Domain Driven Design 101

AgendaWhyBuilding blocksRepositories, entities, specifications etcPutting it to practiceDependency injectionPersistenceValidationArchitectureChallengesWhen not to use DDDResources

We solve complexity in software by distilling our problems

publicboolCanBook(Cargocargo, Voyagevoyage){doublemaxBooking = voyage.Capacity * 1.1;if (voyage.BookedCargoSize + cargo.Size > maxBooking)returnfalse; ...}publicboolCanBook(Cargocargo, Voyagevoyage){if (!overbookingPolicy.IsAllowed(cargo, voyage))returnfalse; ...}DDD is about making concepts explicit

publicinterfaceISapService{doubleGetHourlyRate(intsapId);}ûA poor abstractionpublicinterfaceIPayrollService{doubleGetHourlyRate(Employeeemployee);}üIntention-revealing interfaces

publicclassEmployee{voidApplyForLeave(DateTime start,DateTime end,ILeaveService leaves) { ... }}

publicclassEmployee : IEquatable<Employee>{publicbool Equals(Employee other) {returnthis.Id.Equals(other.Id); }}Entities are the same if they have the same identitypublicclassPostalAddress : IEquatable<PostalAddress>{publicbool Equals(PostalAddress other) {returnthis.Number.Equals(other.Number) && this.Street.Equals(other.Street) && this.PostCode.Equals(other.PostCode) && this.Country.Equals(other.Country); }}Value Types are the same if they have the same value

publicclassColour{publicint Red { get; privateset; }publicint Green { get; privateset; }publicint Blue { get; privateset; } publicColour(int red, int green, int blue) {this.Red = red;this.Green = green;this.Blue = blue; } publicColourMixInTo(Colour other) {returnnewColour(Math.Avg(this.Red, other.Red),Math.Avg(this.Green, other.Green), Math.Avg(this.Blue, other.Blue)); }}Value Types are immutable

publicinterfaceIEmployeeRepository{EmployeeGetById(int id);void Add(Employeeemployee);void Remove(Employeeemployee); IEnumerable<Employee> GetStaffWorkingInRegion(Regionregion);}Repositories provide collection semantics and domain queries

publicinterfaceITripService{floatGetDrivingDistanceBetween(Location a, Location b);}

classGoldCustomerSpecification : ISpecification<Customer>{publicboolIsSatisfiedBy(Customer candidate) {returncandidate.TotalPurchases > 1000.0m; }} if (newGoldCustomerSpecification().IsSatisfiedBy(employee))// apply special discountSpecifications encapsulate a single rule

Specifications can be used…to construct objects

var spec = newPizzaSpecification() .BasedOn(newMargaritaPizzaSpecification()) .WithThickCrust() .WithSwirl(Sauces.Bbq) .WithExtraCheese(); var pizza = newPizzaFactory().CreatePizzaFrom(spec);Constructing objects according to a specification

Specifications can be used…for querying

publicinterfaceICustomerRepository{IEnumerable<Customer> GetCustomersSatisfying(ISpecification<Customer> spec);}vargoldCustomerSpec = newGoldCustomerSpecification(); var customers = this.customerRepository .GetCustomersSatisfying(goldCustomerSpec);Querying for objects that match some specification

Your subsystemAnti-corruption layerOther subsystem

Any 3rd party system that I have to integrate with was written by a drunken monkey typing with his feet.Oren Eini aka Ayende

publicclassLead{publicIEnumerable<Opportunity> Opportunities { get; }publicPerson Contact { get; }}publicclassClient{publicIEnumerable<Invoice> GetOutstandingInvoices();publicAddressBillingAddress { get; }publicIEnumerable<Order> PurchaseHistory { get; }}publicclassCustomer{publicIEnumerable<Ticket> Tickets { get; }}

publicinterfaceINotificationService{void Notify(Employeeemployee, string message);}An interface defines the modelpublicclassEmailNotificationService : INotificationService{ void Notify(Employeeemployee, string message) {var message = newMailMessage(employee.Email, message);this.smtpClient.Send(message); }}Far away, a concrete class satisfies it

publicclassLeaveService{privatereadonlyINotificationService notifications; publicLeaveService(INotificationService notifications) {this.notifications = notifications; } publicvoidTakeLeave(Employeeemployee, DateTime start,DateTime end) {// do stuff this.notifications.Notify(employee, "Leave approved."); }}Dependencies are injected at runtime

