Introducing the domain service concept in "Implementing Domain-Driven Design"

1. SERVICES
IDDD CHAPTER 7

2. THREE MAIN ROLES IN DOMAIN
ENTITY VALUE OBJECT
DOMAIN SERVICE
Unique Identity
Stateful
Immutable
Conceptual whole
Replaceability
Side-effect free
Stateless
Domain object interaction
Complex process

3. SOMETIMES, IT JUST ISN’T A THING - ERIC EVANS
DOMAIN SERVICE
▸ A STATELESS operation that fulfills a domain-specific task
▸ An operation is not easy to be defined on an
Aggregate(Entity) or a Value Object
▸ Make sure the operation name is a part of the
UBIQUITOUS LANGUAGE
▸ The only role can invoke repositories in the domain

4. LET’S SEE AN
EXAMPLE

5. PRODUCT
1..*
BACKLOG ITEM
public class Product {
private Set<BacklogItem> backlogItems
public BusinessPriorityTotals businessPriorityTotals() {
// Iterating backlogItems and calculating priority totals
}
}
SO FAR SO GOOD

6. WE CHANGED THE MODEL
PRODUCT PRODUCT ID
BACKLOG ITEM

7. public class {
public static BusinessPriorityTotals businessPriorityTotals(
Set<BacklogItem> backlogItems
) {
// Iterating backlogItems and calculating priority totals
}
}

8. IS THAT THE BEST
CHOICE ?

9. PRODUCT
OR
BACKLOG ITEM ?

10. DOMAIN SERVICE

11. WHAT IS NOT A DOMAIN SERVICE
▸ Different from application services
▸ Not required to be a remote-capable(SOA, RPC, MoM)
Application Service
Domain Service

12. REASONS OF USING DOMAIN SERVICES
▸ Perform a significant business process
▸ Calculate a Value requiring input from more than one
domain object
▸ Transform a domain object from one composition to
another

13. PERFORM A SIGNIFICANT
BUSINESS PROCESS

14. IDENTITY AND ACCESS CONTEXT
▸ Users of a system must be authenticated but can be
authenticated only if the tenant is active
▸ Passwords must be stored encrypted, not as clear text

15. boolean authentic = false;
Tenant tenant = DomainRegistry.tenantRepository().tenantOfId(aTenantId);
if (tenant == null || !tenant.isActive()) {
return authentic;
}
User user = DomainRegistry
.tenantRepository()
.userWithUsername(aTenantId, aUsername);
if (user == null) {
return authentic;
}
authentic = tenant.authenticate(user, aPassword);
return authentic;

16. boolean authentic = false;
Tenant tenant = DomainRegistry.tenantRepository().tenantOfId(aTenantId);
if (tenant == null || !tenant.isActive()) {
return authentic;
}
User user = DomainRegistry
.tenantRepository()
.userWithUsername(aTenantId, aUsername);
if (user == null) {
return authentic;
}
authentic = tenant.authenticate(user, aPassword);
return authentic;
Tenant knowing how to
encrypt the password
Domain logic was leaked
to the client

17. if {
…
} else { … }
Application Layer
for …
x + y
ENCAPSULATE COMPLEX LOGICS IN DOMAIN SERVICE

18. USING A DOMAIN SERVICE
UserDescriptor userDescriptor = DomainRegistry
.authenticationService()
.authenticate(aTenantId, aUsername, aPassword);
YOU CAN RETURN WHAT YOU NEED INSTEAD OF A WHOLE USER
FIT OUR UBIQUITOUS LANGUAGE

19. SEPARATED
INTERFACE

20. Application
Service
Domain
Service
Domain Layer
Application Layer

21. <interface>
DomainService
Application
Service
Domain
Service
Implementation
Infrastructure Layer
Domain Layer
Application Layer
extends

22. package saasovation.identityaccess.infrastructure.services;
import saasovation.identityaccess.domain.model.identity.AuthenticationService;
import …
public class DefaultEncryptionAuthenticationService implement AuthenticationService {
public UserDescriptor authenticate(TenantId tenantId, String username, String password) {
// Checking null value for all of the properties
UserDescriptor userDescriptor = null;
if (tenant == null || !tenant.isActive()) {
return userDescriptor;
}
String encryptedPassword = DomainRegistry.encryptionService().encryptedValue(password);
User user = DomainRegistry.userRepository().userFromAuthenticCredentials(
tenantId, username, encryptedPassword
);
if (user == null || !user.isEnabled()) {
return userDescriptor;
}
return userDescriptor;
}
}

23. IS SEPARATED INTERFACE A NECESSITY ?
▸ Considering the name of your interface
(ex: DefaultService, AuthenticationServiceImp)
▸ The service in the domain will have multiple
implementation
▸ You can still prevent the client from being aware of the
implementation by using DI or a Factory

24. A CALCULATION
PROCESS

25. package saasovation.agile.domain.model.product;
import …
public class BusinessPriorityCalculator {
public BusinessPriorityTotals businessPriorityTotals(Tenant tenant, ProductId productId) {
int totalBenefit = 0; int totalPenalty = 0; int totalCost = 0; int totalRisk = 0;
Collection<BacklogItem> outstandingBacklogItems = DomainRegistry
.backlogItemRepository()
.allOutstandingProductBacklogItems(tenant, productId);
for (BacklogItem backlogItem: outstandingBacklogItems) {
if (backlogItem.hasBusinessPriority()) {
BusinessPriorityRatings ratings = backlogItem.businessPriority().ratings();
totalBenefit += ratings.benefit();
totalPenalty += ratings.penalty();
totalCost += ratings.cost();
totalRisk += ratings.risk();
}
}
BusinessPriorityTotals businessPriorityTotals = new BusinessPriorityTotals(
…
totalBenefit + totalPenalty
);
return businessPriorityTotals;
}
}

26. TRANSFORMATION
SERVICE

28. saasovation.identityaccess
saasovation.collaboration

29. <interface>
CollaboratorService
TranslatingCollaborator
Service
UserInRoleAdapter CollaboratorTranslator
extends
saasovation.collaboration.infrastructure.services
saasovation.collaboration.domain.model.services

30. MAKE SURE YOU NEED A SERVICE
▸ Do not treat Domain Services as a silver bullet
▸ Using Services overzealously will usually result in the
negative consequences of creating an Anemic Domain
Model

31. SWITCH BETWEEN DIFFERENT KINDS OF DEVELOPER
TESTING
Client Developer Domain Developer

32. THANK YOU FOR COMING HERE !