WeDontNeedNoMockTrouble Devoxx

Adapter contract testing
How to test your integrations with external dependencies and reuse those same tests to build simulators.
The simulators allows you to test without any mocks in your whole codebase

1. Mathieu Cans & Johan Martinsson Adapter contract testing En fi nir avec les problèmes de mocks
2. We don’t need no mock trouble Why do we shift away from using mock Photo de Jonathan Borba sur Unsplash
3. Welcome to mockists anonymous Photo de Sincerely Media sur Unsplash
4. The mock addiction Where does it come from? SUT A B Test D C SUT A B Test D C
5. All tests pass but we have a bug in production Where does it come from?
6. But the tests are passing • A missed use case • Many layers of tests, with a bad mock somewhere • Surprise behavious of a dependency Bugs in prod Photo de Xavi Cabrera sur Unsplash
7. False assumptions Mock problems Assumption: 4G | 5G Real world: - 5G - 4G - 4G - 4G - 4g 💥
8. Mock problems Hinders refactorings across class boundaries SUT A B Test D C
9. Tech language - Not business facing Confusing intent Verbose Mock problems Test SUT Stub Spy Setup Veri fi cation Test SUT Simulator Other classes Setup Veri fi cation
10. Mock problems Mocks bugs Ignores external api changes
11. Domain The hexagonal opportunity Domain and infrastructure segregation Domain Infrastructure Infrastructure code ⛔ ✅
12. Domain The hexagonal opportunity Dependency inversion Domain Adapters Infrastructure Ports Infrastructure code ⛔ Adapters ✅
13. Domain The hexagonal opportunity Plugin behavior Domain Adapters Adapters Adapters Infrastructure Ports Infrastructure code Other adapter ✅
14. Domain Simulators 🤖 Simulators 🤖 Infrastructure Ports Test The hexagonal opportunity Use simulators to test your domain with con fi dence Adapters Adapters
15. Mock vs Simulator value Mocks never compounds Value Time Simulators Mocks
16. Could we make our simulators great immediately?
17. Adapter contract testing
18. Adapter contract test Test the real adapter de fi ne the contract Adapters Infrastructure Test Including anti-corruption layer ✍
19. Use the contract to create strong simulators Adapters Infrastructure Test Including anti-corruption layer Simulator Contract = running the same test Adapter contract test ✍ 💪
20. • Establish a contract with simulator against a DB • Establish a contract with a read- only dependency • Use simulators to test the domain with con fi dence Road-book high level view Photo de Jesse Bowser sur Unsplash
21. How about an example?
22. 🏁 Domain is about snail races 🏁 Register bets Select winners
23. BetApplication
24. Make groups of 2 ou 3 💡 Tips for pair/mob • Driver role : 
 Have the keyboard
 Responsible to translate ideas to code • Navigators role :
 Tells the driver what you want to be done • Switch roles often • If you have an idea : be a kind navigator Hands-on setup Photo de Halogen Condense sur Unsplash
25. Get the code Hands-on setup https://github.com/adapter-contract-testing/snail-race-kata
26. Hands-on setup Setup steps 1. Open the project fi le of your chosen language 2. Follow README instruction : 1. Start mongo db locally
 docker run --rm -p 27017:27017 --name mongo_contract_testing -d mongo 2. Start race server locally
 docker run --rm -d -p8000:8000 --name snail-race-server mathieucans/snail-race-server:latest 3. Execute requirement test If you are done : help others! 🤝
27. Establish a contract with simulator against a DB • Write a test against a database • Extract the contract test • Use contract test to write a simulator Road-book Step 1 Photo de Glenn Carstens-Peters sur Unsplash
28. Exercice 1 - Contract test a database Test the real adapter de fi ne the contract BetRepositoryMongoDb BetRepository MongoDbTest Mongo DB BetRepository
29. Contract test a database 💡 Tips : • Don’t panic with mongo syntax. • Refers to MongoDbCheatSheet.md if needed Exercice 1
30. Use Contract test to implement simulator BetRepositoryMongoDb BetRepository MongoDb Test Mongo DB BetRepository BetRepositorySimulator BetRepository Simulator Test BetRepository Contract