…ordinary classes where you focus on the business problem at hand without adding stuff for infrastructure-related reasons… nothing else should be in the Domain Model.

publicclassCustomer{publicint Id { get; privateset; }publicstringFirstName { get; set; }publicstringLastName { get; set; } publicIEnumerable<Address> Addresses { get; }publicIEnumerable<Order> Orders { get; } publicOrderCreateOrder(ShoppingCart cart) { ... }}Plain Old CLR Object (POCO)

[global::System.Data.Objects.DataClasses.EdmEntityTypeAttribute(NamespaceName="AdventureWorksLTModel", Name="Customer")] [global::System.Runtime.Serialization.DataContractAttribute(IsReference=true)] [global::System.Serializable()]publicpartialclassCustomer : global::System.Data.Objects.DataClasses.EntityObject { [global::System.Data.Objects.DataClasses.EdmScalarPropertyAttribute(EntityKeyProperty=true, IsNullable=false)] [global::System.Runtime.Serialization.DataMemberAttribute()]publicintCustomerID {get {returnthis._CustomerID; }set {this.OnCustomerIDChanging(value);this.ReportPropertyChanging("CustomerID");this._CustomerID = global::System.Data.Objects.DataClasses.StructuralObject.SetValidValue(value);this.ReportPropertyChanged("CustomerID");this.OnCustomerIDChanged(); } }privateint _CustomerID;partialvoidOnCustomerIDChanging(int value);partialvoidOnCustomerIDChanged(); This is not a POCO.

Traditional ArchitecturePresentationBusiness Logic (BLL)InfrastructureData Access (DAL)

Onion ArchitectureUser InterfaceGApplication ServicesMDomain ServicesDatabaseDomain ModelServicesFilesystemInfrastructureTestsetc

Onion ArchitectureEmployeeControllerUser InterfaceGApplication ServicesMIEmailSenderDomain ServicesDatabaseDomain ModelServicesFilesystemEmployee,IEmployeeRepositoryInfrastructureTestsSmtpEmailSenderetcNHibernateEmployeeRepository

Validation ExamplesInput validationIs the first name filled in?Is the e-mail address format valid?Is the first name less than 255 characters long?Is the chosen username available?Is the password strong enough?Is the requested book available, or already out on loan?Is the customer eligible for this policy?Business domain

publicclassPersonRepository : IPersonRepository{publicvoid Save(Person customer) {if (!customer.IsValid())thrownewException(...) }}validation and persistence anti-patterns

The golden rule for validation:The Domain Model is always in a valid state

publicclassNewUserFormValidator : AbstractValidator<NewUserForm>{IUsernameAvailabilityServiceusernameAvailabilityService; publicNewUserFormValidator() {RuleFor(f => f.Email).EmailAddress(); RuleFor(f => f.Username).NotEmpty().Length(1, 32) .WithMessage("Username must be between 1 and 32 characters"); RuleFor(f => f.Url).Must(s => Uri.IsWellFormedUriString(s)) .Unless(f => String.IsNullOrEmpty(f.Url)) .WithMessage("This doesn't look like a valid URL"); RuleFor(f => f.Username) .Must(s => this.usernameAvailabilityService.IsAvailable(s)) .WithMessage("Username is already taken"); }}separation of validation concerns with FluentValidation

Where validation fitsEmployeeControllerUser InterfaceGApplication ServicesMIEmailSenderDomain ServicesDatabaseEmployee,IEmployeeRepositoryDomain ModelServicesNewUserForm,NewUserFormValidatorIOverdraftLimitPolicyFilesystemInfrastructureTestsSmtpEmailSenderIUsernameAvailabilityServiceetcNHibernateEmployeeRepository

LinksDomain Driven Design mailing listhttp://tech.groups.yahoo.com/group/domaindrivendesign/ALT.NET mailing listhttp://tech.groups.yahoo.com/group/altdotnet/DDD Step By Stephttp://dddstepbystep.com/Domain Driven Design Quickly (e-book)http://www.infoq.com/minibooks/domain-driven-design-quickly

Domain Driven Design 101

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