This session is about rules & behavior at the place that holds the data. We will work our way up from value objects to services and consider the pro, con and grey area design choices along the way. Do you want to learn what exactly the function of an aggregate is? Then join me for a discussion on boundaries, message passing, and role play.

1. @pelshoff

2. @pelshoff

3. @pelshoff
aggregate

4. @pelshoff

5. @pelshoff
Rules & Behavior
at the Information

6. @pelshoff
1/3 Objects & Messages

7. @pelshoff
$response = $object->message($input);

8. @pelshoff
Value object Entity Service
Identity X V X
State V V X
Rules V V V
Behavior ? V V

9. @pelshoff
$today = Date::today();
$yesterday = $today->previousDay();
$lastYear = $today->previousYear();
$request = $request->withHeader('x-pim-is', 'awesome');
$fiftyCents = Money::EUR(50);
$oneEuro = $fiftyCents->multiply(Amount::fromFloat(2.0));
https://twitter.com/Pelshoff/status/1266012117230071808 for many more examples!

10. @pelshoff
https://i.imgflip.com/43d4ha.jpg

11. @pelshoff
$today = Date::today();
$yesterday = $today->previousDay();
$lastYear = $today->previousYear();
$request = $request->withHeader('x-pim-is', 'awesome');
$fiftyCents = Money::EUR(50);
$oneEuro = $fiftyCents->multiply(Amount::fromFloat(2.0));

12. @pelshoff
$yesterday = $today->previousDay();
$otherDay = $today->withDay(32);

13. @pelshoff
$yesterday = $today->previousDay();
$otherDay = $today->withDay(32);
$bluesDay = Date::fromString('My favorite color is blue');

14. @pelshoff
$yesterday = $today->previousDay();
$otherDay = $today->withDay(32);
$bluesDay = Date::fromString('My favorite color is blue');
$newBalance = $balance->subtract(Money::EUR(999));

15. @pelshoff
$yesterday = $today->previousDay();
$otherDay = $today->withDay(32);
$bluesDay = Date::fromString('My favorite color is blue');
$newBalance = $balance->subtract(Money::EUR(999));
$currentMeetings = $meetingService->getCurrentMeetings();

16. @pelshoff
Input Local Global
Information AggregateValue object Entity Service

17. @pelshoff
Aggregate
Root entity
Entities

18. @pelshoff
$meeting = $meetingRepository->getMeeting($meetingId);
try {
$meeting->register($attendeeId);
} catch (CouldNotRegisterAttendee $e) {
/**/
}
$meetingRepository->save($meeting);
Single
transaction

19. @pelshoff
$myAggregate->addAnEntity(new AnEntity(/**/));
$myAggregate->addAnEntity($anEntityId, /**/);

20. @pelshoff
$myAggregate->addAnEntity(new AnEntity(/**/));
$myAggregate->addAnEntity($anEntityId, /**/);
$anEntity = $myAggregate->getAnEntity($anEntityId);
$aViewModel = $myAggregate->getAnEntity($anEntityId); // best not?
$aViewModel = $myAggregate->view()->getAnEntity($anEntityId); // my preference
$myAggregate = $anEntityViewModel->getAnEntity($anEntityId); // CQRS

21. @pelshoff
$myAggregate->addAnEntity(new AnEntity(/**/));
$myAggregate->addAnEntity($anEntityId, /**/);
$anEntity = $myAggregate->getAnEntity($anEntityId);
$aViewModel = $myAggregate->getAnEntity($anEntityId); // best not?
$aViewModel = $myAggregate->view()->getAnEntity($anEntityId); // my preference
$myAggregate = $anEntityViewModel->getAnEntity($anEntityId); // CQRS
$myAggregate->getAnEntity($anEntityId)->update(/**/);
$myAggregate->updateAnEntity($anEntityId, /**/);

22. @pelshoff
$myAggregate->addAnEntity(new AnEntity(/**/));
$myAggregate->addAnEntity($anEntityId, /**/);
$anEntity = $myAggregate->getAnEntity($anEntityId);
$aViewModel = $myAggregate->getAnEntity($anEntityId); // best not?
$aViewModel = $myAggregate->view()->getAnEntity($anEntityId); // my preference
$myAggregate = $anEntityViewModel->getAnEntity($anEntityId); // CQRS
$myAggregate->getAnEntity($anEntityId)->update(/**/);
$myAggregate->updateAnEntity($anEntityId, /**/);

23. @pelshoff
query failed: [1213] Deadlock: wsrep aborted transaction

24. @pelshoff
Service - Eventually consistent
Aggregate - Transactionally consistent
Entity
Value
Object

25. @pelshoff
The need for information drives design

26. @pelshoff
Value object Entity Aggregate Service
Identity X V V X
State V V V X
Rules V V V V
Behavior V V V V
Consistency Transactional Eventual

27. @pelshoff
2/3 Context

28. @pelshoff

29. @pelshoff
final class MeetingService
{
private MeetingRepository $repository;
private Clock $clock;
public function __construct(/**/) {/**/}
public function getCurrentMeetings(): array
{
$range = new ClosedDateTimeRange($this->clock->now(), $this->clock->now('+1 month'));
return array_map(
fn (Meeting $meeting) => $meeting->view(),
$this->repository->findMeetingsBySpecification(
new IsPublishedDuring($range)
)
);
}
}
No input!
Context Context
Context

