In his publication describing CUPID principles, Dan North begins with the question, "If I do not think the SOLID principles are useful these days, then what would I replace them with?". This got me thinking about what I think of SOLID and how I look at it. In my presentation, I would like to break down this acronym into its components, test its usefulness with examples (based on a Symfony-based app), and consider what we can take from it. I will also try to answer the question posed in the title of the presentation.

1. @lukaszchrusciel
Is
Is SOLID
SOLID
dead?
dead?
Let's discuss reusable
Let's discuss reusable
software design
software design

2. @lukaszchrusciel

3. @lukaszchrusciel
Context
Context is a King
is a King

4. @lukaszchrusciel
Story
Story

5. @lukaszchrusciel
S
Single
ingle
Responsibility
Responsibility
Principle
Principle

6. @lukaszchrusciel
public function __invoke(string $orderNumber, int $unitId, int $amount,
RefundTypeInterface $refundType): void
{
/** @var OrderInterface|null $order */
$order = $this->orderRepository->findOneByNumber($orderNumber);
Assert::notNull($order);
$remainingTotal = $this->remainingTotalProvider->getTotalLeftToRefund($unitId,
$refundType);
if ($remainingTotal === 0) {
throw UnitAlreadyRefunded::withIdAndOrderNumber($unitId, $orderNumber);
}
$refund = $this->refundFactory->createWithData($order, $unitId, $amount,
$refundType);
$this->refundManager->persist($refund);
$this->refundManager->flush();
}

7. @lukaszchrusciel
Types of classes
Types of classes

8. @lukaszchrusciel
Types of classes
Types of classes
Data structures
Data structures

9. @lukaszchrusciel
Data structures
Data structures
ValueObjects
–

10. @lukaszchrusciel
Data structures
Data structures
ValueObjects
–
DataTransferObjects
–

11. @lukaszchrusciel
Data structures
Data structures
ValueObjects
–
DataTransferObjects
–
Models
–

12. @lukaszchrusciel
Data structures
Data structures
ValueObjects
–
DataTransferObjects
–
Models
–
Entities
–

13. @lukaszchrusciel
Data structures
Data structures
readonly class BlameCart
{
public function __construct(
public string $shopUserEmail,
public string $orderTokenValue
) {
}
}

14. @lukaszchrusciel
Types of classes
Types of classes
Data structures
Data structures
Calculations
Calculations

15. @lukaszchrusciel
Calculations
Calculations
F(X) →
F(X) → Y
Y

16. @lukaszchrusciel
Calculations (pure functions)
Calculations (pure functions)
final class PerUnitRateCalculator implements CalculatorInterface
{
public function calculate(ShipmentInterface $subject, array
$configuration): int
{
return (int) ($configuration['amount'] * $subject-
>getShippingUnitCount());
}
public function getType(): string
{
return 'per_unit_rate';
}
}

17. @lukaszchrusciel
Calculations (impure functions)
Calculations (impure functions)
final class PromotionApplicator implements PromotionApplicatorInterface
{
public function __construct(private ServiceRegistryInterface $registry)
{
}
public function apply(PromotionSubjectInterface $subject, PromotionInterface $promotion): void
{
$applyPromotion = false;
foreach ($promotion->getActions() as $action) {
$result = $this->registry->get($action->getType())->execute($subject, $action-
>getConfiguration(), $promotion);
$applyPromotion = $applyPromotion || $result;
}
if ($applyPromotion) {
$subject->addPromotion($promotion);
}
}
}

18. @lukaszchrusciel
Calculations (impure functions)
Calculations (impure functions)
final class PromotionApplicator implements PromotionApplicatorInterface
{
public function __construct(private ServiceRegistryInterface $registry)
{
}
public function apply(PromotionSubjectInterface $subject, PromotionInterface $promotion): void
{
$applyPromotion = false;
foreach ($promotion->getActions() as $action) {
$result = $this->registry->get($action->getType())->execute($subject, $action-
>getConfiguration(), $promotion);
$applyPromotion = $applyPromotion || $result;
}
if ($applyPromotion) {
$subject->addPromotion($promotion);
}
}
}

19. @lukaszchrusciel
Types of classes
Types of classes
Data structures
Data structures
Calculations
Calculations
Side effects
Side effects

