События являются довольно сильным инструментом для приложений: коммуникация между компонентами системы; история действий над данными; триггеры для операций; интеграция с посторонними системами. Во время доклада я расскажу о применениях событий. На какие грабли можно наступить спеша в выборе инструмента и какое место они имеют в нашей архитектуре.

1. Andrii Dembitskyi, EvoPlay
Events in our applications. Event bus and
distributed systems.

2. CQRS

4. CQRS
(Command query responsibility segregation)

5. Read Model
© An introduction to CQRS (without Event Sourcing) made at MS experiences days in Paris by Thomas Pierrain

6. Write Model

7. How is this related?

8. Sync Read & Write data store

9. Sync Read & Write data store

10. Sync Read & Write data store

11. Command
Triggers an action.
Modify the state of the system.
Expected to be handled immediately or ASAP
at least by one handler.
Can be rejected.

12. Event
Is a message sent by an object to signal the
occurrence of an action.
Doesn’t have restriction on reaction time or
count handlers.

13. Command & Event
Event
1
0..N
HandlersHandler
1
1..N
Command
Handlers

14. Command & Event
Immutability

15. Command & Event
Immutability

16. Ways of event processing
Asynchronous
Synchronous

17. Synchronous (independent handlers)
Return
Program
Handlers

18. Synchronous (chain on handlers)
Return
Program
Handlers

19. Asynchronous
Program
Handlers

20. Distributed applications

21. // we have services

22. // we have a bit more services

23. Event propagation via point to point

24. Event bus
docs.microsoft.com

25. Event Lifecycle in Event Bus
Topic Consumers
“loyalty-p.tracking”
“auth”
“analytics.metrics”
Queues
A
A
A A
A
A
B
B B
B
A
A - auth.registration.success
B - auth.login.success
События, события и ещё раз события / Антон Сухов (Avito)

26. Solutions
● Azure Service Bus
● IBM Integration Bus
● RabbitMQ
● Kafka

27. RabbitMQ
www.cloudamqp.com

28. RabbitMQ
www.cloudamqp.com

29. RabbitMQ
www.cloudamqp.com

30. RabbitMQ
www.cloudamqp.com

31. RabbitMQ
www.cloudamqp.com

32. Example

33. Create producer topic
$this->eventBus-
>registerProducer();

34. Shared libraries

35. Create subscriptions
$descriptors = [
$this->authDescriptorFactory->loginSuccessful(),
$this->authDescriptorFactory->logoutSuccessful(),
$this->authDescriptorFactory->registrationSuccessful(),
];
$this->eventBus->registerSubscription('ev-bus.loyalty-program.tracking', $descriptors);

36. Send event
$this->eventPublisher->loginSuccessful($playerId);

37. Consume events
$this->eventBus->consumeSubscription('ev-bus.loyalty-program.tracking', $this->consumer);
public function consume(EventBusEventInterface $event)
{
$metric = $this->convertToMetricName($event->getType());
$this->incrementMetric($metric, $event->getCreatedAt());
}

38. RabbitMQ

39. RabbitMQ

40. Failure strategies
ok
is this
message
useful if
delivered
late?
process
message
recreate
same
message
with added
retry
metadata
reject and
requeue
reject
without
requeue
dead letter
queue
is the
failure
temporary
?
succeeds
fals
no
no
yes
yes
not sure

41. Saga
Orchestration
distributed decision making
Choreography
centralized decision making
© microXchg: Managing data consistency in a microservice architecture using Sagas from Chris Richardson

42. Without saga
docs.microsoft.com

43. Orchestration-based saga
docs.microsoft.com

44. Orchestration-based saga
docs.microsoft.com

45. Orchestration-based saga
Benefits
● Centralized coordination logic
is easier to understand
● Reduced coupin
● Reduces cyclic dependencies
Drawbacks
● Risk of smart sagas
directing dumb services
© microXchg: Managing data consistency in a microservice architecture using Sagas from Chris Richardson

46. Choreography-based saga

47. Choreography-based saga
Benefits
● Simple
● Participants are loosely coupled
Drawbacks
● Cyclic dependencies - services
listen to each other’s events
● Distributed logic. Hard to find all
participants
© microXchg: Managing data consistency in a microservice architecture using Sagas from Chris Richardson

48. Summary

49. Questions?
Andrii Dembitskyi
Telegram: @andrewDemb