CQRS and Event sourcing

Presentation given at GDG-Natal, January/2017

Transcript

1. @batusa
2. madewithlove
4. CQRS Event sourcing
5. CQRS meaning Command Query Responsibility Segregation
6. CQRS top-level view Write Read
7. CQRS top-level view Write Read Command Query
8. Write model • Command • Command handler • Command bus
9. Command • Represents user intention • Class name is written in the imperative • Data Transfer Object • Immutable • Serializable
10. Command Ubiquitous Language
11. Command class DeleteUserFromClass { … class DisenrollStudentFromSubject { …
12. Command code example final class DisenrollStudentFromSubject { private $student; private $enrollment; public function __construct(StudentId $student, EnrollmentId $enrollment) { $this->student = $student; $this->enrollment = $enrollment; } public function getStudent() : StudentId { return $this->student; } public function getEnrollment() : EnrollmentId { return $this->enrollment; } }
13. Command handler • Accomplish a business task with data from the command; • Can use service objects; • Should affect only one entity/aggregate; • Can fire events; • Command handlers and commands have a relationship 1:1; • Can throw exceptions; • Should not return anything;
14. Command handler code example use Command\DisenrollStudentFromSubject as Command; class DisenrollStudentFromSubject { private $students; public function __construct(RepositoryInterface $students) { $this->students = $students; } /** * @throws EnrollmentNotFound */ public function handle(Command $command) : void { $student = $this->students->find($command->getStudent()); $student->disenroll($command->getEnrollment()); $this->students->save($student); } }
15. Command bus • Take a command and match it to its handler • Ability to queue the command • Apply decorators to run around handlers
16. Command bus $commandBus->dispatch( new DisenrollStudentFromSubject($student, $enrollment) ); • Take a command and match it to its handler • Ability to queue the command • Apply decorators to run around handlers
17. Read model • Can have direct access to the database; • Should report data back to the client using DTO tailored to the specific needs of the view; • It only queries data and can do calculations with the support of domain objects.
18. Putting it all together
19. Event sourcing Storing all the changes (events) to the system, rather than just its current state by using an append-only and immutable store.
20. CQRS + Event sourcing “Event sourcing is commonly combined with the CQRS pattern by performing the data management tasks in response to the events, and by materializing views from the stored events.”
21. CQRS + Event sourcing
22. CQRS + Event sourcing Stand-alone CQRS CQRS + Event sourcing
23. Events • Events have already occurred; • Each event represents a set of changes; • Events are immutable and serializable; • We describe events using Ubiquitous Language and in the past tense; • We can use events to express history; • Events can't be undone; • If something went wrong, we compensate it with other event.
24. Event code example interface DomainEvent { public function getAggregateRootId() : string; public function getOccurredOn() : DateTimeImmutable; }
25. Event code example final class StudentWasDisenrolledFromSubject implements DomainEvent { private $student; private $enrollment; public function __construct( StudentId $student, EnrollmentId $enrollment, DateTimeImmutable $occurredOn ) { /** assign attributes here */ } public function getOccurredOn() : DateTimeImmutable { return $this->occurredOn; } public function getAggregateRootId() : string { return (string) $this->student; } }
26. Aggregates (ES context) • Record events • Do not expose state • Protect invariants • Final state is built by replaying stored events
27. Aggregate code example class Student extends EventSourcedAggregateRoot { public function disenroll(EnrollmentId $enrollment) { $this->apply( new StudentWasDisenrolledFromSubject($this->id, $enrollment) ); } }
28. Aggregate code example class Student extends EventSourcedAggregateRoot { public function disenroll(EnrollmentId $enrollment) { $this->apply( new StudentWasDisenrolledFromSubject($this->id, $enrollment) ); } private function applyStudentWasDisenrolledFromSubject( StudentWasDisenrolledFromSubject $event ) { $this->enrollments ->filter(function ($enrollment) use ($event) { return $enrollment->getId()->equals($event->getEnrollment()); }) ->forget(); } }
29. Loading and saving aggregates // reconstitute aggregate by replaying events $student = $this->students->find($command->student()); // registering events $student->disenroll($command->enrollment()); // appends new events to the event store $this->students->save($student);
30. Loading and saving aggregates // reconstitute aggregate by replaying events $student = $this->students->find($command->student()); // registering events $student->disenroll($command->enrollment()); // appends new events to the event store $this->students->save($student);
31. Projections Produce state representation from stored events
32. Projections Can be run at any given time
33. Projector code example public function projectStudentWasDisenrolledFromSubject( StudentWasDisenrolledFromSubject $event ) { $this->redisClient->incr('disenrolledStudents'); }
34. Projector code example public function projectStudentWasDisenrolledFromSubject( StudentWasDisenrolledFromSubject $event ) { $this->redisClient->incr('disenrolledStudents'); } public function projectStudentWasRegistered( StudentWasRegistered $event ) { $studentProfile = new StudentProfile( $event->getName(), $event->getEmail(), $event->getBirthdate() ); $this->elasticSearchClient->save($studentProfile); }
35. Eventual consistency
36. References • cqrs.nu • msdn.microsoft.com/en-us/library/jj554200.aspx • verraes.net/#blog • github.com/broadway/broadway
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