20. @lukaszchrusciel
Communication outside of the local
Communication outside of the local
scope
scope
Mailing
–

21. @lukaszchrusciel
Communication outside of the local
Communication outside of the local
scope
scope
Mailing
–
Transaction committing
–

22. @lukaszchrusciel
Communication outside of the local
Communication outside of the local
scope
scope
Mailing
–
Transaction committing
–
Making request
–

23. @lukaszchrusciel
public function __invoke(string $orderNumber, int $unitId, int $amount,
RefundTypeInterface $refundType): void
{
/** @var OrderInterface|null $order */
$order = $this->orderRepository->findOneByNumber($orderNumber);
Assert::notNull($order);
$remainingTotal = $this->remainingTotalProvider->getTotalLeftToRefund($unitId,
$refundType);
if ($remainingTotal === 0) {
throw UnitAlreadyRefunded::withIdAndOrderNumber($unitId, $orderNumber);
}
$refund = $this->refundFactory->createWithData($order, $unitId, $amount,
$refundType);
$this->refundManager->persist($refund);
$this->refundManager->flush();
}

24. @lukaszchrusciel
Communication outside of the local scope
Communication outside of the local scope
public function __invoke(string $orderNumber, int $unitId, int $amount, RefundTypeInterface
$refundType): void
{
$order = $this->orderRepository->findOneByNumber($orderNumber);
$remainingTotal = $this->remainingTotalProvider->getTotalLeftToRefund($unitId, $refundType);
if ($remainingTotal === 0) {
throw UnitAlreadyRefunded::withIdAndOrderNumber($unitId, $orderNumber);
}
$refund = $this->refundFactory->createWithData(
$order,
$unitId,
$amount,
$refundType
);
$this->refundManager->persist($refund);
$this->refundManager->flush();
}

25. @lukaszchrusciel
Communication outside of the local scope
Communication outside of the local scope
public function __invoke(string $orderNumber, int $unitId, int $amount, RefundTypeInterface
$refundType): void
{
$order = $this->orderRepository->findOneByNumber($orderNumber);
$remainingTotal = $this->remainingTotalProvider->getTotalLeftToRefund($unitId, $refundType);
if ($remainingTotal === 0) {
throw UnitAlreadyRefunded::withIdAndOrderNumber($unitId, $orderNumber);
}
$refund = $this->refundFactory->createWithData(
$order,
$unitId,
$amount,
$refundType
);
return $refund;
}

26. @lukaszchrusciel

27. @lukaszchrusciel
O
Open-Close
pen-Close
Principle
Principle

28. @lukaszchrusciel
private function getRefundUnitTotal(
int $id,
RefundTypeInterface $refundType
): int

29. @lukaszchrusciel
private function getRefundUnitTotal(int $id,
RefundTypeInterface $refundType): int
{
if ($refundType->getValue() ===
RefundTypeInterface::ORDER_ITEM_UNIT) {
// calculation
return $value;
}
// calculation
return $otherValue;
}

30. @lukaszchrusciel
private function getRefundUnitTotal(int
$id, RefundTypeInterface $refundType): int
{
$refundUnitTotalProvider = $this-
>totalProviderRegistry->get($refundType-
>getValue());
return $refundUnitTotalProvider-
>getRefundUnitTotal($id);
}

31. @lukaszchrusciel
protected function configureOptionsNode(ArrayNodeDefinition
$optionsNode): void
{
$optionsNode
->children()
->integerNode('amount')->isRequired()->min(0)->end()
->scalarNode('channel')->cannotBeEmpty()->end()
->scalarNode('customer')->cannotBeEmpty()->end()
->scalarNode('country')->cannotBeEmpty()->end()
->booleanNode('fulfilled')->defaultValue(false)-
>end()
->end()
;
}

32. @lukaszchrusciel

33. @lukaszchrusciel

34. @lukaszchrusciel

35. @lukaszchrusciel
protected function configureOptionsNode(ArrayNodeDefinition
$optionsNode): void
{
$optionsNode
->children()
->integerNode('amount')->isRequired()->min(0)->end()
->scalarNode('channel')->cannotBeEmpty()->end()
->scalarNode('customer')->cannotBeEmpty()->end()
->scalarNode('country')->cannotBeEmpty()->end()
->booleanNode('fulfilled')->defaultValue(false)-
>end()
->end()
;
}

