Services in Domain Driven Design

Services: Services are reusable classes that encapsulate business logic and operations. They are responsible for performing complex operations, such as business rules validation, calculations, coordination of multiple entities etc..

Services can be classified into three main types:

• Domain services: Domain services encapsulate business logic that is specific to the domain model, but does not belong to any particular entity or value object. For example, a domain service might be responsible for calculating the total price of a shopping cart, or validating a customer's order.
• Application services: Application services mediate between the outside world and the domain model. They are responsible for handling requests from clients, such as web applications or APIs, and translating them into operations on the domain model. For example, an application service might be responsible for placing a customer's order or retrieving a product catalog.
• Infrastructure services: Infrastructure services provide access to external resources, such as databases, messaging systems, and email servers. They are typically used by domain and application services to perform tasks such as persisting data, sending notifications, and calling third-party APIs.

Services should be stateless, meaning that they should not maintain any internal state between calls. This makes them easier to reason about and test. Services should also be designed to be loosely coupled, meaning that they should depend on as few other components as possible. This makes them more reusable and maintainable.