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JBoss Fuse - Error Handling workshop

This time I want to talk about the error handling in Camel, when there is calculation, there will be exceptions. I know 99% of the time, it doesn't go the exception, but we don't want this 1% to screw up our entire application. So it is rather important to handle these exceptions carefully.

In Camel, there are three main ways you can handle exceptions,

Error Handler, any exceptions in the message will trigger this handler, so normally I'll use this for a more general base error handling.

On Exception captures the specific exceptions and can then apply different ways to redeliver messages. I often use this with Error Handler, so the on Exception handles the exception that I want to deal with specifically, and let the Error handler handles the rest of other possible errors.

Try and Catch works exactly like what is does in Java, you can use this to handle errors in any particular sections in your route.

The three error handling methods can be define in different scopes. For both Error handler and On Exception, you can define them in both Camel Context scope or Route scope.But there is a little difference, if you want to configure Error Handler to the Camel Context, you have to place your configuration outside of the camel context tag, For On Exception in the camel context scope, you place it inside the camel context tag.

All of three exception handling can be define in the route scope, for Error handler, you place the configuration outside the route tag, and then reference it in the route. For both On Exception and Try n Catch all you have to do is simply add them to your route.

So how about the routes that did not reference anything, well, it's quite simple, the route will inherit the error handler of the camel context.

So there are 5 different types of Error Handler, but here I am only going over the most commonly used ones.

First up is the DefaultErrorHandler, as it's name suggested, this handler is camel's default handler, basically what is does, is to capture the exception, log and redeliver the messages if configured, when everything fails, it'll return the message to caller, which is the very beginning of your route.
The other very commonly used on is the DeadLetterChannel handler, the major difference between this handler and the default one is that this handler moves the message to another location instead of returning it to the caller. The location can be any thing from a messaging queue, database or simple a file folder.

For on exceptions, camel will automatically finds the closest exception it can find and then apply it's redelivery policy to handle the error.
So what are the redeliver policies? Redeliver policies are sets of rules you can configure to control your redelivery methods, such as what is the maximum time you can re-send you messages, what is the gap time between each attempts? Set special patterns to each delivery also sets the log level for different situations.

Try and Catch are exactly like java, where you are going to surround the process that you want to capture the errors with a doTry tag, and each doCatch can catch one or more exception and handles them individually. Note there are several definition you can use inside catch to help you, such as onWhen, onRedeliver and retryWhile.

So here are my slides,

And this time in our lab. we are going to create a brand new project. In this project, we are receiving xml files from partner, and before we actually process the data, we are going to validate it with a xml schema. If the validation failed, I want to place the file into a garbage folder. If it's a valid xml file, I am going to send it to another route to start processing.

In side my other route, I have a process, that is going to randomly throw Exception, it has 30% chance of throwing IOException, 60% of throwing FileNotFoundExceptions and 10% chance of passing the process. For each different exception, we will handle it differently, and if all redelivery failed, we are going to send the xml to a data file for later investigation.

Here are the lab instructions.


Chawki said…
Nice Post.

Can you please share the code?

john jersy said…
Thank you for your is very nice article.
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