36. @lukaszchrusciel
L
Liskov
iskov
Substitution
Substitution
Principle
Principle

37. @lukaszchrusciel
Behavioural
Behavioural
sub-typing
sub-typing

38. @lukaszchrusciel
Covariance
Covariance
abstract class Animal
{
}
class Cat extends Animal
{
}
interface AnimalShelter
{
public function adopt(): Animal;
}
class CatShelter implements AnimalShelter
{
public function adopt(): Cat // instead of returning class type Animal, it can return class type Cat
{
return new Cat();
}
}

39. @lukaszchrusciel
abstract class Animal
{
}
class Cat extends Animal
{
}
interface CatShelter
{
public function adopt(Cat $cat);
}
class AnimalShelter implements CatShelter
{
public function adopt(Animal $animal) // instead of accepting class type Cat, it can
accept any Animal
{
return new Cat();
}
}

40. @lukaszchrusciel
abstract class Animal
{
}
class Cat extends Animal
{
}
interface CatShelter
{
public function adopt(Animal $cat);
}
class AnimalShelter implements CatShelter
{
public function adopt(Cat $animal) // instead of accepting class type Cat, it can accept any Animal
{
return new Cat();
}
}
Fatal error: Declaration of AnimalShelter::adopt(Cat $animal) must be compatible with
CatShelter::adopt(Animal $cat) in /tmp/preview on line 19

41. @lukaszchrusciel
interface CalculatorInterface
{
public function
calculate(ShipmentInterface $subject,
array $configuration): int;
public function getType(): string;
}

42. @lukaszchrusciel
final class PerUnitRateCalculator implements CalculatorInterface
{
public function calculate(ShipmentInterface $subject, array
$configuration): int
{
return (int) ($configuration['amount'] * $subject-
>getShippingUnitCount());
}
public function getType(): string
{
return 'per_unit_rate';
}
}

43. @lukaszchrusciel
{
/**
* @param array{'amount': int}
$configuration
*/
public function calculate(ShipmentInterface
$subject, array $configuration): int;
public function getType(): string;
}

44. @lukaszchrusciel
interface CalculatorInterface
{
/**
* @throws
InvalidCalculatorConfigurationException
*/
public function calculate(ShipmentInterface
$subject, array $configuration): int;
public function getType(): string;
}

45. @lukaszchrusciel
public function configureOptions(OptionsResolver $resolver)
{
parent::configureOptions($resolver);
// some logic
$resolver->setDefined([
'data',
]);
$resolver->setDefaults([
'data_class' => $dataClass,
'empty_data' => $emptyData,
'...' =>
]);

46. @lukaszchrusciel
interface CalculatorInterface
{
/**
* @throws
InvalidCalculatorConfigurationException
*/
public function calculate(Configuration
$configuration): int;
public function getType(): string;
}

47. @lukaszchrusciel
$resources = $this->resourcesResolver-
>getResources($requestConfiguration, $repository);
$paginationLimits = [];
if ($resources instanceof ResourceGridView) {
$paginator = $resources->getData();
$paginationLimits = $resources-
>getDefinition()->getLimits();
} else {
$paginator = $resources;
}

48. @lukaszchrusciel
I
Interface
nterface
Segregation
Segregation
Principle
Principle

