Architecture
in the Small
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Hi!
Vinai Kopp
Freelance Developer and Trainer
@VinaiKopp
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My PHP Evolution:
1. Quick Hacks (1999)
2. Typo3 Plugin Developer (2003)
3. Magento Developer (2008)
4. Experienced Magento Developer (2012)
5. Discovering Code Beyond Magento (2014)
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I hope some of the following
will be useful and interesting
to play with for you, too
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Some of the many People
who inspired and helped me
• Rich Hickey
• Robert C. Martin
• Martin Fowler
• Kent Beck
All brilliant people with a sense of humor!
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Disclaimer
The following thoughts are all mine, but influenced by
reading these great peoples work.
If I get something wrong, it is all my fault.
I might change my mind about something at any time.
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Topics
• Extraction & Encapsulation
• Managing State with Immutable Objects
• The Value of Validity
• Immutable Variables
• Reasoning about Code with Idempotence
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Extraction
&
Encapsulation
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Extraction?
WTF?
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A subset of
Refactoring
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„Refactoring is the process of changing a software
system in such a way that it does not alter the external
behavior of the code yet improves its internal
structure“
-- Martin Fowler, Refactoring (Addison-Wesley)
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The usual purpose of refactoring:
Get rid of smelly code
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But in case of extraction
It can be more
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Simplicity
vs.
Amount of code
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Simplicity
Can make things
visible
that where hidden in the code
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Extract Method Example - Before
// MagentoCatalogModelProduct
protected function convertToMediaGalleryInterface(array $mediaGallery)
{
$entries = [];
foreach ($mediaGallery as $image) {
$entry = $this
->mediaGalleryEntryConverterPool
->getConverterByMediaType($image['media_type'])
->convertTo($this, $image);
$entries[] = $entry;
}
return $entries;
}
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Extract Method Example - After (1/2)
// MagentoCatalogModelProduct
protected function convertToMediaGalleryInterface(array $mediaGallery)
{
$entries = [];
foreach ($mediaGallery as $image) {
$entries[] = $this->convertImageToMediaGalleryEntry($image);
}
return $entries;
}
private function convertImageToMediaGalleryEntry(array $image)
{
$converter = $this->mediaGalleryEntryConverterPool
->getConverterByMediaType($image['media_type']);
return $converter->convertTo($this, $image);
}
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Extract Method Example - After (2/2)
// MagentoCatalogModelProduct
protected function convertToMediaGalleryInterface(array $mediaGallery)
{
$entries = [];
foreach ($mediaGallery as $image) {
$entries[] = $this->mediaGalleryEntryConverterPool
->convertImageToMediaGalleryEntry($image, $this);
}
return $entries;
}
// MagentoCatalogModelProductAttributeBackendMediaEntryConverterPool
public function convertImageToMediaGalleryEntry(array $image, Product $product)
{
$converter = $this->getConverterByMediaType($image['media_type']);
return $converter->convertTo($product, $image);
}
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Encapsulation
„In general, encapsulation is the inclusion of one thing
within another thing so that the included thing is not
apparent.“
-- TechTarget SearchNetworking
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Encapsulation
„Other objects ... use the object without having to be
concerned with how the object accomplishes it.“
-- TechTarget SearchNetworking
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Tell, don't ask!
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Extraction enables us to properly encapsulate
functionality.
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Proposition:
It is good to write code that is simple to extract.
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Topics
• Extraction & Encapsulation
• Managing State with Immutable Objects
• The Value of Validity
• Immutable Variables
• Reasoning about Code with Idempotence
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Managing State
with Immutable
Objects
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Immutawhat?
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„In object-oriented and functional programming, an
immutable object is an object whose state cannot be
modified after it is created.“
-- Wikipedia, Immutable object
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Yes, but... why?
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I've read it helps avoid a whole class of bugs.
So, what are they?
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Reasonability
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Reading ➡ Understanding
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Understanding ➡ Knowing
• Is it correct
or
• Is it incorrect
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When objects change in distant code, it becomes more
complex to track what changed where and why.
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Immutability helps to avoid
temporal coupling
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For Example:
// MagentoBraintreeModelPaymentMethod::partialCapture()
$this->config->initEnvironment($payment->getOrder()->getStoreId());
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Temporal Coupling:
// MagentoBraintreeModelPaymentMethod::partialCapture()
$this->config->initEnvironment($payment->getOrder()->getStoreId());
What state does the config start out in before it is
initialized?
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Temporal Coupling:
// MagentoBraintreeModelPaymentMethod::partialCapture()
$this->config->initEnvironment($payment->getOrder()->getStoreId());
What happens if the config is initialized twice with
different store ids?
