Service granularity

Service granularity
Service granularity refers to the scope of functionality a service exposes.

Fine-grained services might be services that provide a small amount of business-process usefulness, such as basic data access.

Coarse-grained services offer a set of related business functions rather than a single function so they are constructed from lower-level services, components, and objects that are intelligently structured to meet specific business needs. These coarse-grained services can be created from one or more existing systems by defining and exposing interfaces that meet business-process requirements.

Services as composite interfaces
Using coarse-grained interfaces, a system of services controls access to the objects referenced by each service. While each service may be implemented as an abstraction on a group of finer-grained objects, the objects themselves can be hidden from public access. Each service can be implemented by grouping objects, components, and fine-grained services, and exposing them as a single unit through the use of facades or interfaces.

Service composition
Service composition allows you to compose applications and processes using services from heterogeneous environments without regard to the details and differences of those environments. Service composition is much more achievable if the services are constructed with coarse-grained interfaces. With effectively designed and composed coarse-grained services, a business expert can productively compose new business processes and applications.

Finally regarding to Web Service s, I want to mention that Web services technology provides a way of defining coarse-grained services that access the right amount of business logic.