49. @lukaszchrusciel
interface OrderRepositoryInterface extends BaseOrderRepositoryInterface
{
public function createListQueryBuilder(): QueryBuilder;
public function createSearchListQueryBuilder(): QueryBuilder;
public function createByCustomerIdQueryBuilder($customerId): QueryBuilder;
public function createByCustomerAndChannelIdQueryBuilder($customerId, $channelId): QueryBuilder;
public function countByCustomerAndCoupon(CustomerInterface $customer, PromotionCouponInterface $coupon): int;
public function countByCustomer(CustomerInterface $customer): int;
public function findOrderById($id): ?OrderInterface;
public function findByCustomer(CustomerInterface $customer): array;
public function findForCustomerStatistics(CustomerInterface $customer): array;
public function findOneForPayment($id): ?OrderInterface;
public function findOneByNumberAndCustomer(string $number, CustomerInterface $customer): ?OrderInterface;
public function findCartByChannel($id, ChannelInterface $channel): ?OrderInterface;
public function findLatestCartByChannelAndCustomer(ChannelInterface $channel, CustomerInterface $customer): ?OrderInterface;
public function findLatestNotEmptyCartByChannelAndCustomer(ChannelInterface $channel, CustomerInterface $customer): ?OrderInterface;
public function getTotalSalesForChannel(ChannelInterface $channel): int;
public function getTotalPaidSalesForChannel(ChannelInterface $channel): int;
public function getTotalPaidSalesForChannelInPeriod(ChannelInterface $channel, DateTimeInterface $startDate, DateTimeInterface $endDate): int;
public function countFulfilledByChannel(ChannelInterface $channel): int;
public function countPaidByChannel(ChannelInterface $channel): int;
public function countPaidForChannelInPeriod(ChannelInterface $channel, DateTimeInterface $startDate, DateTimeInterface $endDate): int;
public function findLatestInChannel(int $count, ChannelInterface $channel): array;
public function findOrdersUnpaidSince(DateTimeInterface $terminalDate, ?int $limit = null): array;
public function findCartForSummary($id): ?OrderInterface;
public function findCartForAddressing($id): ?OrderInterface;
public function findCartForSelectingShipping($id): ?OrderInterface;
public function findCartForSelectingPayment($id): ?OrderInterface;
public function findCartByTokenValue(string $tokenValue): ?BaseOrderInterface;
public function findCartByTokenValueAndChannel(string $tokenValue, ChannelInterface $channel): ?BaseOrderInterface;
}

50. @lukaszchrusciel
OrderRepositoryInterface extends
BaseOrderRepositoryInterface

51. @lukaszchrusciel
D
Dependency
ependency
Inversion
Inversion
Principle
Principle

52. @lukaszchrusciel
public function __invoke(VerifyCustomerAccount $command): JsonResponse
{
/** @var UserInterface|null $user */
$user = $this->shopUserRepository->findOneBy(['emailVerificationToken' =>
$command->token]);
if (null === $user) {
throw new InvalidArgumentException(
sprintf('There is no shop user with %s email verification token',
$command->token),
);
}
$user->setVerifiedAt(new DateTime());
$user->setEmailVerificationToken(null);
$user->enable();
return new JsonResponse([]);
}

53. @lukaszchrusciel
public function __construct(
private RepositoryInterface $shopUserRepository,
private DateTimeProviderInterface $calendar,
) {
}
public function __invoke(VerifyCustomerAccount $command): JsonResponse
{
/** @var UserInterface|null $user */
$user = $this->shopUserRepository->findOneBy(['emailVerificationToken' => $command->token]);
if (null === $user) {
throw new InvalidArgumentException(
sprintf('There is no shop user with %s email verification token', $command->token),
);
}
$user->setVerifiedAt($this->calendar->now());
$user->setEmailVerificationToken(null);
$user->enable();
return new JsonResponse([]);
}

54. @lukaszchrusciel
Sylius/Calendar
–
PSR-20 Clock interface +
Symfony/Clock
–

55. @lukaszchrusciel

56. @lukaszchrusciel

57. @lukaszchrusciel

58. @lukaszchrusciel
FINAL
FINAL

59. @lukaszchrusciel
What the horse is, everyone can see
1
ks. Benedykta Chmielowskiego
1.

60. @lukaszchrusciel
What the address is, everyone can
see 1
Łukasz Chruściel
1.

61. @lukaszchrusciel
Łukasz Chruściel
Wólczańska 125
90-521 Łódź
Poland

62. @lukaszchrusciel
Name Surename
Street StreetNumber (?Flat)
PostCode City
Country

