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From Daniel Mikusa <dmik...@vmware.com>
Subject Re: JDBC Pool - Error handling during connection creation
Date Thu, 08 Mar 2012 19:14:44 GMT
On Thu, 2012-03-08 at 08:55 -0800, Amit wrote:
> 
> 
> 
> On 08-Mar-2012, at 6:37 PM, Daniel Mikusa <dmikusa@vmware.com> wrote:
> 
> > On Thu, 2012-03-08 at 00:34 -0800, amit shah wrote:
> >> Comment below
> >> 
> >> On Wed, Mar 7, 2012 at 8:53 PM, Christopher Schultz <
> >> chris@christopherschultz.net> wrote:
> >> 
> >>> -----BEGIN PGP SIGNED MESSAGE-----
> >>> Hash: SHA1
> >>> 
> >>> Amit,
> >>> 
> >>> On 3/7/12 12:12 AM, amit shah wrote:
> >>>> I am using tomcat-jdbc.jar and tomcat-juli.jar from version
> >>>> 7.0.26.
> >>>> 
> >>>>> I don't see any place in setupConnection where an exception is
> >>>>> swallowed, do you?
> >>>>> 
> >>>>> 
> >>> http://svn.apache.org/viewvc/tomcat/trunk/modules/jdbc-pool/src/main/java/org/apache/tomcat/jdbc/pool/ConnectionPool.java?view=markup
> >>>> 
> >>>>> 
> >>>> Have a look at the public boolean validate(int
> >>>> validateAction,String sql) method in PooledConnection class line no
> >>>> - 445.
> >>> 
> >>>> The validate method ignores any exception which is thrown while
> >>>> validating the connection or executing the initSQL query.
> >>> 
> >>> If you have debug logging enabled, it will log the exception. So,
> >>> enable debug logging and try again. *shrug*
> >>> 
> >>>> The createConnection() method in ConnectionPool class which calls
> >>>> the validate() method returns null in such a case and hence it
> >>>> leads to a null pointer exception in setupConnection method.
> >>>> 
> >>>> Ignoring the exception in the validate() method may sound
> >>>> appropriate (not sure for what reason though) but my point is that
> >>>> it makes troubleshooting the issue a lot harder.
> >>> 
> >>> Just turn on debug logging and you'll see the original exception. I
> >>> think in this case, the error is quite fatal so checking for null and
> >>> emitting some other error message would only be slightly more helpful
> >>> to you, but the effect would be the same.
> >>> 
> >> 
> >> I understand that turning the log level to debug will display the original
> >> exception. The point is in production environments the log level would be
> >> by default info and it would take a longer time to understand the root
> >> cause.
> > 
> > Why not just set the log level to debug for that particular class?  The
> > logging in that class seems to be minimal, even at debug level.
> 
> Enabling debug level would add some extra handling
> 
> 1. We use slf4j & logback as our logging framework & tomcat uses jul logging.
We would have to add jul-to-slf4j.jar to direct the jul messages to logback. This adds a performance
overhead since the validation code & this class would be executed frequently.

I would be a little surprised if it added significant overhead as there
is only a very small amount of logging in the PooledConnection class.
In addition, what logging does exist in that class seems to reside in
catch blocks.  In other words, it doesn't seem like it would log
anything unless there is a problem.

That being said, I did not try this or do any performance testing on it,
so I can't really say exactly how it would affect performance.  If
you've tested and the performance overhead is unacceptable then I'll
take your word for it.

> 
> 2. Specially adding a logger in logback.xml for this particular class. We have multiple
web applications which do not share the logback.xml currently.
> 
> If the exception is not ignored and thrown right where it occurred, the stack trace would
be enough to understand the root cause of the issue. 
> 
> > 
> >> 
> >> In our case, we execute a SP as an initSQL query, so the reasons why the
> >> query fails would increase. Hence it would been easier if the validate()
> >> method throws the exception instead of ignoring it.
> > 
> > I understand that you would like to get access to the exception, but can
> > you explain why you would want to do this?  What would you do with the
> > exception?  Where would you handle the exception?
> > 
> 
> I do not want to handle the exception. It would just help in understanding the root cause
in secs instead of mins since there could many reasons why establishing a connection could
fail. Seeing a null pointer exception does not help at all.

I get that the null pointer exception doesn't help. I've been there and
experienced that fun myself.  However, it was just a quick matter of
enabling debug logging to see what is really happening.

Can't really say much more about this.  It would seem that you have two
choices: 

1.) Enable debug logging and see the exception.

2.) Make an adjustment to the source code and modify the exception
handling to fit your needs.

Dan



> 
> I would appreciate if someone could detail on the benefits on having the code the way
it is currently implemented i.e. why the exception is ignored instead of just throwing it
there right away?
> 
> > Dan
> > 
> >> 
> >> Do you see any issues in throwing the exception right there?
> >> 
> >> 
> >>> 
> >>> - -chris
> >>> -----BEGIN PGP SIGNATURE-----
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> >>> =AzPZ
> >>> -----END PGP SIGNATURE-----
> >>> 
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