Event-Driven Architecture Traps by Jakub Pilimon at SpringOne Tour 2019

1. JakubPilimon
EVENT-DRIVEN ARCHITECTURE
TRAPS
… AND HOW TO AVOID THEM

2. SZCZEBRZESZYN
CHRZĄSZCZYŻEWOSZYCE

3. JakubPilimon
DISCLAIMERS
• Traps based on private experiences
• Solutions worked in my context
• Haven’t pushed anything to production since … 11
months
• … But this is based on previous experiences
• Completely new content …
• … Please don’t throw tomatoes

4. JakubPilimon
WHOAMI
DEVELOPER ADVOCATE
AND PROGRAMMER
@PIVOTAL
•Domain-Driven Design,
•Event Storming,
•Test-Driven Development,
•Spring,
•Architecture
pillopl.github.io
Co-founder of #dddbyexamples initiative: github.com/ddd-by-examples

5. JakubPilimon

6. JakubPilimon
PATTERN THAT PROMOTES DETECTION
AND REACTION TO STATE CHANGES
REPRESENTED BY DOMAIN EVENTS
ME
EVENT-DRIVEN TO ME

7. JakubPilimon
EVENT-DRIVEN TO ME
• EDA != “I have microservices”
• EDA != “I have a message broker”
• Events might be transported for instance via HTTP
• Events can live only in application memory
• … Events might not be even visible in the code
(solution space)
• … but are present in the business process (problem
space)
• Event Sourcing counts too

8. JakubPilimon
EVENT-DRIVEN TO ME
EVENT 1
EVENT 2
EVENT 3
JMS/AMQP
HTTP
IN MEMORY
SERVICE #1
SERVICE #2
SERVICE #1
SERVICE #1 SERVICE #2

9. TRAP #1
A MISSING EVENT

10. JakubPilimon
2. Start transaction
1. Customer plugged in
4. Stop transaction
3. Customer finished charging
5. Customer unplugged
BACKEND
BACKEND

11. JakubPilimon
@PostMapping("/starts")
public ResponseEntity startSession
(@RequestBody StartTransaction startTransaction) {
chargingSessionRepository
.save(new ChargingSession(startTransaction.getId(), Instant.now()))
return ResponseEntity.ok().build();
}
@PostMapping(“/ends”)
public ResponseEntity stopSession
(@RequestBody StopTransaction stopTransaction) {
ChargingSession session =
chargingSessionRepository
.findById(stopTransaction.getId())
.orElseThrow(
() -> new IllegalStateException("Session was never started!"));
session.finishedAt(Instant.now());
return ResponseEntity.ok().build();
}

12. JakubPilimon

13. JakubPilimon
TRAP #1: MISSING AN EVENT
REASONS:
▸ Boundary of software != Boundary of business process
▸ Sometimes Software != Source of Truth
▸ Protocol’s semantics != protocol’s implementation
SOLUTIONS:
▸ Event Storming
▸ Don’t try to be consistent no matter what (sometimes it is impossible)
▸ Attach to semantics, not to implementation (focus on an intention of
any communication)

14. JakubPilimon
@PostMapping("/ends")
public ResponseEntity stopSession(@RequestBody StopTransaction stopTransaction) {
try {
ChargingSession session = chargingSessionRepository.findById(id)
.orElseGet(() -> new ChargingSession(id, null));
session.finishedAt(Instant.now());
} catch (Exception e) {
//DO WHAT YOU CAN - ASK BUSINESS
}
return ResponseEntity.ok().build();
}
MAKE IT SAFE

15. TRAP #2
VERSIONING OF BEHAVIOR

16. JakubPilimon
1. ChargingSessionFinished
3. Calculate and insert tax
BACKEND
INVOICES
INVOICES
EVENT 1
EVENT 2
EVENT 3
EVENT 1
EVENT 2
EVENT 3
2. ChargingSessionFinished

17. JakubPilimon
@StreamListener(“sessions”)
public void handle(ChargingSessionFinished event) {
Money rate = getRate();
Tax tax = new Tax(event.getSessionId(), rate.multiply(event.getCost()));
taxRepository.save(tax);
}
private Money getRate() {
return new Money(0.15);
}

18. JakubPilimon
TRAP #2: VERSIONING OF BEHAVIORS
REASONS:
▸ Temporal values
▸ OccurredAt vs ArrivedAt mismatch!
SOLUTIONS:
▸ Calculation logic in projection should be done at time of
event creation and the result should be inside the event
▸ Same with external calls

19. TRAP #3
EXTERNAL CALLS

20. JakubPilimon
TAXES
INVOICES
ChargingSessionFinished
BACKEND

21. JakubPilimon
TRAP #3: EXTERNAL CALLS
REASONS:
▸ Relying on a single source of truth
SOLUTIONS:
▸ External calls should be done at time of event creation
and the result should be kept inside the event
▸ Stream/Stream Join

