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Unbreakable Domain Models PHPUK 2014 London

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Mathias Verraes
Mathias Verraes

Data Mappers (like Doctrine2) help us a lot to persist data. Yet many projects are still struggling with tough questions: -Where to put business logic? -How to protect our code from abuse? Where to put queries, and how test them? Let’s look beyond the old Gang of Four design patterns, and take some clues from tactical Domain Driven Design. At the heart of our models, we can use Value Objects and Entities, with tightly defined consistency boundaries. Repositories abstract away the persistence. Encapsulated Operations helps us to protect invariants. And if we need to manage a lot of complexity, the Specification pattern helps us express business rules in the language of the business. These patterns help us evolve from structural data models, to rich behavioral models. They capture not just state and relationships, but true meaning. The presentation is a fast paced introduction to some patterns and ideas that will make your Domain Model expressive, unbreakable, and beautiful.

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Unbreakable Domain Models @mathiasverraes
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All models are wrong, but some are useful.
I’m Mathias Verraes I'm an independent consultant. I help teams build enterprise web applications.
Blog verraes.net ! Podcast with @everzet elephantintheroom.io ! DDD in PHP bit.ly/dddinphp
Domain Problem Space Domain Model Solution Space
Data Model ~= Structural Model ~= State ! Domain Model ~= Behavioral Model !
Protect your invariants
The domain expert says “A customer must always have an email address.” * Could be different for your domain ** All examples are simplified
Test fails class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer();! ! assertThat(! $customer->getEmail(),! equalTo('jim@example.com') ! );! ! }! }
Test passes class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer();! $customer->setEmail('jim@example.com');! assertThat(! $customer->getEmail(),! equalTo('jim@example.com') ! );! }! }
Test fails class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer();! assertThat(! $customer->getEmail(),! equalTo(‘jim@example.com') ! );! $customer->setEmail(‘jim@example.com’);! ! }! }
final class Customer! {! private $email;! ! public function __construct($email)! {! $this->email = $email;! }! ! public function getEmail()! {! return $this->email;! }! }
Test passes class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer(‘jim@example.com’);! ! assertThat(! $customer->getEmail(),! equalTo(‘jim@example.com') ! );! }! }
Use objects as consistency boundaries
final class ProspectiveCustomer ! {! public function __construct()! {! // no email! }! }! ! final class PayingCustomer ! { ! public function __construct($email)! {! $this->email = $email;! }! }
Make the implicit explicit
final class ProspectiveCustomer ! {! /** @return PayingCustomer */! public function convertToPayingCustomer($email)! { ! //...! }! }! ! final class PayingCustomer ! { ! //...! }
The domain expert meant “A customer must always have a valid email address.”
$customerValidator = new CustomerValidator;! if($customerValidator->isValid($customer)){! // ...! }
Test fails class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_a_valid_email()! {! ! $this->setExpectedException(! 'InvalidArgumentException'! );! ! new Customer('malformed@email');! ! }! }
Test passes final class Customer ! {! public function __construct($email)! {! if( /* boring validation stuff */) {! throw new InvalidArgumentException();! }! $this->email = $email;! }! }
Violates Single Responsibility Principle
Test passes final class Email! {! private $email;! ! public function __construct($email)! {! if( /* boring validation stuff */) {! throw new InvalidArgumentException();! }! $this->email = $email;! }! ! public function __toString() ! {! return $this->email;! } ! }
Test passes final class Customer! {! /** @var Email */! private $email;! ! public function __construct(Email $email)! {! $this->email = $email;! }! }
Test passes class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_a_valid_email()! {! ! $this->setExpectedException(! ‘InvalidArgumentException’! );! ! new Customer(new Email(‘malformed@email’));! ! }! }
Entity ! Equality by Identity Lifecycle Mutable Value Object Equality by Value ! Immutable
Encapsulate state and behavior with Value Objects
The domain expert says “A customer orders products and pays for them.”
$order = new Order;! $order->setCustomer($customer);! $order->setProducts($products);! $order->setStatus(Order::UNPAID);! ! ! // ...! ! ! $order->setPaidAmount(500);! $order->setPaidCurrency(‘EUR’);! ! $order->setStatus(Order::PAID);! !
