Unbreakable Domain Models - PHPUK 2014 London

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Unbreakable Domain Models @mathiasverraes

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A Map of the World

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London Paris Amsterdam Kortrijk, Belgium 3h train rides

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All models are wrong, but some are useful.

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I'm an independent consultant. I help teams build enterprise web applications. I’m Mathias Verraes

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Blog verraes.net ! Podcast with @everzet elephantintheroom.io ! DDD in PHP bit.ly/dddinphp

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Domain Problem Space Domain Model Solution Space

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Data Model ~= Structural Model ~= State ! Domain Model ~= Behavioral Model !

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Protect your invariants

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The domain expert says “A customer must always have an email address.” * Could be different for your domain ** All examples are simpliﬁed

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class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer();! ! assertThat(! $customer->getEmail(),! equalTo('[email protected]') ! );! ! }! } Test fails

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class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer();! $customer->setEmail('[email protected]');! assertThat(! $customer->getEmail(),! equalTo('[email protected]') ! );! }! } Test passes

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class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer();! assertThat(! $customer->getEmail(),! equalTo(‘[email protected]') ! );! $customer->setEmail(‘[email protected]’);! ! }! } Test fails

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final class Customer! {! private $email;! ! public function __construct($email)! {! $this->email = $email;! }! ! public function getEmail()! {! return $this->email;! }! }

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class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer(‘[email protected]’);! ! assertThat(! $customer->getEmail(),! equalTo(‘[email protected]') ! );! }! } Test passes

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Use objects as consistency boundaries

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final class ProspectiveCustomer ! {! public function __construct()! {! // no email! }! }! ! final class PayingCustomer ! { ! public function __construct($email)! {! $this->email = $email;! }! }

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Make the implicit explicit

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final class ProspectiveCustomer ! {! /** @return PayingCustomer */! public function convertToPayingCustomer($email)! { ! //...! }! }! ! final class PayingCustomer ! { ! //...! }

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The domain expert meant “A customer must always have a valid email address.”

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$customerValidator = new CustomerValidator;! if($customerValidator->isValid($customer)){! // ...! }

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class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_a_valid_email()! {! ! $this->setExpectedException(! '\InvalidArgumentException'! );! ! new Customer('malformed@email');! ! }! } Test fails

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final class Customer ! {! public function __construct($email)! {! if( /* boring validation stuff */) {! throw new \InvalidArgumentException();! }! $this->email = $email;! }! } Test passes

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Violates Single Responsibility Principle

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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

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final class Customer! {! /** @var Email */! private $email;! ! public function __construct(Email $email)! {! $this->email = $email;! }! } Test passes

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class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_a_valid_email()! {! ! $this->setExpectedException(! ‘\InvalidArgumentException’! );! ! new Customer(new Email(‘malformed@email’));! ! }! } Test passes

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Entity ! Equality by Identity Lifecycle Mutable Value Object Equality by Value ! Immutable

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Encapsulate state and behavior with Value Objects

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The domain expert says “A customer orders products and pays for them.”

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$order = new Order;! $order->setCustomer($customer);! $order->setProducts($products);! $order->setStatus(Order::UNPAID);! ! ! // ...! ! ! $order->setPaidAmount(500);! $order->setPaidCurrency(‘EUR’);! ! $order->setStatus(Order::PAID);! !

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$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)! );

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$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)! );

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$order = new Order($customer, $products);! // set PaymentStatus in Order::__construct()! ! ! ! ! ! ! ! $order->setPaidMonetary(! new Money(500, new Currency(‘EUR’))! );! $order->setStatus(! new PaymentStatus(PaymentStatus::PAID)! );

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$order = new Order($customer, $products);! ! ! ! ! ! ! ! ! $order->pay(! new Money(500, new Currency(‘EUR’))! );! // set PaymentStatus in Order#pay()! !

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Encapsulate operations

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$order = $customer->order($products);! ! ! ! ! ! ! ! ! $customer->payFor(! $order,! new Money(500, new Currency(‘EUR’))! );! !

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The domain expert says “Premium customers get special offers.”

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if($customer->isPremium()) {! // send special offer! }

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The domain expert says “Order 3 times to become a premium customer.”

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interface CustomerSpecification ! {! /** @return bool */! public function isSatisfiedBy(Customer $customer); ! }

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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! }!

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$customerIsPremium = new CustomerIsPremium;! ! $aCustomerWith2Orders = ...! $aCustomerWith3Orders = ...! ! assertFalse(! $customerIsPremium->isSatisfiedBy($aCustomerWith2Orders)! );! ! assertTrue(! $customerIsPremium->isSatisfiedBy($aCustomerWith3Orders)! );! ! !

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The domain expert says “Different rules apply for different tenants.”

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interface CustomerIsPremium ! extends CustomerSpecification! ! final class CustomerWith3OrdersIsPremium ! implements CustomerIsPremium! ! final class CustomerWith500EuroTotalIsPremium! implements CustomerIsPremium! ! final class CustomerWhoBoughtLuxuryProductsIsPremium! implements CustomerIsPremium! ! ...!

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final class SpecialOfferSender! {! private $customerIsPremium;! ! ! public function __construct(! CustomerIsPremium $customerIsPremium) {...}! ! ! public function sendOffersTo(Customer $customer) ! {! if($this->customerIsPremium->isSatisfiedBy(! $customer! )) ! {! // send offers...! }! }! }!

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! ! ! ! ! ! ! ! ! ! !

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Use speciﬁcations to encapsulate rules about object selection

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The domain expert says “Get a list of all premium customers.”

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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);! }!

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Use repositories to create the illusion of in-memory collections

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interface CustomerRepository! {! ! /** @return Customer[] */! public function findSatisfying(! CustomerSpecification $customerSpecification! );! ! }! ! ! // generalized:! $objects = $repository->findSatisfying($specification);!

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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);! } ! );! ! }! }!

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final class CustomerWith3OrdersIsPremium! implements CustomerSpecification! {! public function asSql() {! return ‘SELECT * FROM Customer...’;! }! }! ! ! // class DbCustomerRepository ! public function findSatisfying($specification) ! {! return $this->db->query($specification->asSql()); ! }

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Use double dispatch to preserve encapsulation

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$expectedCustomers = array_filter(! $repository->findAll(),! // filter…! );! ! $actualCustomers = ! $repository->findSatisfying($specification);! ! assertThat($expectedCustomers, equalTo($actualCustomers));

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Test by comparing different representations of the same rule

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Protect your invariants ! Objects as consistency boundaries ! Encapsulate state and behavior

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