22. JakubPilimon
@StreamListener(target = "sessions")
public void handle(ChargingSessionFinished event) {
BigDecimal rate = getTaxRate();
taxRepository.save(new Tax(event.getSessionId(), rate.multiply(event.getCost())));
}
@StreamListener(target = "taxRates")
public void handle(TaxRateChanged event) {
taxRateRepository.save(new TaxRate(event));
}
private TaxRate createNewRate(TaxRateChanged event) {
return taxRateRepository.save(new TaxRate(event));
}
private BigDecimal getTaxRate(ChargingSessionFinished event) {
return taxRateRepository.findFor(event);
}
STREAM/STREAM JOIN

23. TRAP #4
IGNORING CAP THEOREM

24. JakubPilimon
@Transactional
public void stopSession(StopTransaction stopTransaction) {
ChargingSession session =
chargingSessionRepository.getOne(stopTransaction.getId());
session.finishedAt(now());
eventPublisher.publish(sessionFinished(session));
}

25. JakubPilimon
TRANSACTION
eventPublisher.publish(…);
proxy.commit(…);chargingSessionRepository
.findById(…)
session.finish(…);

26. JakubPilimon
TRAP #4: IGNORING CAP THEOREM
REASONS:
▸ Ignoring the fact that every now and then things will not work
▸ Trying to be consistent IMMEDIATELY!
▸ Experience with 2PC/DTC
SOLUTIONS:
▸ Event Sourcing
▸ At-least once (and deduplicating) or at-most once
▸ Listen to yourself

27. JakubPilimon
@Transactional
public ResponseEntity stopSession(StopTransaction stopTransaction) {
//change database
TransactionSynchronizationManager.registerSynchronization(new
TransactionSynchronization() {
@Override
public void afterCommit() {
//eventPublisher.publish(...);
}
});
return ResponseEntity.ok().build();
}
@Transactional
public ResponseEntity stopSession(StopTransaction stopTransaction) {
ChargingSession session = database.findById(stopTransaction.getId());
session.finishedAt(now());
database.save(new ChargingSessionFinished());
return ResponseEntity.ok().build();
}
AT LEAST ONCE
AT MOST ONCE

28. JakubPilimon
@Transactional
public ResponseEntity stopSession(StopTransaction stopTransaction) {
//..
eventPublisher.publish(sessionFinished(session));
return ResponseEntity.ok().build();
}
@StreamListener(target = "sessions")
public ResponseEntity handle(ChargingSessionFinished event) {
ChargingSession session = repo.findById(event.sessionId());
session.finishedAt(now());
return ResponseEntity.ok().build();
}
LISTEN TO YOURSELF

29. TRAP #5
ANEMIC EVENTS

30. JakubPilimon
ChargingSessionStateChanged
{
“oldState": {
"finishedAt": null,
"field2": "b",
"field3": "c"
},
"newState": {
"finishedAt": “2019-01-11:00:00GMT“,
"field2": "e",
"field3": “f"
}
}

31. JakubPilimon
TRAP #5: ANEMIC-EVENTS
REASONS:
▸ Focusing on data instead of behavior
SOLUTIONS:
▸ Domain events (with behavior)
▸ Thinking about business intent of any change, not only
about the representation of that change
▸ Event Storming

32. TRAP #6
VERSIONING OF EVENTS

33. JakubPilimon
I WILL JUST FIND IT AND
UPDATE IT. WHAT CAN
POSSIBLY GO WRONG?
EVENT 1
EVENT 2
EVENT 3

34. JakubPilimon
1
2
3
“sessions”
4
5
ChargingSessionFinished
ChargingSessionFinishedV2
4 5
BACKEND
CONSUMER #2CONSUMER #1

35. JakubPilimon
class ChargingSessionFinishedV2 {
private String sessionId;
private BigDecimal cost;
private String customerName;
}
class ChargingSessionFinished {
private String sessionId;
private BigDecimal cost;
}

36. JakubPilimon
class ChargingSessionFinished {
private String sessionId;
private BigDecimal cost;
private String customerName = “I don’t care”;
public String getType() {
return "session-finished";
}
}

37. JakubPilimon
class ChargingSessionFinishedV2 {
private String sessionId;
private BigDecimal totalCost;
}
class ChargingSessionFinished {
private String sessionId;
private BigDecimal cost;
}

38. JakubPilimon
1
2
3
1’
2’
3’
“sessions” “sessions-v2”
TRANSLAT
OR

39. JakubPilimon
TRAP #6: VERSIONING OF EVENTS
REASONS:
▸ Using strong schema
▸ Canonical shared model of events - coupling
SOLUTIONS:
▸ Double-write
▸ Weak schema
▸ Copy and replace
▸ Schema server

40. TRAP #7
GDPR

41. JakubPilimon
TRAP #7: VERSIONING OF EVENTS
REASONS:
▸ Regulations
▸ Not enough BDUF
SOLUTIONS:
▸ Pointer events (Claim check pattern)
▸ Encryption

42. JakubPilimon
MORE INFORMATION

43. JakubPilimon
QUESTIONS?
https://github.com/pilloPl/edatraps
https://github.com/ddd-by-examples