$order = new Order;! $order->setCustomer($customer);! $order->setProducts($products);! $order->setStatus(! new PaymentStatus(PaymentStatus::UNPAID)! );! ! ! ! $order->setPaidAmount(500);! $order->setPaidCurrency(‘EUR’);! ! $order->setStatus(! new PaymentStatus(PaymentStatus::PAID)! );
$order = new Order;! $order->setCustomer($customer);! $order->setProducts($products);! $order->setStatus(! new PaymentStatus(PaymentStatus::UNPAID)! );! ! ! ! $order->setPaidMonetary(! new Money(500, new Currency(‘EUR’))! );! $order->setStatus(! new PaymentStatus(PaymentStatus::PAID)! );
$order = new Order($customer, $products);! // set PaymentStatus in Order::__construct()! ! ! ! ! ! ! ! $order->setPaidMonetary(! new Money(500, new Currency(‘EUR’))! );! $order->setStatus(! new PaymentStatus(PaymentStatus::PAID)! );
$order = new Order($customer, $products);! ! ! ! ! ! ! ! ! $order->pay(! new Money(500, new Currency(‘EUR’))! );! // set PaymentStatus in Order#pay()! !
Encapsulate operations
$order = $customer->order($products);! ! ! ! ! ! ! ! ! $customer->payFor(! $order,! new Money(500, new Currency(‘EUR’))! );! !
The domain expert says “Premium customers get special offers.”
if($customer->isPremium()) {! // send special offer! }
The domain expert says “Order 3 times to become a premium customer.”
interface CustomerSpecification ! {! /** @return bool */! public function isSatisfiedBy(Customer $customer); ! }
class CustomerIsPremium implements CustomerSpecification ! {! private $orderRepository;! public function __construct(! OrderRepository $orderRepository! ) {...}! ! /** @return bool */! public function isSatisfiedBy(Customer $customer) ! {! $count = $this->orderRepository->countFor($customer);! return $count >= 3;! }! }! ! $customerIsPremium = new CustomerIsPremium($orderRepository)! if($customerIsPremium->isSatisfiedBy($customer)) {! // send special offer! }!
$customerIsPremium = new CustomerIsPremium;! ! $aCustomerWith2Orders = ...! $aCustomerWith3Orders = ...! ! assertFalse(! $customerIsPremium->isSatisfiedBy($aCustomerWith2Orders)! );! ! assertTrue(! $customerIsPremium->isSatisfiedBy($aCustomerWith3Orders)! );! ! !
The domain expert says “Different rules apply for different tenants.”
interface CustomerIsPremium ! extends CustomerSpecification! ! final class CustomerWith3OrdersIsPremium ! implements CustomerIsPremium! ! final class CustomerWith500EuroTotalIsPremium! implements CustomerIsPremium! ! final class CustomerWhoBoughtLuxuryProductsIsPremium! implements CustomerIsPremium! ! ...!
final class SpecialOfferSender! {! private $customerIsPremium;! ! ! public function __construct(! CustomerIsPremium $customerIsPremium) {...}! ! ! public function sendOffersTo(Customer $customer) ! {! if($this->customerIsPremium->isSatisfiedBy(! $customer! )) ! {! // send offers...! }! }! }!
! <!-- if you load services_amazon.xml: -->! <service id="customer.is.premium"! class="CustomerWith500EuroTotalIsPremium"> ! ! <!-- if you load services_ebay.xml: -->! <service id="customer.is.premium"! class="CustomerWith3OrdersIsPremium"> ! ! ! <!-- elsewhere -->! <service ! id=”special.offer.sender”! class=”SpecialOfferSender”>! <argument type=”service” id=”customer.is.premium”/>! </service>
Use specifications to encapsulate rules about object selection
The domain expert says “Get a list of all premium customers.”
interface CustomerRepository! {! public function add(Customer $customer);! ! public function remove(Customer $customer);! ! /** @return Customer */! public function find(CustomerId $customerId);! ! /** @return Customer[] */! public function findAll();! ! /** @return Customer[] */! public function findRegisteredIn(Year $year);! }!
Use repositories to create the illusion of in-memory collections
interface CustomerRepository! {! ! /** @return Customer[] */! public function findSatisfying(! CustomerSpecification $customerSpecification! );! ! }! ! ! // generalized:! $objects = $repository->findSatisfying($specification);!
class DbCustomerRepository implements CustomerRepository! {! /** @return Customer[] */! public function findSatisfying(! CustomerSpecification $specification) ! {! ! return array_filter(! $this->findAll(),! function(Customer $customer) use($specification) {! return $specification->isSatisfiedBy($customer);! } ! );! ! }! }!
final class CustomerWith3OrdersIsPremium! implements CustomerSpecification! {! public function asSql() {! return ‘SELECT * FROM Customer...’;! }! }! ! ! // class DbCustomerRepository ! public function findSatisfying($specification) ! {! return $this->db->query($specification->asSql()); } !
