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Global Design
Principles
The Quest for
Matthias Noback
Stability
Change
Stability versus change
• Backwards compatibility of APIs
• Semantic versioning
• HTTP status codes
• XML schemas
For internal APIs
only!
The key is in the concept of communication
ReceiverSender
What's being communicated?
Message
Type ("BookHotel")
Value (“Ter Elst", “29-01-2016“, “30-01-2016”)
Message flavours
Command Event
Imperative "BookHotel"
Informational "HotelWasBooked"
Message flavours
Query Document
Interrogatory "GetRecentBookings"
Neutral "BookingsCollection"
Sender
Construct the message
Message
Translate
Construct
(Prepare fortransport)
Receiver
Deconstruct the message
(Unwrap)
Translate
Construct
Global design principles can be
discovered
if we recognise the fact that communication between objects and applications are (more or less) equal
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
class AccountService{
public function deposit($accountId, Money $amount) {
...}
}Opportunity for
sending a message
Inter-object communication
$money = new Money(20000, 'EUR');$userId = 123;
$accountService->deposit($userId, $money
);
Translation
The sender prepares the message for the receiver
Prepare the message
Send the message
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, ...
PUT /accounts/deposit/ HTTP/1.1Host: localhost
{"accountId": "123","currency": "EUR","amount": 20000
}
Inter-application communication
Opportunity for sending a message
$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
/** @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
Global design principles
Should be applicable to both inter-object and inter-application communication
Part IStability
III Implementation
II Information
Sender Receiver
Communication
Dependency relation!
Stable Unstable
What is safer?
Unstable Stable
OR
To be stable"Steady in position or balance;
firm"
Irresponsible
What makes something unstable?
1. Nothing depends on it
It doesn't need to stay the same for anyone
Dependent
What makes something unstable?
2. It depends on many things
Any of the dependencies could change at any time
Stability is not a quality of one thing
It emerges from the environment of that thing
Responsible
What makes something stable?
1. Many things depend on it
It has to stay the same, because change would break dependents
Independent
What makes something stable?
2. It depends on nothing
It is not affected by changes in anything
–The Stable dependencies principle
Depend in the direction of stability
Stable package Unstable package
What to do when this happens?
Stable package
Depend on something that you own
Unstable packageStable package
–The Dependency inversion principle
Always depend on abstractions, not on concretions
Dependency inversion for objects
• Depend on an interface instead of a class
• Separate a task from its implementation
Stable thing
Communication between systems
External application (unstable)
Mediator
Stable thing
Slightly better
External application (unstable)
ConsumerApplication
Communication between systems
Message queue
External application (unstable)
Dependency inversion for systems
• Depend on your own (messaging) system
• Communicate with other applications through a message queue
Part IIInformation
I Stability
III Implementation
Knowledge, data, duplication
Nameplate order system
Nameplate machine
Business automation for creating nameplates
<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
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
<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
<nameplate><name>Ibuildings</name>
</nameplate>
Serialized Nameplate object
// 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
<nameplate><name>Ibuildings</name><width>120</width>
</nameplate>
Deduplication
Knowledge is in one place, facts can be everywhere
$nameplate = deserialize($xml);
$width = $nameplate['width'];
Accepting messages
No duplicate knowledge anymore
–The Don't repeat yourself principle
“Every piece of knowledge must have a single, unambiguous, authoritative representation
within a system.”
Even across applications!
"Don't repeat yourself"
• Doesn't mean you can't repeat data
• It means you can't have knowledge in multiple locations
Mutability
What's the difference between...
class Money{
private $amount;private $currency;
public function setAmount($amount) {$this->amount = $amount;
}
public function getAmount() {return $this->amount;
}
...}
... and this
class Money{
public $amount;public $currency;
}
Inconsistent data$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');
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
Immutability
• Once created, can not be modified
• Can only be replaced
Consistent data!
$savings = new Money(1000, 'USD');// we already have consistent data
// we can't change anything anymore
Immutable data!
Using immutable values
• Prevents bugs
• Prevents invalid state
What about API messages?
