Global design principles for stable APIs
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The document discusses global design principles that can be applied to both inter-object and inter-application communication. It advocates treating objects like applications and applying similar design rules to both. Specifically, it recommends: depending on abstractions rather than concretions to achieve stable dependencies; avoiding duplication of knowledge while allowing duplication of facts; using immutable objects to prevent inconsistent states; hiding implementation details to avoid tight coupling; and ensuring APIs are maximally discoverable. The overall message is that communication between systems, whether objects or applications, can be improved by recognizing them as equivalent and applying cross-cutting principles of stability, information hiding, and implementation independence.
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Global design principles for stable APIs
- 1. Global Design Principles Matthias Noback Dutch PHP Conference, Amsterdam, June 26th, 2015 The Quest for
- 3. Stability versus change ⢠Backwards compatibility of APIs ⢠Semantic versioning ⢠HTTP status codes ⢠XML schemas
- 6. The key is in the concept of communication ReceiverSender
- 7. What's being communicated? Message Type ("BookHotel") Value ("Inntel Hotels", "25-06-2015", "27-06-2015")
- 10. Sender Construct the message Message Translate Construct (Prepare for⨠transport)
- 12. Global design principles can be discovered if we recognise the fact that communication between objects and applications are (more or less) equal
- 13. Communication between objects ⢠Calling a function is like sending a message ⢠The function and its parameters are the message type ⢠The arguments constitute the value of the message
- 14. class AccountService! {! ! public function deposit($accountId, Money $amount) ! {! ! ! ...! ! }! } Opportunity for sending a message Inter-object communication
- 15. $money = new Money(20000, 'EUR');! $userId = 123;! ! $accountService->deposit(! ! $userId, ! ! $money! ); Translation The sender prepares the message for the receiver Prepare the message Send the message
- 16. Communication between applications ⢠An HTTP request is a message ⢠The HTTP method and URI are the type of the message ⢠The HTTP request body constitutes the value of the message Or AMQP, Stomp, Gearman, ...
- 17. PUT /accounts/deposit/ HTTP/1.1! Host: localhost! ! {! ! "accountId": "123",! ! "currency": "EUR",! ! "amount": 20000! } Inter-application communication Opportunity for sending a message
- 18. $uri ='/accounts/deposit/';! ! $data = json_encode([! ! 'accountId' => 123! ! 'currency' => 'EUR',! ! 'amount' => 20000! ]);! ! $httpClient! ! ->createRequest('PUT', $uri, $data)! ! ->send(); Translation Prepare the message Send the message
- 19. /** @Route("/accounts/deposit") */! function depositAction(Request $request)! {! ! $request = json_decode($request->getContent());! ! $money = new Money(! ! ! $request['amount'],! ! ! $request['currency']! ! );! ! ! $this->get('account_service')->deposit(! ! ! ! $request['accountId'],! ! ! ! $money! ! ! );! ! ! return new Response(null, 200);! }! Translation Prepare the message Send the message
- 20. Global design principles Should be applicable to both inter-object and inter-application communication
- 21. Part I Stability III Implementation II Information
- 23. Stable Unstable What is safer? Unstable Stable OR
- 24. To be stable "Steady in position or balance; firm"
- 25. Irresponsible What makes something unstable? 1. Nothing depends on it It doesn't need to stay the same for anyone
- 26. Dependent What makes something unstable? 2. It depends on many things Any of the dependencies could change at any time
- 27. Stability is not a quality of one thing It emerges from the environment of that thing
- 28. Responsible What makes something stable? 1. Many things depend on it It has to stay the same, because change would break dependents
- 29. Independent What makes something stable? 2. It depends on nothing It is not aďŹected by changes in anything
- 30. âThe Stable dependencies principle Depend in the direction of stability
- 31. Stable thing Unstable thing What to do when this happens?