63. @lukaszchrusciel
Antigua and Barbuda
–
Aruba
–
Ascension island
–
Bahamas
–
Belize
–
Benin
–
Botswana
–
Bolivia
–
Bonaire, Sint Eustatius and Saba
–
Burkina Faso
–
Burundi
–
Cameroon
–
Central African Republic
–
Comoros
–
Congo
–
Congo the Democratic Republic of the
–
Cook Islands
–
Cote d’Ivoire
–
Curaçao
–
Djibouti
–
Dominica
–
East Timor
–
Equatorial Guinea
–
Eritrea
–
Fiji
–
French Southern Territories
–
Gambia
–
Gamon
–
Ghana
–
Grenada
–
Guyana
–
Heard and McDonald Islands
–
Hong Kong
–
Kiribati
–
Libya
–
Macau
–
Malawi
–
Mali
–
Mauritania
–
Nauru
–
Netherlands Antilles
–
Niue
–
North Korea
–
Qatar
–
Rwanda
–
Saint Kitts and Nevis
–
Sao Tome and Principe
–
Seychelles
–
Sierra Leone
–
Solomon Islands
–
Suriname
–
Syria
–
Timor-Leste
–
Togo
–
Tokelau
–
Tonga
–
Tuvalu
–
Uganda
–
United Arab Emirates
–
Vanuatu
–
Yemen
–
Zimbabwe
–

64. @lukaszchrusciel
Antigua and Barbuda
–
Aruba
–
Ascension island
–
Bahamas
–
Belize
–
Benin
–
Botswana
–
Bolivia
–
Bonaire, Sint Eustatius and Saba
–
Burkina Faso
–
Burundi
–
Cameroon
–
Central African Republic
–
Comoros
–
Congo
–
Congo the Democratic Republic of the
–
Cook Islands
–
Cote d’Ivoire
–
Curaçao
–
Djibouti
–
Dominica
–
East Timor
–
Equatorial Guinea
–
Eritrea
–
Fiji
–
French Southern Territories
–
Gambia
–
Gamon
–
Ghana
–
Grenada
–
Guyana
–
Heard and McDonald Islands
–
Hong Kong
Hong Kong
–
Kiribati
–
Libya
–
Macau
Macau
–
Malawi
–
Mali
–
Mauritania
–
Nauru
–
Netherlands Antilles
–
Niue
–
North Korea
–
Qatar
Qatar
–
Rwanda
–
Saint Kitts and Nevis
–
Sao Tome and Principe
–
Seychelles
–
Sierra Leone
–
Solomon Islands
–
Suriname
–
Syria
–
Timor-Leste
–
Togo
–
Tokelau
–
Tonga
–
Tuvalu
–
Uganda
–
United Arab Emirates
United Arab Emirates
–
Vanuatu
–
Yemen
–
Zimbabwe
–

65. @lukaszchrusciel
Name Surename
Street StreetNumber (?Flat)
PostCode City
Country

66. @lukaszchrusciel
Outro
Outro

67. @lukaszchrusciel
SOLID works perfectly
SOLID works perfectly in
in
the context of reusable
the context of reusable
software
software

68. @lukaszchrusciel
SOLID gives useful
SOLID gives useful
directions how to design
directions how to design
any code
any code

69. @lukaszchrusciel
Benefits of these
Benefits of these
approaches
approaches

70. @lukaszchrusciel
Is there anything
Is there anything
more in
more in CUPID
CUPID?
?

71. @lukaszchrusciel
Resources
Resources
https://symfonycasts.com/screencast/solid/
https://symfonycasts.com/screencast/solid/
https://stackoverflow.blog/2021/11/01/why-solid-principles-are-still-the-foundation-for-modern-
software-architecture/
–
https://dannorth.net/2022/02/10/cupid-for-joyful-coding/
–
https://speakerdeck.com/tastapod/why-every-element-of-solid-is-wrong
–
https://www.reddit.com/r/programming/comments/5qto27/why_every_element_of_solid_is_wrong/
–
https://solid-is-not-solid.com/
–
Norbert Wójtowicz (@pithyless) - Functional Programming
–
https://www.youtube.com/watch?v=-Z-17h3jG0A
–

72. @lukaszchrusciel
Takeaways
Takeaways If you see clickbait title with
If you see clickbait title with
question in it
question in it
Then the answer is always no
no
Functional programming may
Functional programming may
help us
help us
Deliver better ObjectOriented code
SOLID is a solid fundament for
SOLID is a solid fundament for
packages that will be reused by
packages that will be reused by
other devs
other devs
But end applications may benefit from it as well
Doing eCommerce? Give a
Doing eCommerce? Give a
Sylius try ;)
Sylius try ;)
Don't be discouraged by this presentation

73. @lukaszchrusciel

74. @lukaszchrusciel
Thank you!
Thank you!