not be closed 
but put back to the pool waiting to be used. This way the pool will only 
close DBConnection that cant be trusted.
This have nothing wrong at all but if for some reason an application 
that usually use 10 DBConnection is forced to open 50, a lot of them 
will stay at the pool and never be used. They will just waste resources 
at the aplication and the  database.
This is something really easy to happen when the db is used by more than 
one application and the database is on heavy load during just a few 
seconds per day. When this happen you can have hundreds of DBConnections 
open and only a few used.

To solve this "problem" i made a patch that allow de pool to choose to 
close a "good" connection because it has already too many connections 
waiting to be used. This way you can control  the pool to keep just 
enough connections needed to handle a burst but avoid wasting resources. 
I also added support to read the value of maximum number of 
DBConnections waiting from the configuration file.

If someone think that this can be usefull let me know.

Regards.
João Paulo Ribeiro

PS: below is a diff of the 
com.lutris.appserver.server.sql.standard.StandardConnectionAllocator to 
apply the patch.
I will send another mail about the same problem with the 5.1.7 dods 
version and to discuss the change made the pool: stack to linkedlist

diff StandardConnectionAllocator.java.old 
StandardConnectionAllocator.java -b
189a190,194
>     /** JP Patch
>      * Maximum number of DBConnections that will be waiting in the pool
>      */
>     protected int maxWaitingConnections;
>
246a252
>             maxWaitingConnections = 
conConfig.getInt("MaxWaitingConnections", 10);
350c356
<           || dbConnection.isMarkedForDrop()) {
---
>           || dbConnection.isMarkedForDrop() || pool.size() >= 
maxWaitingConnections) {