- 32. Also stable Half stable thingStable thing Unstable thing Depend on something that you own
- 33. âThe Dependency inversion principle Always depend on abstractions, not on concretions
- 34. Dependency inversion for objects ⢠Depend on an interface instead of a class ⢠Separate a task from its implementation
- 38. Dependency inversion for systems ⢠Depend on your own (messaging) system ⢠Communicate with other applications via a message queue
- 39. Part II Information I Stability III Implementation
- 41. Nameplate order system Nameplate machine Business automation for creating nameplates
- 42. <h1>You are about to order a nameplate!</h1>! ! <p>Name on the plate: {{ name }}<br/>! Width of the plate: {{ 12*(name|length) }} cm.</p>! Calculating the width of a nameplate Knowledge
- 43. class Nameplate! {! ! private $name;! ! ! function __construct($name) {! ! ! $this->name = $name;! ! }! ! ! function widthInCm() {! ! ! return strlen($this->name) * 12;! ! }! } Data object Knowledge is close to the subject
- 44. <h1>You are about to order a nameplate!</h1>! ! <p>Name on the plate: {{ nameplate.name }}<br/>! Width of the plate: {{ nameplate.widthInCm) }} cm.</p>! No knowledge in the template
- 46. // calculate the width of the nameplate! ! $nameplate = deserialize($xml);! $name = $nameplate['name'];! ! $width = strlen($name);! ! $widthPerCharacterInCm = 12;! ! $widthInCm = $width * $widthPerCharacterInCm;! ! // configure the nameplate machine ;) Accepting messages Duplication of knowledge
- 47. <nameplate>! ! <name>Buildings</name>! ! <width>120</width>! </nameplate> Deduplication Knowledge is in one place, facts can be everywhere
- 48. $nameplate = deserialize($xml);! ! $width = $nameplate['width']; Accepting messages No duplicate knowledge anymore
- 49. âThe Don't repeat yourself principle âEvery piece of knowledge must have a single, unambiguous, authoritative representation within a system.â Even across applications!
- 50. "Don't repeat yourself" ⢠Doesn't mean you can't repeat data ⢠It means you can't have knowledge in multiple locations
- 51. Mutability
- 52. What's the difference between... class Money! {! ! private $amount;! ! private $currency;! ! ! public function setAmount($amount) {! ! ! $this->amount = $amount;! ! }! ! ! public function getAmount() {! ! ! return $this->amount;! ! }! ! ! ...! }
- 53. ... and this class Money! {! ! public $amount;! ! public $currency;! }
- 54. Inconsistent data $savings = new Money();! $savings->setAmount(1000);! ! // what's the currency at this point?! ! $savings->setCurrency('USD');! ! // only now do we have consistent data! ! $savings->setCurrency('EUR');! ! // we have a lot more money now!! ! $savings->setAmount('Amsterdam');
- 55. Making something better of this class Money! {! ! private $amount;! ! private $currency;! ! ! public function __construct($amount, $currency) {! ! ! $this->setAmount($amount);! ! }! ! ! private function setAmount($amount) {! ! ! if (!is_int($amount) || $amount < 0) {! ! ! ! throw new InvalidArgumentException();! ! ! }! ! ! ! $this->amount = $amount;! ! }! } Private Required
- 56. Immutability ⢠Once created, can not be modiďŹed ⢠Can only be replaced
- 57. Consistent data! $savings = new Money(1000, 'USD');! // we already have consistent data! ! // we can't change anything anymore Immutable data!
- 58. Using immutable values ⢠Prevents bugs ⢠Prevents invalid state
- 59. What about API messages? <money>! ! <amount>1000</amount>! ! <currency>USD</currency>! </money> PUT /savings/ <money>! ! <currency>EUR</currency>! </money> POST /savings/
- 60. Large object graphs <user>! ! <first-name/>! ! <last-name/> ! ! <mobile-phone-number/> ! ! <email-address/> ! ! <homepage/> ! ! <orders>! ! ! <order/>! ! ! ...! ! </orders>! ! <tickets>! ! ! <ticket/>! ! ! ...! ! </tickets>! ! <payments>! ! ! <payment/>! ! ! ...! ! </payments>! </user>
- 61. Forms & Doctrine ORM $form = $this->createForm(new MoneyType());! $form->handleRequest($request);! ! if ($form->isValid()) {! ! $em = $this->getDoctrine()->getManager();! ! $em->persist($form->getData());! ! ! // calculates change set and executes queries! ! ! $em->flush();! }
- 62. If you allow your users to change every field at any time ⢠You end up with inconsistent data ⢠You loose the why of a change ⢠You end up with the what of only the last change ⢠You ignore the underlying real-world scenario
- 64. Commands and events Register ConďŹrmRegistration SendMessage RegistrationConďŹrmed MessageSent UserRegistered Application
- 65. Tomorrow!