Use double dispatch to preserve encapsulation
$expectedCustomers = array_filter(! $repository->findAll(),! // filter…! );! ! $actualCustomers = ! $repository->findSatisfying($specification);! ! assertThat($expectedCustomers, equalTo($actualCustomers));
Test by comparing different representations of the same rule
Protect your invariants ! Objects as consistency boundaries ! Encapsulate state and behavior
Thanks! Questions? I ♥ Feedback joind.in/10690 ! Blog, Slides, other talks: verraes.net @mathiasverraes

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Unbreakable Domain Models PHPUK 2014 London

  • 2. A Map of the World
  • 3. London 3h train rides Kortrijk, Belgium Paris Amsterdam
  • 4. All models are wrong, but some are useful.
  • 5. I’m Mathias Verraes I'm an independent consultant. I help teams build enterprise web applications.
  • 8. Data Model ~= Structural Model ~= State ! Domain Model ~= Behavioral Model !
  • 10. The domain expert says “A customer must always have an email address.” * Could be different for your domain ** All examples are simplified
  • 11. Test fails class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer();! ! assertThat(! $customer->getEmail(),! equalTo('jim@example.com') ! );! ! }! }
  • 12. Test passes class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer();! $customer->setEmail('jim@example.com');! assertThat(! $customer->getEmail(),! equalTo('jim@example.com') ! );! }! }
  • 13. Test fails class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer();! assertThat(! $customer->getEmail(),! equalTo(‘jim@example.com') ! );! $customer->setEmail(‘jim@example.com’);! ! }! }
  • 14. final class Customer! {! private $email;! ! public function __construct($email)! {! $this->email = $email;! }! ! public function getEmail()! {! return $this->email;! }! }
  • 15. Test passes class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer(‘jim@example.com’);! ! assertThat(! $customer->getEmail(),! equalTo(‘jim@example.com') ! );! }! }
  • 17. final class ProspectiveCustomer ! {! public function __construct()! {! // no email! }! }! ! final class PayingCustomer ! { ! public function __construct($email)! {! $this->email = $email;! }! }
  • 19. final class ProspectiveCustomer ! {! /** @return PayingCustomer */! public function convertToPayingCustomer($email)! { ! //...! }! }! ! final class PayingCustomer ! { ! //...! }
  • 20. The domain expert meant “A customer must always have a valid email address.”
  • 21. $customerValidator = new CustomerValidator;! if($customerValidator->isValid($customer)){! // ...! }
  • 22. Test fails class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_a_valid_email()! {! ! $this->setExpectedException(! 'InvalidArgumentException'! );! ! new Customer('malformed@email');! ! }! }
  • 23. Test passes final class Customer ! {! public function __construct($email)! {! if( /* boring validation stuff */) {! throw new InvalidArgumentException();! }! $this->email = $email;! }! }
  • 25. Test passes final class Email! {! private $email;! ! public function __construct($email)! {! if( /* boring validation stuff */) {! throw new InvalidArgumentException();! }! $this->email = $email;! }! ! public function __toString() ! {! return $this->email;! } ! }
  • 26. Test passes final class Customer! {! /** @var Email */! private $email;! ! public function __construct(Email $email)! {! $this->email = $email;! }! }
  • 27. Test passes class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_a_valid_email()! {! ! $this->setExpectedException(! ‘InvalidArgumentException’! );! ! new Customer(new Email(‘malformed@email’));! ! }! }
  • 30. The domain expert says “A customer orders products and pays for them.”
  • 31. $order = new Order;! $order->setCustomer($customer);! $order->setProducts($products);! $order->setStatus(Order::UNPAID);! ! ! // ...! ! ! $order->setPaidAmount(500);! $order->setPaidCurrency(‘EUR’);! ! $order->setStatus(Order::PAID);! !
  • 32. $order = new Order;! $order->setCustomer($customer);! $order->setProducts($products);! $order->setStatus(! new PaymentStatus(PaymentStatus::UNPAID)! );! ! ! ! $order->setPaidAmount(500);! $order->setPaidCurrency(‘EUR’);! ! $order->setStatus(! new PaymentStatus(PaymentStatus::PAID)! );
  • 33. $order = new Order;! $order->setCustomer($customer);! $order->setProducts($products);! $order->setStatus(! new PaymentStatus(PaymentStatus::UNPAID)! );! ! ! ! $order->setPaidMonetary(! new Money(500, new Currency(‘EUR’))! );! $order->setStatus(! new PaymentStatus(PaymentStatus::PAID)! );
  • 34. $order = new Order($customer, $products);! // set PaymentStatus in Order::__construct()! ! ! ! ! ! ! ! $order->setPaidMonetary(! new Money(500, new Currency(‘EUR’))! );! $order->setStatus(! new PaymentStatus(PaymentStatus::PAID)! );
  • 35. $order = new Order($customer, $products);! ! ! ! ! ! ! ! ! $order->pay(! new Money(500, new Currency(‘EUR’))! );! // set PaymentStatus in Order#pay()! !
