Hello all,
Here is my problem. I have a very classical configuration where a webapp
with some Hibernate code in it accesses a MySQL DB. Everything is
installed in Tomcat 6.
After some time, Hibernate complains about dropped connections, the
error message looks like this :
com.mysql.jdbc.CommunicationsException : Communications link failure The
last packet successfully received from the server was 204 734
milliseconds ago. The last packet sent successfully to the server was 0
milliseconds ago.
java.io.EOFException : Can not read response from server. Expected to
read 4 bytes, read 0 bytes before connection was unexpectedly lost.
Refeshing the page once or twice usually fixes the problem.
I could narrow the problem and reproduce it on demand, juste by setting
the wait timeout parameter of MySQL to a very low value : 10s (the
default value is 8 hours). If I use my webapp fast enough (less than 10s
between clicks) everything runs fine. If I wait just 12s, this problem
shows up, 100% of the time. Reloading the page once or twice fixes it.
So my guess here is that Hibernate is provided with a closed connection
(please correct me if you think I'm wrong on that point). Setting
autoReconnect=true on the connection string doesnt help (I red somewhere
that it was not expected to help in the case of MySQL for some reason,
so I guess it's normal).
The configuration is the following : the datasource is configured in a
context.xml file in the META-INF dir of the webapp.
<Resource name="jdbc/myds" auth="Container" type="javax.sql.DataSource"
maxActive="20" maxIdle="10" maxWait="3"
username="***" password="***"
driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost/mydb?useUnicode=true&characterEncoding=utf8"/>
Then the webapp uses it through a declaration in a persistence.xml file
(JPA standard file).
<persistence-unit name="MyDataService">
<provider>org.hibernate.ejb.HibernatePersistence</provider>
<non-jta-data-source>java:/comp/env/jdbc/myds</non-jta-data-source>
<!-- JPA stuff removed -->
</persistence-unit>
To be sure that DBCP is not providing closed connections to Hibernate, I
added the following to the Resource element, but with no success :
<Resource name="jdbc/myds" auth="Container" type="javax.sql.DataSource"
maxActive="20" maxIdle="10" maxWait="3"
username="***" password="***"
driverClassName="com.mysql.jdbc.Driver"
url="jdbc:mysql://localhost/mydb?useUnicode=true&characterEncoding=utf8"
validationQuery="select 1" testOnBorrow="true"/>
From what I understand in the DBCP docs :
- the validation query has to be set to some legal value for DBCP to
test the connection, which is the case
- testOnBorrow is true by default, setting it again cant harm can it ?
I tried to tune timeBetweenEvictionRunsMillis, numTestsPerEvictionRun
and minEvictableIdleTimeMillis with testWhileIdle set to true, but with
no more success.
Any hint would be much appreciated, especially on the logging side
(Tomcat / DBCP), since I cant see how to get more information on what is
going on with those buggy connections.
Thank you,
José
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