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From "Craig R. McClanahan" <craig...@apache.org>
Subject RE: [DBCP] Please let us start to improve the connection pool
Date Mon, 24 Mar 2003 23:49:59 GMT


On Mon, 24 Mar 2003, Danny Angus wrote:

> Date: Mon, 24 Mar 2003 22:28:29 -0000
> From: Danny Angus <danny@apache.org>
> Reply-To: Jakarta Commons Developers List <commons-dev@jakarta.apache.org>
> To: Jakarta Commons Developers List <commons-dev@jakarta.apache.org>
> Subject: RE: [DBCP] Please let us start to improve the connection pool
>
> Craig,
>
> > IMHO, any application that depends on the connection pool for recovering
> > abandoned connections (whether or not it recycles them) is broken.  Far
> > better is to focus your energy on avoiding all the cases where you grab a
> > connection from the pool but fail to return it for some reason.
> >
> > One simple way to do this is to encapsulate your JDBC-using code in a
> > try/catch/finally block, something like this:
>
> I haven't used DBCP for anthing yet, though we're proposing to use it
> for James in place of our homespun pool.
>
> However from what I understand of this discussion there are two things
> going on here;
>
> Thing one, I agree with you, code failing to return connections to the
> pool should lead to failure, and the sooner this can be done the better
> for identifying such broken code.
>
> Thing two, DBCP will apparently make a value judgement about an assigned
> connection, and is capable of recycling it with no notification to the
> code which has checked it out.
>

will --> can be configured to

> In my opinion thing two is wrong or incomplete as it creates a situation
> where potential failure is built in, difficult to reproduce and
> difficult identify the cause of.
>

There is a lot of flexibility in how you configure DBCP -- most of it
inherited from the commons-pool capabilities that lie underneath.  In all
cases, the use of these features is configurable.

* Test On Borrow -- The pool can perform some tests on an object instance
  that the pool is about to return to the application that asked for it.

* Test On Return -- The pool can perform the same sorts of tests, but this
  time when the application returns the instance to the pool.

* Test While Idle -- The pool can perform checks on connections that are
  in the pool (not currently allocated to an application) to make sure
  that it is still valid.

DBCP (in particular, the BasicDataSource class found there) implements
flags to enable each of these behaviors individually, along with a
validation query that is used to perform the test.  In addition, there are
tuning parameters to control things like how often the "test when idle"
checks are performed, and how many connections are tested each time.

> In the case of JDBC connection pooling may be reasonable to want to keep
> a connection even when it is idle, because connections can aquire state
> which is expensive to reproduce. Is it not, then, unresonable to allow
> the pool to silently and forcably recycle apparently idle but valuable
> connections?
>

Keeping physical JDBC connections open between uses by the app, and
allowing those connections to be shared, is the whole point of DBCP :-).

But I think we might be using the term "recycle" differently.

By "recycle", do you mean "if a connection has been setting in the pool
for a long time and is not allocated to an application, so we can close it
now"?  If so, that behavior is already configurable -- just set the
maxIdle property to zero and no idle connection will ever get harvested.
There's no application to notify in this case, because the instance being
harvested wasn't allocated to an application at the time.

Or, by "recycle", do you mean "if a connection has been allocated to an
application but not returned for a long time, the pool is allowed to grab
it back again -- but you want to notify the application first".  The "grab
it back" behavior is already configurable (and not enabled by default) --
and it's this functionality that I object to having at all.  If there was
no "grab it back" we wouldn't have to worry about notifying anyone that it
was about to happen :-).

> My solution would either be to make it possible to turn off the forcable
> recycling of connections, or to make the pool capable of notifying code
> that its connection has been recycled.
>

You can already turn off the forcible recycling -- in fact, you explicitly
have to turn it *on* (i.e., for BasicDataSource, you have to explicitly
set the "removeAbandoned" property and related values to enable it).

But, I contend that no one should ever do that :-).

> Is that reasonable?

If we're talking about the second use of "recycled" above, IMHO, I think
adding support for recovering abandoned connections at all was a mistake.
Doing anything to make it work "better" (knowing all the while that it
cannot be made "perfect") simply perpetuates the mistake.  I'd much rather
see this whole area of functionality deprecated, rather than continuing to
mislead people into believing that its OK to depend on something that
cannot ever work reliably 100% of the time.

But that's just my opinion.

>
> d.
>

Craig

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