Unbreakable Domain Models - PHPUK 2014 London

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.

Transcript

1. Unbreakable
   Domain Models @mathiasverraes
2. A Map of the World
3. London Paris Amsterdam Kortrijk, Belgium 3h train rides
4. All models are wrong, but some are useful.
5. I'm an independent consultant.
   I help teams build enterprise web applications.
   I’m Mathias Verraes
   
6. Blog
   verraes.net ! Podcast with @everzet elephantintheroom.io ! DDD in PHP bit.ly/dddinphp
7. Domain
   Problem Space Domain Model
   Solution Space
8. Data Model ~= Structural Model ~= State
   ! Domain Model ~= Behavioral Model
   !
9. Protect your invariants
10. The domain expert says “A customer must always have an
    email address.” * Could be different for your domain ** All examples are simplified
11. 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 fails
12. 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 passes
13. 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’);! ! }! } Test fails
14. final class Customer! {! private $email;! ! public function __construct($email)! {! $this->email = $email;! }! ! public function getEmail()! {! return $this->email;! }! }
15. class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_an_email()! {! ! $customer = new Customer(‘jim@example.com’);! ! assertThat(! $customer->getEmail(),! equalTo(‘jim@example.com') ! );! }! } Test passes
16. Use objects as
    consistency boundaries
17. final class ProspectiveCustomer ! {! public function __construct()! {! // no email! }! }! ! final class PayingCustomer ! { ! public function __construct($email)! {! $this->email = $email;! }! }
18. Make the implicit
    explicit
19. final class ProspectiveCustomer ! {! /** @return PayingCustomer */! public function convertToPayingCustomer($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. class CustomerTest extends PHPUnit_Framework_TestCase! {! /** @test */! public function should_always_have_a_valid_email()! {! ! $this->setExpectedException(! '\InvalidArgumentException'! );! ! new Customer('malformed@email');! ! }! } Test fails
23. final class Customer ! {! public function __construct($email)! {! if( /* boring validation stuff */) {! throw new \InvalidArgumentException();! }! $this->email = $email;! }! } Test passes
24. Violates
    Single Responsibility Principle
25. 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
26. final class Customer! {! /** @var Email */! private $email;! ! public function __construct(Email $email)! {! $this->email = $email;! }! } Test passes
27. 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
28. Entity ! Equality by Identity Lifecycle Mutable Value Object Equality by Value ! Immutable
29. Encapsulate
    state and behavior
    with Value Objects
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()! !
36. Encapsulate operations
37. $order = $customer->order($products);! ! ! ! ! ! ! ! ! $customer->payFor(! $order,! new Money(500, new Currency(‘EUR’))! );! !
38. The domain expert says “Premium customers
    get special offers.”
39. if($customer->isPremium()) {! // send special offer! }
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? ! Blog, Slides, other talks: verraes.net @mathiasverraes I ♥ Feedback joind.in/10690