30. @pelshoff
final class MeetingServiceTest extends TestCase
{
public function testThatItOnlyFindsCurrentMeetings(): void
{
$meetingService = new MeetingService(
new InMemoryMeetingRepository(),
new Clock('1997-01-01')
);
$meetingService->planNewMeetingAt(new DateTimeImmutable('1996-12-01'));
$meetingService->planNewMeetingAt(new DateTimeImmutable('1997-01-11'));
$meetingService->planNewMeetingAt(new DateTimeImmutable('1997-05-11'));
$actual = $meetingService->getCurrentMeetings();
$expected = [new MeetingView(new DateTimeImmutable('1997-01-11'))];
$this->assertEquals($expected, $actual);
}
}

31. @pelshoff
final class RegistrationService
{
private MeetingRepository $meetingRepository;
private AttendeeRepository $attendeeRepository;
private RegistrationRepository $registrationRepository;
public function __construct(/**/) {/**/}
public function registerAttendee(Uuid $meetingId, Uuid $attendeeId): void
{
$meeting = $this->assertMeertingExists($meetingId);
$this->assertAttendeeIsNotImaginary($attendeeId);
$this->assertAttendeeIsNotRegistered($meetingId, $attendeeId);
$registration = $meeting->register($attendeeId);
$this->registrationRepository->save($registration);
}
}
Context
Context
Context
Side-effect
Input

32. @pelshoff
final class RegistrationServiceTest extends TestCase
{
public function testThatItSavesRegistrations(): void
{
/**/
$this->getMockBuilder(RegistrationRepository::class)
->getMock()
->expects($this->once())
->method('sav')
->with(new Registration($meetingId, $attendeeId));
}
}
Oops

33. @pelshoff
$client->expects($this->any())
->method('request')
->willReturnOnConsecutiveCalls(
new JsonResponse(['access_token' => 'testAccessToken']), // authenticateClient
new JsonResponse(['refresh_token' => 'testToken']),
new JsonResponse(['data' => [
[
'standingInstructionType' => StandingInstruction::STANDING_INSTRUCTION_PURCHASE,
'fundCode' => 'testFundCode',
'endDate' => date('Y-m-d', strtotime('+99 year')),
'standingInstructionNumber' => 3,
],
]]), // get
new JsonResponse([]), // le remove
new JsonResponse([]), // put
);
100%
converage!
:D
Real
code!

34. @pelshoff
Mock and spy
make me cry :’(

35. @pelshoff

36. @pelshoff
MeetingService::registerAttendee()
R
R
R
M

37. @pelshoff

38. @pelshoff
MeetingService::registerAttendee()
R
R
M

39. @pelshoff

40. @pelshoff
MeetingService::registerAttendee()
R
R
R
RegisterNewAttendee::execute

41. @pelshoff
final class RegistrationService
{
private MeetingRepository $meetingRepository;
private AttendeeRepository $attendeeRepository;
private RegistrationRepository $registrationRepository;
public function __construct(/**/) {/**/}
public function registerAttendee(Uuid $meetingId, Uuid $attendeeId): void
{
$meeting = $this->assertMeertingExists($meetingId);
$attendee = $this->assertAttendeeIsNotImaginary($attendeeId);
$listOfAttendees = $this->attendeeRepository->listAttendeesFor($meetingId);
$context = new RegisterNewAttendee($meeting, $listOfAttendees);
$registration = $context->register($attendee);
$this->registrationRepository->save($registration);
}
}

42. @pelshoff
final class RegisterNewAttendee
{
private Meeting $meeting;
private ListOfRegistrations $registrations;
/**/
public function register(Attendee $attendee): Registration
{
$this->assertAttendeeIsNotRegistered($attendee);
return $this->meeting->register($attendee->getId());
}
private function assertAttendeeIsNotRegistered(Attendee $attendee)
{
if ($this->registrations->isAttendeeRegistered($attendee->getId())) {
throw CouldNotRegisterAttendee::becauseAttendeeWasPreviouslyRegistered(
$this->meeting->getId(),
$attendee->getId()
);
}
}
}

43. @pelshoff
1, 2...
// Integration/DB or Unit+Mock
function testThatNewRegistrationsAreSaved()
function testThatRegistrationsAreUpdatedAndSaved()
function testThatXyAndZAndSaved()
many
// Integration/DB
function testThatTheAggregateCanBeSaved()
function testThatSpecialCircumstancesAlsoIntegrateWell()
// Unit
function testThatBusinessLogicWorksAsExpected()
function testThatTheyDontRequireManyMocks()
function testThatIfEvenAnyAtAll()
function testThatItMakesYouHappierAndMoreProductive()

44. @pelshoff
Service Aggregate Service+Context
Performance + - +/-
Code overhead +/- + -
Infra complexity + +/- +
Unit testing - + +
Consistency Eventual Transactional It depends
Conclusion For simple cases
(most)
For transactional
boundaries
For complex cases

45. @pelshoff
3/3 Heuristics

46. @pelshoff
Heuristics
anything that provides a plausible aid or direction in the solution of a problem but is in the final analysis
unjustified, incapable of justification, and potentially fallible.
-Billy Vaughn Koen
https://www.dddheuristics.com/

47. @pelshoff
Everything is a value (object) until it's not

48. @pelshoff
Aggregates guard transactional
consistency, services guard eventual
consistency

49. @pelshoff
Programmers make theoretical rules

50. @pelshoff
Keep aggregates small

51. @pelshoff
Push context and side-effects to the edge
of the domain

52. @pelshoff
Rules and Behavior at the Information

53. @pelshoff
Why is there so much
confusion about aggregates?

54. Pim Elshoff
Work: developer.procurios.com
Follow: @pelshoff
Coaching: pelshoff.com
Slides: speakerdeck.com/pelshoff/final-class-aggregate
Diagrams: diagram.codes