Component Implementation Guidelines

I've recently worked on a couple Mule ESB projects that make heavy use of components. The projects were similar in the sense that the component logic was implemented from scratch. We were not, for instance, exposing existing Java classes out over a transport.

It was challenging at times to implement these components while maintaining best-practices with decoupling the code from Mule and unit-testing it. The below are some approaches / guidelines when implementing components that keep these goals in mind:

• Be careful when implementing Callable. Implementing the Callable interface gives you access to the MuleEventContext when the "onCall" method is invoked. This allows you to invoke the endpoint's transformers, access the MuleMessage directly, stop message processing, etc. It also tightly couples your component code to the Mule infrastructure, making the component more difficult to unit-test in isolation as well as refactor if the Mule API changes. See if your component's reliance on the MuleEventContext can be refactored out to transformers of exception-strategies.
• Favor using a canonical data model over interacting with MuleMessage directly. You can use a transformer to move the external data into a common format, like a domain model class, an XML document or a hash-map. This again decouples your code from the Mule API, making unit-testing, refactoring, etc easier.
• Hide MuleClient usage behind a service interface. MuleClient is an extremely useful facility to send and receive messages over arbitrary transports. Its usage introduces the same difficulties as the items above. This can be mitigated by abstracting the MuleClient invocations into a service class that can be injected into the component. A mock implementation of this class can then be used in unit tests.
• Consider using jBPM to orchestrate activity and maintain state. Component code, along with MuleClient, can be a tempting place to orchestrate, compose and maintain state across endpoint invocations. But be careful when considering this approach. You'll need to consider what happens if the component logic is interrupted during execution, if the state needs to be transient across Mule restarts, if the state needs to be transient between Mule nodes, etc. Many, if not most, of these issues can be addressed by using jBPM in conjunction with Mule's BPM transport.
• Avoid using hardcoded strings in endpoint names with MuleClient. This is obvious but is worth mentioning. Try to centralize all the endpoint names in one place (in an enum, for instance) and inject this into your service facades. This helps avoid issues with typos in endpoint addresses.

Hopefully some of the above will give you component implementation / configurations that are easy to unit-test and refactor. Any other tips / guidelines are welcomed in the comments :)