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Temporal Coupling:
// MagentoBraintreeModelPaymentMethod::partialCapture()
$this->config->initEnvironment($payment->getOrder()->getStoreId());
What happens if another method is called before this
one and the other one also relies on the config
environment being initialized?
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Temporal Coupling:
// MagentoBraintreeModelPaymentMethod::partialCapture()
$this->config->initEnvironment($payment->getOrder()->getStoreId());
What happens if the call to initEnvironment is missed?
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Temporal Coupling:
// MagentoBraintreeModelPaymentMethod::partialCapture()
$this->config->initEnvironment($payment->getOrder()->getStoreId());
Extracting methods is tricky because the extracted part
might rely on a different method being called first.
This might not be apparent.
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Temporal Coupling:
// MagentoBraintreeModelPaymentMethod::partialCapture()
$this->config->initEnvironment($payment->getOrder()->getStoreId());
What happens if another object has a reference to the
same config instance, but it doesn't know about or
expect the config change?
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Temporal Coupling:
// MagentoBraintreeModelPaymentMethod::partialCapture()
$this->config->initEnvironment($payment->getOrder()->getStoreId());
What happens if a response of the payment instance is
cached and then the config environment is changed?
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External
Initialization methods
and
Setters
add complexity
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External immutability
&
Internal immutability
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Internal mutability allows for memoization
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Memoization :
„An optimization technique to speed up expensive
method calls by storing the result after the first call and
returning the cached result if the same inputs occur
again.“
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Memoization Example
public function getAltitudeAvg(Location $loc, Date $date)
{
$coords = (string) $loc->getCoords();
$dateStr = (string) $date;
if (! @$this->memoizedResults[$coords][$dateStr]) {
$avgAltitude = $this->doExpensiveCalculation($loc, $date);
$this->memoizedResults[$coords][$dateStr] = $avgAltitude;
}
return $this->memoizedResults[$coords][$dateStr];
}
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Anyway...
Memoization is a lot simpler to do with
externally immutable objects
since nothing can invalidate the result.
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Testing code which uses
immutable objects is simpler
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Test Double: Mutable Object
$initWasCalled = false;
$mockConfig = $this->getMock(MagentoBraintreeModelConfig, [], [], '', false);
$mockConfig->method('initEnvironment')
->willReturnCallback(function () use (&$initWasCalled) {
$initWasCalled = true;
});
$mockConfig->method('canUseForCountry')
->willReturnCallback(function ($country) use (&$initWasCalled) {
return $initWasCalled ? false : true;
});
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Yuck!
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Incomplete Test Double:
Mutable Object
$initializedConfigMock = $this->getMock(MagentoBraintreeModelConfig, [], [], '', false);
$initializedConfigMock->method('canUseForCountry')->willReturn(true);
Easy to miss calling initEnvironment in the system
under test!
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Test Double: Immutable Object
$configMock = $this->getMock(MagentoBraintreeModelConfig, [], [], '', false);
$configMock->method('canUseForCountry')->willReturn(true);
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Simpler to cache
No need to worry that an object changes after it is
written to cache.
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Example:
Magento EE PageCache
<controller_front_send_response_before>
<observers>
<enterprise_pagecache>
<class>enterprise_pagecache/observer</class>
<method>cacheResponse</method>
</enterprise_pagecache>
</observers>
</controller_front_send_response_before>
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Example:
Magento EE PageCache
Events after cache write:
• controller_front_send_response_before
(in a later Observer)
• controller_front_send_response_after
• http_response_send_before
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Once valid
always valid
Can something be „half valid“?
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Example: invalidation of a mutable object
$product = $productCollection->getFirstItem();
$product->getPrice(); // 24.99
$product->getSpecialPrice(); // 19.99
$product->setPrice(11.95);
$product->setSpecialPrice(10.95);
$product->getFinalPrice() // 19.99
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So how do the values get into an immutable object?
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Constructor Injection
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__construct()
$request = new HttpRequest(
HttpRequest::METHOD_GET,
HttpUrl::fromString('http://example.com/lookie/here'),
HttpHeaders::fromArray($httpHeaders),
HttpRequestBody::fromString($httpRequestBodyString)
);
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Named constructor
$request = HttpRequest::fromGlobalState();
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Example 1: named constructor
public static function fromGlobalState($requestBody = '')
{
$method = $_SERVER['REQUEST_METHOD'];
$protocol = isset($_SERVER['HTTPS']) && $_SERVER['HTTPS']) ?
