Service

A service is a technical, autonomous unit that bundles related functionality, typically scoped to a single topic or capability, and exposes it to other building blocks through a well-defined interface.

A well-designed service abstracts its internal implementation to such a degree that a consumer need not know or understand how the service works internally in order to use it.

Web services, network services, operating system services, and telecommunications services are all familiar examples of externally accessible services.

Within domain-driven design, service takes on a more specific meaning. It refers to a stateless building block used to implement business logic or processes that don’t naturally belong to any single entity, aggregate, or value object, or which act across several of them at once. Because a domain service holds no state of its own, its operations take domain objects as parameters and return domain objects as results, rather than accumulating any internal data between calls.

The general and DDD senses of service both describe the same underlying idea – a unit of behavior, hidden behind an interface, that other parts of a system can call without depending on its internals – at different scales. That same idea, applied to whole systems composed from many independently deployable services, is what service-oriented architecture and its finer-grained descendant, microservices, are built around.