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With so many integration points and applications of , troubleshooting issues can be a daunting task for those just starting out with Mule.  This post will provide a few tips on how to get to the root of issues you may encounter.

Set up your IDE for debugging
One of the “biggest bang for the buck” things you can do is to take a few minutes to set up your favorite IDE for Mule debugging.  My IDE of choice is Eclipse, but there are also directions on setting up IntelliJ here.  I would highly recommend installing the MuleIDE eclipse plugin.  allows you to quickly create new Mule projects and to run Mule configurations against an installation with a few clicks of the mouse.

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Log4J settings
In addition to attaching a debugger for troubleshooting, you can bump up the logging level in order to dig deeper into what's going on inside of Mule.  If you are running Mule standalone, there is a log4j configuration file located at MULE_HOME/conf/  If you have narrowed down your issue to a certain package, you can enable debug logging just for that package so you are not overwhelmed by a barrage of non-related logging messages.  For instance, if you just want to get more logging for the http transport, you can add the following line to your file:

You can read more about configuring log4j properties here

Check your environment
If you are like me, you have several JVMs installed on your system and they can easily get mixed up when executing Mule on the command line, from an IDE, from an application server, etc. It is especially important to know which jvm you are using when you are building Mule code as the jvm needs to have JAXP implementation jars under the jre/lib/endorsed directory.

I run on Linux and for working on the command line I set up an environment file which gets automatically sourced when I log in.  I put this at the end of my ~/.bashrc file:

export M2_HOME=~/apache-maven-2.0.9
export ANT_HOME=~/apache-ant-1.7.1
export JAVA_HOME=/usr/lib/jvm/java-1.5.0-sun-
#export JAVA_HOME=/usr/lib/jvm/java-6-sun-
export PATH=$JAVA_HOME/bin:$M2_HOME/bin:$ANT_HOME/bin:$PATH

In an IDE such as Eclipse, you can add additional JREs under Window -> Preferences -> Java -> Installed JREs.

In an application server, there are usually shell files which you can edit to add the appropriate environment settings to.  For example, in Tomcat you can specify your jvm and jvm options in the TOMCAT_HOME/conf/ file.

Peel back the layers; take a step back
It's very easy to lose site of the forest for the trees when running Mule.  For example, take a configuration file which you embed in a war file and deploy it to an application server.  When the app server starts up, it may simply log that it cannot deploy the war.  In some cases you can start your troubleshooting by trying to run your configuration file in a standalone Mule installation so you can rule out your application server configuration as the culprit.  When that's not an option such as when your mule configuration is tightly integrated with app server resources or the war file, try changing the logging levels and make sure that the necessary ports are open.  The key is to try and simplify your environment so you can quickly get to the root of the issue.

Check Jira for known issues
Sometimes an issue you are encountering has already been reported by someone else.  Make use of the Jira search function to see if you can find your issue and if it's already been fixed in a later version.  Jira can be accessed here.  The ‘Quick Search' tool at the top right of the page works very well to search for exception messages or keywords.

When all else fails, don't be afraid to ask for help!
If you find yourself struggling with an issue, remember that this is a community project.  There are lots of experienced people reading the mailing lists ready to help out.  This includes both community members and MuleSource engineers who constantly monitor and respond to posts on the mailing lists.  Here are a few tips before you post