Bill de hÓra: Foundations for component and service models:

The core problem of component and service architectures is easy to express: how do I change a published interface without breaking the callers?

...Use highly generic method calls that are qualified with metadata - this is how SMTP and HTTP work and is often cited as a reason for their phenomenal success. You'll notice that these are the polar opposite of models such as the EJB spec, which actively encourage you to pile on the methods and go n^2.