What is small system? Need to change design if system grows
Entity – objects with a distinct identity. Core of domain model. ValueObject – objects that are primarily defined by the value of their attributes. Often immutable. Two instances whose attributes have the same values can be used interchangeably. Service – implement the workflow of the application. Fulfill a use case. Generally include behaviors that cannot be assigned to a single entity and consist of methods that act on multiple objects. Can be in domain, infrastructure or application level.
Facade – hides complexities of accessing domain model. Like application level services. Define use cases. Contain no business logic themselves. Specification – need to check that only suitable objects are used for a certain role. Keep business rules inside the domain model not leak it out into application layer. If there is large collection it can be same as Query object. Repository – manage collections of entities and define methods for finding and deleting entities. So that clients don't have to care where entities are coming from. Other == DAO but in can be special cases like query some stuff first from some service and then from db. Factory – for creating NEW complex entities. Not like Repositories.
![Introduction to Domain Driven Design ,[object Object],[object Object]](https://image.slidesharecdn.com/ddd-100430010449-phpapp02/85/Domain-Driven-Design-1-320.jpg)
![Sample use case – enter activity ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],[object Object]](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)