'https' :
'http';
$path = $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
$url = $protocol . '://' . $path;
$headers = self::getGlobalRequestHeaders();
return static::fromScalars($method, $url, $headers, $requestBody);
}
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Example 2: named constructor
public static function fromScalars(
$methodString,
$urlString,
$headersArray,
$bodyString
) {
$url = HttpUrl::fromString($urlString);
$headers = HttpHeaders::fromArray($headersArray);
$body = HttpRequestBody::fromString($bodyString);
return new self($methodString, $url, $headers, $body);
}
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But IRL
things change
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Modeling change with
immutable objects
Change is introduced by creating a new instance.
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Looks like mutation, but it ain't
$today = new DateTimeImmutable();
echo $today->format('Y-m-d H:i:s'); // 2015-10-29 21:32:06
$day = DateInterval::createFromDateString('24 hours');
$tomorrow = $today->add($day);
echo $today->format('Y-m-d H:i:s'); // 2015-10-29 21:32:06
echo $tomorrow->format('Y-m-d H:i:s');// 2015-10-30 21:32:06
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Contraindications
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Does immutability make code more
complex
or
simple?
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A thing that changes over time.
$room = new ConferenceRoom(
$location,
$event,
$attendees
)
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Is it a new conference room just because the number of
people change?
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A mutable conference room model:
$nPeople = $room->countPeople();
$room->addPerson($attendee);
$room->countPeople() === $nPeople + 1;
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A immutable conference room model:
$nPeople = $room->countPeople();
$updatedRoom = $room->addPerson($attendee);
$room->countPeople() === $nPeople;
$updatedRoom->countPeople() === $nPeople + 1;
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Temporal modeling
An instance represents the object
at one moment in time.
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Temporal modeling
(Immutable object at a moment in time)
(Immutable object at a moment in time)
(Immutable object at a moment in time)
(Immutable object at a moment in time)
...
All the same Entity that changes over time.
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Temporal modeling
Keeps History !
Not a natural way to do OOP, adds complexity !
Unaccustomed way to think ! "
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Bottom Line
If you don't have to model changes over time, using
immutability will probably make your code simpler.
If you do have to worry about changes over time...
it depends.
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Topics
• Extraction & Encapsulation
• Managing State with Immutable Objects
• The Value of Validity
• Immutable Variables
• Reasoning about Code with Idempotence
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The Value of
Validity
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Enforce object validity at instantiation.
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Validation at Instantiation
• __construct()
• Named constructor (aka Factory Method)
• Builder or Factory
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Constructor validation
The safest place to enforce validity
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Example Constructor Validation
public function __construct($amount)
{
if (!is_int($amount)) {
$type = gettype($amount);
$msg = sprintf('Can not create price from "%s"', $type);
throw new InvalidAmountTypeException($msg);
}
$this->amount = $amount;
}
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When it gets more complex...
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Example Delegation of Validation
// ProductAttributeListBuilder
public static function fromArray(array $attributesArray)
{
$attributes = array_map(function (array $attributeArray) {
return ProductAttribute::fromArray($attributeArray);
}, $attributesArray);
return new self(...$attributes);
}
public function __construct(ProductAttribute ...$attributes)
{
$this->validateAllAttributesHaveCompatibleContextData(...$attributes);
$this->attributes = $attributes;
}
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Each thing is a class
Each object validates it's data
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Plug: Value Objects
Introduction slides on Value Objects
by Tim Bezhashvyly
http://vin.ai/tims_value_objects
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The cost of validity
Classes require time to write, test and maintain.
• Each thing is a new class
• Lots of code
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What about scalars?
• string
• int
• bool
• float
• null
• arrays of scalars
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Code Smell:
Primitives Obsession
„Primitives Obsession is using primitive data types to
represent domain ideas.“
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Primitives Obsession
No Type Safety
(until PHP 7)
Tests help
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Primitives Obsession
Easy to end up with Code Duplication
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Primitives Obsession
Magic values
„I knew at once what process(1.34, true) meant!“
-- Noone, ever
Class names have documentary value
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Primitives Obsession
No built-in validation
Tests help (again)
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Benefits of scalar types
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Benefits of scalar types
They are immutable
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Benefits of scalar types
They are easily created
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Benefits of scalar types
They are comparable
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Benefits of scalar types
Serializable
They work over the wire
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Benefits of scalar types
Completely upgrade safe
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Benefits of scalar types
They are fast
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Benefits of scalar types
They are language independent
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Scalars
vs.