- 66. True ⢠Messages should serve actual use cases instead of patch operations ⢠After processing them, data should be in a valid state
- 67. Part III Implementation I Stability II Information IV Conclusion
- 68. Implementation
- 70. class Person! {! ! /**! ! * @return PhoneNumber[]! ! */! ! public function getPhoneNumbers() {! ! ! return $this->phoneNumbers;! ! }! } Initial implementation
- 71. class Person! {! ! /**! ! * @return ArrayCollection! ! */! ! public function getPhoneNumbers() {! ! ! return $this->phoneNumbers;! ! }! } Implementation leakage (Doctrine)
- 72. class Person! {! ! /**! ! * @return PhoneNumber[]! ! */! ! public function getPhoneNumbers() {! ! ! return $this->phoneNumbers->toArray();! ! }! } Hiding implementation
- 73. class NameplateController! {! ! function getAction($id) {! ! ! $nameplate = $this! ! ! ! ->getDoctrine()! ! ! ! ->getManager()! ! ! ! ->getRepository(Nameplate::class)! ! ! ! ->findOneBy(['id' => $id]);! ! ! ! if ($nameplate === null) {! ! ! ! throw new NotFoundHttpException();! ! ! }! ! ! ...! ! }! } More implementation hiding Actual ďŹeld names! null or false? "ďŹnd"?
- 74. class NameplateRepository! {! ! function fromId($id) {! ! ! $nameplate = $this! ! ! ! ->findOneBy(['id' => $id]);! ! ! ! if ($nameplate === null) {! ! ! ! throw new NameplateNotFound($id);! ! ! }! ! ! ! return $nameplate;! ! }! } Push it out of sight Domain-speciďŹc exception Hide speciďŹc return value No "ďŹnd"
- 75. class NameplateController! {! ! function getAction($id) {! ! ! try {! ! ! ! $nameplate = $this! ! ! ! ! ->nameplateRepository! ! ! ! ! ->fromId($id);! ! ! } catch (NameplateNotFound $exception) {! ! ! ! throw new NotFoundHttpException();! ! ! }! ! ! ! ...! ! }! } Respect layers Convert domain exception to web speciďŹc exception
- 76. Plain old OOP ⢠Encapsulation ⢠Abstraction
- 78. <ticket>! ...! <priority type="integer">1</priority>! ... Assembla API We do "switch ($priority)"
- 80. <ticket>! ...! <is-story type="boolean">false</is-story>! <total-estimate type="float">0.0</total-estimate>! ... Assembla API Table "tickets" has a column "is_story"
- 82. Design your messages in such a way that ⢠You hide your implementation ⢠Clients won't need to reimplement your application ⢠Clients get the information they need
- 83. API discovery
- 84. /**! * @param string $password! * @param integer $algo! * @param array $options! */! function password_hash($password, $algo, array $options = array()); Undiscoverable API What are my options here? And here?
- 85. class Algorithm extends SplEnum! {! ! const __default = self::BCRYPT;! ! ! const BCRYPT = 1;! } Allow no mistakes
- 86. [! ! 'salt' => '...'! ! 'cost' => 10! ] Options for bcrypt hashing
- 87. class Bcrypt implements HashingStrategy! {! ! /**! * @param integer $cost! * @param string|null $salt! */! ! public function __construct($cost, $salt = null) {! ! ! ...! ! }! } Inject a strategy
- 88. function password_hash(! ! $password, ! ! HashingStrategy $hashingStrategy! ); Inject a strategy More explicit and... discoverable!
- 89. Discoverability of an API ⢠Full ReďŹection capabilities :) ⢠Basic knowledge of English
- 90. // I've got this password! $password = ...;! // I want to hash it...! // I found a function for this: password_hash()! password_hash($password, HashingStrategy $hashingStrategy);! // It requires an argument: a password (string)! // I already got a password right here:! password_hash($password);! // Wait, it requires a hashing strategy (a HashingStrategy object)! // I just found a class implementing that interface:! $hashingStrategy = new BcryptStrategy();! // That doesn't work, BcryptStrategy needs a cost! $hashingStrategy = new BcryptStrategy(10);! password_hash($password, $hashingStrategy); Example of API discovery Who is talking? How stupid are they? How do you ďŹnd out a valid range?
- 91. /**! * @param array $options! */! function some_function(array $options);! ! /**! * @param integer $type! */! function some_other_function($type);! ! /**! * @param object $command! */! function handle($command); Undiscoverable APIs
- 92. Any kind of API should be maximally discoverable
- 93. Everything should be an object A class is a type
- 94. Define lots of interfaces An interface deďŹnes the public API of functions
- 95. <?xml version="1.0" encoding="UTF-8"?>! <ticket>! <reporter>! <id>43</id>! <link rel="self" href="/api/reporters/43" />! <link rel="index" href="/api/reporters/" />! </reporter>! ... HATEOAS Links are a way to explain an "object"
- 96. In summary ⢠Objects are just like applications ⢠Try to apply the same rules to them
- 97. Think about ⢠Stable dependencies ⢠Duplication of facts, not knowledge ⢠Immutability over mutability ⢠No leakage of implementation details ⢠Everything should be maximally discoverable