<money><amount>1000</amount><currency>USD</currency>
</money>
PUT /savings/<money>
<currency>EUR</currency></money>
POST /savings/
Large object graphs<user>
<first-name/><last-name/> <mobile-phone-number/> <email-address/> <homepage/> <orders>
<order/>...
</orders><tickets>
<ticket/>...
</tickets><payments>
<payment/>...
</payments></user>
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();}
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
Commands and events
Register ConfirmRegistration
SendMessage
RegistrationConfirmed
MessageSent
UserRegisteredApplication
True
• Messages should serve actual use cases instead of patch operations
• After processing them, data should be in a valid state
Part IIIImplementation
I StabilityII Information
IV Conclusion
Implementation
Leaking implementation details
class Person{
/** * @return PhoneNumber[] */public function getPhoneNumbers() {
return $this->phoneNumbers;}
}
Initial implementation
class Person{
/** * @return ArrayCollection */public function getPhoneNumbers() {
return $this->phoneNumbers;}
}
Implementation leakage (Doctrine)
class Person{
/** * @return PhoneNumber[] */public function getPhoneNumbers() {
return $this->phoneNumbers->toArray();}
}
Hiding implementation
class NameplateController{
function getAction($id) {$nameplate = $this
->getDoctrine()->getManager()->getRepository(Nameplate::class)->findOneBy(['id' => $id]);
if ($nameplate === null) {throw new NotFoundHttpException();
}...
}}
More implementation hiding
Actual field names!
null or false?
"find"?
class NameplateRepository{
function byId($id) {$nameplate = $this
->findOneBy(['id' => $id]);
if ($nameplate === null) {throw new NameplateNotFound($id);
}
return $nameplate;}
}
Push it out of sight
Domain-specific exception
Hide specific return value
No "find"
class NameplateController{
function getAction($id) {try {
$nameplate = $this->nameplateRepository->byId($id);
} catch (NameplateNotFound $exception) {throw new NotFoundHttpException();
}
...}
}
Respect layers
Convert domain exception to web specific exception
Basically just plain old OOP
• Encapsulation
• Abstraction
Implementation details and usability
<ticket> ... <priority type="integer">1</priority> ...
Assembla API
<ticket> ... <priority key="highest"> <label>Highest</label> </priority> ...
Why not...
<ticket> ... <is-story type="boolean">false</is-story> <total-estimate type="float">0.0</total-estimate> ...
Assembla API
<story> ... <total-estimate type="float">0.0</total-estimate> ...
Why not...
Design your messages in such a way that
• You hide your implementation
• Clients won't need to reimplement your logic
• Clients get the information they need
API discovery
/** * @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?
class Algorithm extends \SplEnum{
const __default = self::BCRYPT;
const BCRYPT = 1;}
Allow no mistakes
['salt' => '...''cost' => 10
]
Options for bcrypt hashing
class Bcrypt implements HashingStrategy{
/** * @param integer $cost * @param string|null $salt */
public function __construct($cost, $salt = null) {...
}}
Inject a strategy
function password_hash($password, HashingStrategy $hashingStrategy
);
Inject a strategy
More explicit and... discoverable!
Discoverability of an API
• Full Reflection capabilities :)
• Basic knowledge of English
// I've got this password I want to hash…$password = ...;
// Look, 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 discoveryWho is talking?
How stupid are they?
How do you find out a valid range?
/** * @param array $options */function some_function(array $options);
/** * @param integer $type */function some_other_function($type);
/** * @param object $command */function handle($command);
Undiscoverable APIs
Any kind of API should be maximally discoverable
Everything should be an object
A class is a type
Define lots of interfaces
An interface defines the public API of functions
<?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"
Summary/Hypothesis
• Objects are just like applications
• Try to apply the same rules to them
Think about
• Stable dependencies
• Duplication of facts, not knowledge
• Immutability over mutability
• No leakage of implementation details
• Everything should be maximally discoverable
–Chris Hadfield
“… I should do things that keep me moving in the right direction, just in case — and I should be sure
those things interest me, so whatever happens, I’m happy.”