  • 38. The domain expert says “Premium customers get special offers.”
  • 40. The domain expert says “Order 3 times to become a premium customer.”
  • 41. interface CustomerSpecification ! {! /** @return bool */! public function isSatisfiedBy(Customer $customer); ! }
  • 42. class CustomerIsPremium implements CustomerSpecification ! {! private $orderRepository;! public function __construct(! OrderRepository $orderRepository! ) {...}! ! /** @return bool */! public function isSatisfiedBy(Customer $customer) ! {! $count = $this->orderRepository->countFor($customer);! return $count >= 3;! }! }! ! $customerIsPremium = new CustomerIsPremium($orderRepository)! if($customerIsPremium->isSatisfiedBy($customer)) {! // send special offer! }!
  • 43. $customerIsPremium = new CustomerIsPremium;! ! $aCustomerWith2Orders = ...! $aCustomerWith3Orders = ...! ! assertFalse(! $customerIsPremium->isSatisfiedBy($aCustomerWith2Orders)! );! ! assertTrue(! $customerIsPremium->isSatisfiedBy($aCustomerWith3Orders)! );! ! !
  • 44. The domain expert says “Different rules apply for different tenants.”
  • 45. interface CustomerIsPremium ! extends CustomerSpecification! ! final class CustomerWith3OrdersIsPremium ! implements CustomerIsPremium! ! final class CustomerWith500EuroTotalIsPremium! implements CustomerIsPremium! ! final class CustomerWhoBoughtLuxuryProductsIsPremium! implements CustomerIsPremium! ! ...!
  • 46. final class SpecialOfferSender! {! private $customerIsPremium;! ! ! public function __construct(! CustomerIsPremium $customerIsPremium) {...}! ! ! public function sendOffersTo(Customer $customer) ! {! if($this->customerIsPremium->isSatisfiedBy(! $customer! )) ! {! // send offers...! }! }! }!
  • 47. ! <!-- if you load services_amazon.xml: -->! <service id="customer.is.premium"! class="CustomerWith500EuroTotalIsPremium"> ! ! <!-- if you load services_ebay.xml: -->! <service id="customer.is.premium"! class="CustomerWith3OrdersIsPremium"> ! ! ! <!-- elsewhere -->! <service ! id=”special.offer.sender”! class=”SpecialOfferSender”>! <argument type=”service” id=”customer.is.premium”/>! </service>
  • 48. Use specifications to encapsulate rules about object selection
  • 49. The domain expert says “Get a list of all premium customers.”
  • 50. interface CustomerRepository! {! public function add(Customer $customer);! ! public function remove(Customer $customer);! ! /** @return Customer */! public function find(CustomerId $customerId);! ! /** @return Customer[] */! public function findAll();! ! /** @return Customer[] */! public function findRegisteredIn(Year $year);! }!
  • 51. Use repositories to create the illusion of in-memory collections
  • 52. interface CustomerRepository! {! ! /** @return Customer[] */! public function findSatisfying(! CustomerSpecification $customerSpecification! );! ! }! ! ! // generalized:! $objects = $repository->findSatisfying($specification);!
  • 53. class DbCustomerRepository implements CustomerRepository! {! /** @return Customer[] */! public function findSatisfying(! CustomerSpecification $specification) ! {! ! return array_filter(! $this->findAll(),! function(Customer $customer) use($specification) {! return $specification->isSatisfiedBy($customer);! } ! );! ! }! }!
  • 54. final class CustomerWith3OrdersIsPremium! implements CustomerSpecification! {! public function asSql() {! return ‘SELECT * FROM Customer...’;! }! }! ! ! // class DbCustomerRepository ! public function findSatisfying($specification) ! {! return $this->db->query($specification->asSql()); } !
  • 55. Use double dispatch to preserve encapsulation
  • 56. $expectedCustomers = array_filter(! $repository->findAll(),! // filter…! );! ! $actualCustomers = ! $repository->findSatisfying($specification);! ! assertThat($expectedCustomers, equalTo($actualCustomers));
  • 57. Test by comparing different representations of the same rule
  • 58. Protect your invariants ! Objects as consistency boundaries ! Encapsulate state and behavior
  • 59. Thanks! Questions? I ♥ Feedback joind.in/10690 ! Blog, Slides, other talks: verraes.net @mathiasverraes