Friday, April 30, 2010

Grafting Mule Endpoints

Note: The following code samples are applicable to Mule 2.

In Mule ESB, outbound dispatching to a destination whose address is known at runtime only is a pretty trivial endeavor. A less frequent practice consists in programmatically defining inbound service endpoints.

I recently had to do such thing for a little side project I'm running where Mule is used as a frontal bus and load throttler in front of a R nodes exposed over RMI. The goal was to have a non-fixed number of file inbound endpoints defined in a simple properties file and declare them on a particular service during the initialization sequence of Mule.

As an integration framework, Mule ESB exposes all its moving parts and lets you configure them easily with its Spring-powered XML DSL: that's all we need to achieve the above goal.

Let's first look at the resulting service configuration:

As you can see the inbound router doesn't have any endpoint configured on it. This is where we will programmatically graft the file endpoints configured in an external properties file.

Before digging into the code used for this grafting operation, let's look at how the grafter itself is configured:

Unsurprisingly, we use a Spring configured POJO to perform the endpoints generation. Notice how the service and the file connector are referenced: instead of using names I'm directly referencing Mule configuration elements. Because Spring is used consistently being the scene, this kind of cross referencing is possible and the key to many advanced tricks!

Now take a deep breath and take a look at the code in charge of grafting the endpoints to the target service:

The important things to pay attention to are the following:
  • The class implements MuleContextAware in order to receive an instance of the MuleContext, which is the key to the gate of Mule's innermosts. Some might consider fetching this class from the connector object that gets injected in this class too: I personally find this less desirable for design reasons that I'll let Demeter explain.
  • The endpoint is bound to the desired connector by passing its name in the URI used to create it. This allows picking up the right connector, which is compulsory for any Mule configuration with more than one instance of a particular connector (file connectors in this case).
  • Endpoint specific configuration parameters, like moveToDirectory, are configured as extra URI parameters. You can also add other parameters, as key/value pairs: they will be automatically added to the message properties dispatched from this endpoint.
And voila, though you may never have to do this kind of things in your Mule ESB projects, you've gained some deeper experience into what a reasonably skilled gardener can do with this powerful platform.