Value Objects
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Scalars make great
boundary interfaces
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Scalar Boundary Interfaces
• Web API
• Components API
• Message Queue Payload
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Scalar Boundary Interfaces
• Less interface dependencies
• But implicit coupling to data format
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Topics
• Extraction & Encapsulation
• Managing State with Immutable Objects
• The Value of Validity
• Immutable Variables
• Reasoning about Code with Idempotence
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Immutable Variables
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In PHP?
No
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But we can treat variables as immutable.
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Example 1: reusing local variables
public function testTwoRequestsAddTwoMessages()
{
$request = $this->createProductUpdateRequest();
$app = new WebFront($request, $this->testFactory);
$app->runWithoutSendingResponse();
$request = $this->createStockUpdateRequest();
$app = new WebFront($request, $this->testFactory);
$app->runWithoutSendingResponse();
$queue = $this->testFactory->createQueue();
$this->assertCount(2, $queue);
}
// Multiple assignments to $request and $app
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What is the problem?
• Messy to move or extract
• Complex to reason about
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Example 1: treating local variables as immutable
public function testTwoRequestsAddTwoMessages()
{
$productUpdateRequest = $this->createProductUpdateRequest();
$firstApp = new WebFront($productUpdateRequest, $this->testFactory);
$firstApp->runWithoutSendingResponse();
$stockUpdateRequest = $this->createStockUpdateRequest();
$secondApp = new WebFront($stockUpdateRequest, $this->testFactory);
$secondApp->runWithoutSendingResponse();
$queue = $this->testFactory->createQueue();
$this->assertCount(2, $queue);
}
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Example 2: local variable mutation in a loop
// MagentoCatalogModelProduct
protected function convertToMediaGalleryInterface(array $mediaGallery)
{
$entries = [];
foreach ($mediaGallery as $image) {
$entries[] = $this->mediaGalleryEntryConverterPool
->convertImageToMediaGalleryEntry($image, $this);
}
return $entries;
}
// Multiple assignments to $image, mutation of $entries
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Example 2: map/reduce helps avoiding local variable
mutation
// MagentoCatalogModelProduct
protected function convertToMediaGalleryInterface(array $mediaGallery)
{
return array_map(
function ($image) {
$this->mediaGalleryEntryConverterPool
->convertImageToMediaGalleryEntry($image, $this);
},
$mediaGallery
);
}
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Immutable Variables
++Reasonability
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Immutable Variables
Less bugs
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Immutable Variables
Simple to refactor
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Immutable Variables
My new friends:
• Closure
• array_map()
• array_reduce()
• array_merge()
• array_filter()
• recursion
• ...
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Topics
• Extraction & Encapsulation
• Managing State with Immutable Objects
• The Value of Validity
• Immutable Variables
• Reasoning about Code with Idempotence
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Reasoning about
Code with
Idempotence
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Another of those words...
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Idempotence
„In computer science, the term idempotent is used ... to
describe an operation that will produce the same
results if executed once or multiple times.“
-- Wikipedia, Idempotence, Computer Science
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What it ain't
Example 1
class Logger
{
public function log($level, $message)
{
$f = fopen($this->file . '-' . $level, 'a');
flock($f, LOCK_EX);
fwrite($f, message);
flock($f, LOCK_UN);
fclose($f);
}
}
// repeated calls add additional records
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Idempotent logger
Example 1
class Logger
{
public function log($level, $message)
{
$f = fopen($this->file . '-' . $level, 'a');
flock($f, LOCK_EX);
if ($this->getLastLineFromFile() !== $message) {
fwrite($f, message);
}
flock($f, LOCK_UN);
fclose($f);
}
}
// repeated calls add NO additional records
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What it ain't
Example 2
protected function validateIsFoo($object)
{
if (! $object instanceof Foo) {
echo "$object is not an instance of Foon";
return false;
}
return true;
}
// repeated calls may produce repeated output
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Idempotent validateIsFoo
Example 2
protected function validateIsFoo($object)
{
return $object instanceof Foo;
}
// no output
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Pure Functions
All pure functions are idempotent
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Pure Functions
No side effects
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Pure Functions
Rely only on input arguments
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Pure Functions
No dependency on global state such as
• $_SERVER, $_SESSION
• getcwd()
• file_exists()
• Mage::getIsDeveloperMode()
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Pure Functions
For the same input they always return the same output
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And my point is?
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Idempotent Functions are
Simpler to reuse
Simpler to compose
Pure Functions are
Simpler to refactor
Simpler to change
Simpler to reason about
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It pays to focus on the
dependencies of every
method
OOP is all about dependency management
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I'm still learning
But so far results are very encouraging.
I'm happy for every tool that helps to reduce complexity
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Thanks for your attention!
Please,
Ask questions, and
I'm eager to hear your thoughts and comments!
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