This talk elaborates our awesome journey migrate all our micro services from Java to Kotlin.
We will take a closer look at the challenges we faced as a team and the impact on our day to day work in an agile environment and how we overcame technical hurdles integrating with Spring Boot and introducing async workflows using Kotlin coroutines.
Finally we discuss at the actual benefits that we gained by moving all our micro services to Kotlin and do a critical review of our journey and an outlook at things to come.

1. Migrating our micro services
from Java to Kotlin
Yet another awesome journey

2. 2
Björn Wendland
Full-Stack Developer
@BjoernWendland

3. 3
M|SHOP Picture

4. Our Migration Journey

5. Why we migrated from Java to Kotlin
5

6. Controversial
Java Features

7. Checked exceptions
7
public static void refundExcessMoney() throws ExternalPspFailureException {
// try to refund money
throw new ExternalPspFailureException();
}
try {
refundExcessMoney()
} catch (ExternalPspFailureException e) {
// log or rethrow
}

8. Single abstract method types
8
public interface Comparator<T> {
public boolean compare(T a, T b);
}
public class PaymentComparator implements Comparator<PaymentInformation> {
@Overwrite
public boolean compare(PaymentInformation a, PaymentInformation b) {
...
}
}

9. Wildcard-types
9
// use-site wildcard
List<?> paymentRawTypes = new ArrayList<Integer>();
void original(List<Number> paymentRawTypes) { }
void withWildcard(List<? extends Number> paymentRawTypes) { }
// Kotlin use-site variance
var paymentRawTypes = listOf(1, 2, 3)

10. Beneficial
KotlinFeatures

11. Null-safety
11
// non null-able
var paymentInformationId: UUID = UUID.randomUUID()
paymentInformationId = null // compilation error
// null-able
var paymentInformationId : UUID? = UUID.randomUUID()
paymentInformationId = null // ok

12. Extension functions
12
// Java
List<OrderId> orderIds = new ArrayList( Arrays.asList(id1, id2, id3) );
return JsonUtils.toJson(orderIds);
// Kotlin
val orderIds = listOf(id1, id2, id3)
return orderIds.toJson()

13. Smart casts
13
// Java
if (payment instanceof CreditCardPayment) {
CreditCardPayment ccPayment = (CreditCardPayment) payment;
return ccPayment.getCardHolder();
}
// Kotlin
if (payment is CreditCardPayment) {
return payment.cardHolder
}

14. Coroutines

15. Payment capture flow
15
PAY FEED PSPOM
write
feed
poll
status
request
capture
capture
poll
feed

16. fun capturePayment() {
val lastKnownEventId = feedsService.getLatestEventId()
paymentService.capture()
val feedStatus = getFeedStatus(lastKnownEventId)
feedStatus.whenError {
throw CreditCardCaptureException()
}
}
Request capture
16

17. Asynchronous feed reader
17
fun getFeedStatus(lastKnownEventId: UUID?): PaymentFeedStatus {
return runBlocking {
withTimeout(syncClientTimeout) {
var paymentFeedEvent: PaymentFeedEvent?
do {
paymentFeedEvent = feedsService.getSuccessorEventFor(lastKnownEventId)
delay(200)
} while (paymentFeedEvent.isNull())
paymentFeedEvent?.getStatus()
}
}
}

18. 18
Our Journey Continues

19. What did we migrate
19

20. How did we migrate
20
refactor to
idiomatic
Kotlin
verify
using tests
fix invalid
code
automatic
code
conversion

21. Rewrite
everything

22. Switching to Kotlin frameworks
22
 KTOR - asynchronous Kotlin web framework
 Spring Boot - established Java web framework
 Often less matured
 Often less documented
No need to forcefully use
Kotlin libraries and frameworks

23. Review and Outlook

24. Review
24
 Challenge which we mastered as team
 Migrating was worth the effort
 Improved design and new focus on language
 Quality of life update
10/10 would use Kotlin again

25. A heads up
25
 Test coverage is a must have
 Consider your people and eco system
 Consider language and framework maturity level
 Amount of new things can be overwhelming

26. Outlook
26
 Establish Kotlin at METRONOM
 Help others introduce Kotlin
 Introduce more native Kotlin frameworks
 Find new passionate developers

27. Follow us on Twitter and Instagram
@bjoernwendland
@joinmetronomnow
#METRONOMians #wearedevs19 #passion #ambition #purpose
Hall A
Booth A11

28. @BjoernWendland
28
Thank you for
your attention!