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From "Kevin Ross (JIRA)" <>
Subject [jira] Commented: (DBCP-345) NumActive is off-by-one at instantiation and causes premature exhaustion
Date Sat, 09 Oct 2010 15:39:31 GMT


Kevin Ross commented on DBCP-345:

Before borrowing the first connection, {{numActive}} should report 0 correct?

Do we agree, that if you have never borrowed a connection, numActive should not be greater
than 0, regardless of pool settings?

If we agree, then there is a bug.

My code attached exposes this, I'm not sure why you are not able to use it or see it.  The
commons-pool package is commonly used for many things and generic knowledge of that project
is very useful in diagnosing this problem.

Put a breakpoint in the borrowObject() method of the pool object (use your implementation,
or mine), and before the super call to borrowObject() in the pool packages, inspect the {{numActive}}.

If numActive == 1 before the call to the pool to make the first borrow, there is a bug.  AFTER
we borrow the first connection, numActive should be 1.

If you still don't see it, change the line of my code in from:
{code:java}log.debug( "BORROWING:  from " + toId( delegate ) + stats( delegate ) );{code}
{code:java}log.debug( "BORROWING:  holy cow this is num active before borrowing a connection!!!!!!!!!!!!!!!!!!!!!
" + getNumActive();{code}

I think that should get you there.

> NumActive is off-by-one at instantiation and causes premature exhaustion
> ------------------------------------------------------------------------
>                 Key: DBCP-345
>                 URL:
>             Project: Commons Dbcp
>          Issue Type: Bug
>    Affects Versions: 1.4
>            Reporter: Kevin Ross
>         Attachments:,
>   Original Estimate: 1h
>  Remaining Estimate: 1h
> Scenario: we have some code that we had thought was potentially leaking connections.
 In our unitTest/integrationTest environment, we know we can *lock down connections to a total
of 2* and a full run should pass.  We had no such luck with a {{maxActive}} of 2.    
> We created/attached a {{DebugBasicDataSource}} which initializes a  {{DebugConnectionPool}}
for logging purposes and delegates into the DBCP hierarchy.  BTW - consistent use of accessors
would have made this a cleaner affair ;) 
> {code}        // num active starts at one! Here is the original unmodified log message:
>         //          BORROWING:  from AbandonedObjectPool@10f0f6ac (1 of 2) 0 idle: threadStats[
]: all-time uniques{ (empty)  }
>         // SEE! no borrows ever, and the first pre-borrow already has a count of 1!{code}
> Before borrowing the first connection - {{numActive}} is 1!  
> The gorier details below, I hope they help someone else!
> Constraining the pool was the best way to uncover the leakage.  
> Thinking it was our error, we went after our code to find the problem.  We had such a
hard time understanding who was using connections, in which Spring context.  The confusion
stemmed from the fact that our unitTests run against REST resources deployed as Jersey components
in a Grizzly container.  Where they using the same connection pool or not?  Was the unitTest
setup side exhausting more connections, or was it leaking on the REST service side.
> Answers: 
> 1.  Our unitTests executing Jersey with in-VM Grizzly container do indeed utilize the
same pool (and same Spring context).
> 2.  Our unitTest (side) was not using more than one connection for data setup, and it
returned the connection for reuse.
> 3.  Our REST service side was only using one connection, but was a Grizzly threaded container
and we have AcitveMQ running as well.  Practically, one server connection could handle everything,
but the REST service and ActiveMQ listener could potentially claim 2.
> Note, the attached DebugBasicDataSource was quite useful to determine which threads were
claiming which connections in a leak situation.  Certainly do not configure it on the production
side, but it might be nice to see something like this offered up on the DBCP site somewhere
to help developers find or confirm their misconfiguration or bad code.

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