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From "Steve Biondi" <Ste...@schemalogic.com>
Subject Implicit thread-local SqlMapSessions causing hang in WAS
Date Mon, 13 Jun 2005 18:29:33 GMT
Hello all,
  I wanted to share a hairy issue we discovered recently and see if
people had other solutions, experiences or enhancement ideas. I recently
joined a Servlet project using iBatis and Hibernate. When iBatis
statements were needed, they would attach the current Connection object
to the SqlMapClient object using setUserTransaction and execute
insert/update, etc. This worked great. 
 
One of the first things I had to do was to test the app in Websphere. In
Tomcat everything worked great, but we found that the app would hang in
WAS after a while. After much pulling of hair, an IBM support engineer
showed us how to dump the JVM state to disk. We found that after WAS
hung, all the Servlet threads were in the wait state inside the class
common.util.Throttle. Apparently, if you simply call setUserConnection
and not openSession, the client *implicitly* creates a SqlMapSession and
stores it in a ThreadLocal variable (and hence its thread-safe
internally). The problem was that nothing would call the close method on
this after it was created! Because of this, each new Thread object would
cause a new SqlMapSession to be created and stored. Once we hit the
magic number of 129 distinct threads over time, the app would hang. (The
default number of active sessions is 128.) As it turns out, WAS
continuously creates new threads, whereas Tomcat does so much less
often. The Thread creation logic was even more rapid is WAS 6.0.
 
My fix was to instead call openSession(Conn) and then call session.close
at the end of the request to "checkin" the SqlMapSession an decrement
the Throttle counter for SqlMapSessions. I couldn't figure out how to
close the implicit session using the SqlMapClient interface.  Perhaps it
would be useful to add a method to SqlMapClient such as
"closeImplicitThreadSession" or something like that?
 
Thanks much,
 
Steve

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