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MySql connections autodrop after a certain hours


MySql is configured to drop any connection which has been Idle for more than 8 hours. What is the implication of this? After you return to your deployed app after a gap of 8 hours (If default SQL parameters have not been changed), you will be greeted with an exception.

How to solve this issue?

  1. Increase the wait_time parameter -Not a good Idea, it might unnecessarily hold on to the resources and not might be a sure shot way. Apart from that, being dependent on an “external” configuration for failover is not a very good idea -what if the server itself crashes, what if this configuration is missed in one of the instnaces, and many such issues will pop up against this approach.
  2. Use the parameter autoReconnect=true with JDBC URL -My SQl itself does not recommend this, have a look at link and people have reported that this does not work as well, refer link.
  3. Custom handling -have your code identify that connection has been lost and then recover it and try to reconnect, but then it would be a lot of fail over mechanism in code.
  4. The best way I found was to configure pooling mechanism as c3p0. See this post how to configure c3p0 in JPA for hibernate, it’s simple, easy and reliable.

So how do you test that issue is solved?

  1. Change wait_timeout in MySql to just 2 minutes, this is how it can be done from MySql workbench admin console mysql_timeout
  2. Keep value of idleTestPeriod less than wait_timeout -A quick recap what idleTestPeriod signifies
  3. idleTestPeriod:  default value=0; If this is a number greater than 0, c3p0 will test all idle, pooled but unchecked-out connections, every this number of seconds
    
  4. Login after wait_timeout has passed -it should not throw a exception

Solving classloading issue while adding pooling support using c3p0 in JPA with hibernate underneath


I added c3p0 for pooling in JPA, but encountered an exception

Unable to load class [org.hibernate.service.jdbc.connections.internal.C3P0ConnectionProvider]

My configuration looked like

<property name="hibernate.connection.provider_class"
          value="org.hibernate.connection.C3P0ConnectionProvider" />
<property name="hibernate.c3p0.max_size" value="10" />
<property name="hibernate.c3p0.min_size" value="0" />
<property name="hibernate.c3p0.acquire_increment" value="5" />
<property name="hibernate.c3p0.idle_test_period" value="60" />
<property name="hibernate.c3p0.max_statements" value="0" />
<property name="hibernate.c3p0.timeout" value="100" />	    

Details about these properties and other defined at link

Looking at log trace it’s not difficult to figure out that jar is not correct, so first change, upgrade to latest c3p0 artifact.

Previous

<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-c3p0</artifactId>
    <version>3.6.10.Final</version>
</dependency>

Latest

<dependency>
    <groupId>org.hibernate</groupId>
    <artifactId>hibernate-c3p0</artifactId>
    <version>4.1.9.Final</version>
</dependency>

After changing this, it worked but printed an Warning

WARN - HHH000208: org.hibernate.connection.C3P0ConnectionProvider has been deprecated in favor of org.hibernate.service.jdbc.connections.internal.C3P0ConnectionProvider; that provider will be used instead.

This indicates that provider_class should be changed to remove the depricated class, so it should be

<property name="hibernate.connection.provider_class"
          value="org.hibernate.service.jdbc.connections.internal.C3P0ConnectionProvider" />

This cleanly integrated the c3p0 implementation.

How to log query parameters passed to JPA queries


I am using JPA with hibernate underneath. Using the property

<property name="hibernate.show_sql" value="true"/>

Shows just the SQL and not the values of parameters passed -it displays ? marks for query parameter as incomeexpe0_.transactiondate)>=?
Want to know what was the exact parameters passed -This is real helpful for debugging.
This can be achieved using hibernate logging -Add following two lines in your log4J.properties file

log4j.logger.org.hibernate.SQL=TRACE
log4j.logger.org.hibernate.type=TRACE

If this is used “log4j.logger.org.hibernate.SQL=TRACE” -No need to use property hibernate.show_sql, it wil take care of dumping queries.
The secondstatement, “log4j.logger.org.hibernate.type”, logs the JDBC parameters passed to a query

TRACE [main] (BasicBinder.java:83) - binding parameter [1] as [TIMESTAMP] - Thu Jan 03 01:00:18 IST 2013
TRACE [main] (BasicBinder.java:83) - binding parameter [2] as [TIMESTAMP] - Thu Jan 03 23:00:19 IST 2013

For even more advanced analysis and precise JDBC formatted queries (Non in an ORM form, but REAL sql), jdbc proxy driver like P6Spy can be used.

Using hbm2ddl.auto in hibernate


What does this tag do : hbm2ddl.auto

It tells hibernate to automatically sync up the table structure –Pojo to DB layer schema. There are couple of options which can go here:

validate: validate the schema, makes no changes to the database.
update: update the schema.
create: creates the schema, destroying previous data.
create-drop: drop the schema at the end of the session.

I suggest not to use this tag even for development, using it in production is any way nothing less than sin.

In a stressed environment (as it happened with me today), one might forget that this small tag is doing some magic and “automatically” changing the DB structure/deleting/ records et all.

1. I was running a test (DB layer test) for validating a schema change –and it was supposed to fail, because SQL has not been run.
2. Instead all the tests were passing, see even great news sometime smell bad.
3. A couple of tests later, When I verified the DB schema I saw all the DB “Automatically” updated with new schema.
4. As I had earlier worked on hibernate, It clicked and I verified the config file and got the answer to mystery.
5. Same would not have happened if I did not had experience in ORM, or at least it would have taken some time/help.