If you have a large piece of functionality to develop, with a complex Service layer and Visualforce client logic, it can be an advantage to decouple these two streams of development activity so that developers can continue in parallel, meeting up sometime in the future to combine efforts.

An approach to this is sometimes referred to as Contract Driven Development. This is where there is an agreement on a contract (or service definition) between the two development streams before they start their respective developments. Naturally, the contract can be adjusted over time, but having a solid starting point will lead to a smoother parallel development activity.

This type of development can be applied by implementing a small factory pattern within the Service layer class. The main methods on the service class are defined as normal, but their internal implementation can be routed to respective inner classes to provide an initial dummy implementation of the service, which...