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 with 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. Björn Wendland
Migrating our micro services from
Java to Kotlin
Backend

2. Migrating our micro services from
Java to Kotlin - yet another awesome
journey

3. 3
Introduction

4. What is this Talk NOT about?
4
▪ Teaching Kotlin
▪ Best practices for migrating Java to Kotlin
▪ Best practices for implementing coroutines

5. What is this Talk about?
▪ Sharing our experiences
▪ Differences between Java and Kotlin
▪ Sharing what we do at METRONOM
5

6. 6
The Migration Story

7. Our technology stack
7
Gradle
Kotlin
Spring Boot
Postgres
Docker (Kubernetes)
Gitlab CI/CD
Grafana

8. Why we migrated from Java to Kotlin
▪ Self-perception as driver in technology
▪ Controversial design of Java
▪ Kotlin as officially supported language for Android
8

9. How we migrated
▪ Partially migrated classes
▪ Kotlin and Java side by side
▪ Migrated corresponding tests
▪ Migration of libraries
9

10. Migrating to native libraries
10
▪ No need to force migration
▪ Less matured
▪ Less documented
▪ KTOR – asynchronous Web framework
▪ Spring Boot as an established and reliable framework

11. 11
Java or Kotlin

12. Controversial Java features
▪ Checked exceptions
▪ SAM-Types
▪ Wildcard-Types
12

13. Checked exceptions
13

14. SAM-Types
14

15. Wildcard-Types
15

16. Important new Kotlin features
▪ Null-safety
▪ Extension functions
▪ Smart casts
▪ Data classes
▪ Coroutines
16
▪ (now stable !)

17. Null-safety
17

18. Extension functions
18

19. Data classes
19

20. Smart casts
20

21. 22
Coroutines

22. Coroutines and Threads
23
Coroutine
▪ Asynchronous execution
▪ Programmer scheduled
▪ Typically single thread
▪ Lightweight
Thread
▪ Asynchronous execution
▪ Operating system scheduled
▪ Heavyweight

23. Sequential
24
Step 1 Step 2 Step 3

24. Thread - Parallel
25
Part 1
Part 2
Part 3

25. Coroutine – Concurrent
26
Part 1 Part 2 Part 3

26. Enabling Coroutines on the JVM
27
▪ Modified JVM
▪ Modified bytecode
▪ Platform-specific JNI mechanisms
▪ Thread abstractions

27. Callbacks
▪ Passing executable code as argument
▪ Less readable with each call

28. Reactive Code
▪ Oriented around data flows
▪ Automatically propagate changes

29. Kotlin's Coroutines
▪ Based on suspending functions
▪ Dispatched using launch wrapper
▪ Async functions returning deferred result when calling await()

30. 31
The Migration Story

31. Emulating a sync endpoint using coroutines
32
▪ Remove redundant codebase
▪ Remove old synchronous service
▪ Reuse existing codebase

36. Migration Challenges
37
▪ First team to introduce Kotlin
▪ Learning a new language’s concepts
▪ Introducing native Kotlin frameworks

37. Ongoing Challenges
38
▪ Java is the most popular backend language
▪ Little documentation for native frameworks
▪ Find new developers open to learn new technologies
▪ Sharing our experience with others

38. 39
Review and Outlook

39. Review
40
▪ Migrating was worth the effort
▪ Kotlin will stay
▪ Number of new features can be overwhelming
▪ Coroutines are a more complex feature of Kotlin

40. Outlook
41
▪ RabbitMQ as queueing solution
▪ Spiking Blockchain as possible Storage Solution
▪ Automated Service Setup using Ansible / Puppet
▪ Share our experience with others

41. Thank you for
your attention!
Björn Wendland
42