Services

Services

After you complete the domain models, you can start building the services. The following describe a service:

• It performs a significant piece of business logic within the domain.
• It calculates some values.
• It interacts with the repository layer.

Try to push down as much logic as possible into your domain objects (i.e. create service functions within the domain .go files.)

Domain services provide services within the domain, while application services are used to compose other services and repositories–they do not contain domain logic.

Example

A student wants to register for a class. You have the Student entity:

type Student struct {
	ID        int
	firstName string
	lastName  string
	courses   []Class
}

And you have the Class entity:

type Class struct {
	ID            int
	Name          string
	students      []Student
	maxEnrollment int
}

Next, we want to register a student for the class. You might consider creating a Student.AddClass or Class.AddStudent method, but either of those methods would require that you reference another entity and perform business logic. For example:

// This is bad
func (s *Student) AddClass(c *Class) error {
    // validation logic
    if len(c.students) > c.maxEnrollment {
        return errors.New("class is full")
    }
    // ...
    // Class business logic
    c.append(c.students, s)
    return nil
}

The solution is to add a RegistrationService. First, define the struct:

// RegistrationService registers students for the class.
type RegistrationService struct {
	class *Class
}

Next, create a constructor that injects a Class instance into the service:

func NewRegistrationService(c *Class) *RegistrationService {
	return &RegistrationService{class: c}
}

Finally, define the RegisterStudent method on the RegistrationService:

func (r *RegistrationService) RegisterStudent(s *Student) error {
	if len(r.class.students) > r.class.maxEnrollment {
		return errors.New("class is full")
	}
	if r.class.contains(s) {
		return errors.New("student is already enrolled in this class")
	}
	r.class.students = append(r.class.students, *s)
	return nil
}