31. Use Contract test to implement simulator JAVA WAY BetRepositoryMongoDb BetRepository MongoDb Test Mongo DB BetRepository BetRepositorySimulator BetRepository Simulator Test BetRepository Contract Inherit Inherit Setup only Setup only All tests are here ➡
32. Use Contract test to implement simulator JS WAY
33. Exercice Extract contract test and use it to implement simulator
34. Contract test BetRepositoryMongoDb Conclusions • Did you test the boundaries? • Gotcha: in-memory does not copy objects
35. Solved problems Photo de Bozhin Karaivanov sur Unsplash
36. Solved problems External change of the API SOLVED
37. Solved problems Refactorability of the adapter SUT A B Test D C SOLVED
38. Solved problems No more technical tests Test SUT Stub Spy Setup Veri fi cation SOLVED
39. Pause Photo de Radek Grzybowski sur Unsplash 5’
40. Demo - Establish contract with read-only dep. Contract test a read only provider RaceResultProviderHttp RaceResultProvider Http Test RaceResultProvider SnailRaceServer
41. Read-only api 1. Make a contract with the dependency 1. Explore the endpoint with a unit test 2. Use the json response to create infrastructure types 3. Write a contract test that uses the desired port 4. Unit test the mapping (infra -> domain) with pure unit tests 2. Write simulator Roadbook Step 2
42. Orange Restricted Anti corruption layer Domain side RaceResultProvider (Port) snail- race-server Infrastructure side SnailRaces SnailRace Podium Snail * 1 3
43. Use Contract test to implement simulator RaceResultProviderHttp RaceResultProvider HttpTest RaceResultProvider RaceResultSimulatorSimulator RaceResultProvider SimulatorTest RaceResultProvider Contract Simulation SnailRaceServer
44. Anti-Corruption Layer On steroids 1. Perform utterly daring validations and restructurings on the data from providers 👉 Allowing to build highly typed domain objects 2. Use the tests to run them on massive amounts of data 3. You’ve simpli fi ed the job for the domain and you can be certain the simpli fi cation is valid Tranformation Validation Normalisation Dirty DTO Clean Domain Object
45. • ✅ External api changes • ✅ Refactorability of the adapter • ✅ No more technical tests Solved problems Photo de Bozhin Karaivanov sur Unsplash
46. Solved problems Complexity isolation SOLVED Photo de Michael Pointner sur Unsplash
47. Real world: - 5G - 4G - 4G - 4G - 4g 💥 Solved problems False assumptions SOLVED Assumption: - 4G - 5G
48. Exercice - Test application with confidence Use simulator to test your domain with con fi dence Domain RaceResultProviderSimulator BetRepositorySimulator Ports BetApplication Test
49. Use simulators to test the domain with con fi dence • Use the simulators • Write a f….g test 😂 • Take time to appreciate: • domain language in tests • test brievety • fast feedback • con fi ence because we’re utterly close to the production behaviour 
 and interaction Roadbook Step3
50. Domain rules The application can register bets • registering a bet is done by providing • gambler name • the 3 numbers of snails on the podium The application lists the winners according to last race ran. To win a bet : • The podium match exactly the race result • The bet has been registered at least 3 seconds before the race timestamp • A bet is valid only for the next race
51. Exercice Use simulator to test your domain • ➡ Checkout branch start-step3 • Start writing tests
52. • ✅ External api changes • ✅ Refactorability of the adapter • ✅ No more technical tests • ✅ Complexity isolation • ✅ False assumptions … Solved problems Photo de Bozhin Karaivanov sur Unsplash
53. Solved problems Refactorability of the domain SOLVED SUT A B Test D C
54. Solved problems No more technical tests Test SUT Stub Spy Setup Veri fi cation SOLVED
55. Solved problems Lightning ⚡ fast 🚀 tests BONUS Photo de Bill Jelen sur Unsplash
56. Contract tests in the CI • Put in a separate pipeline (possibly di ff erent triggers/periodicity) • Put in an optional pipeline/step • Use as uptime-bot • No CI ! - the main use is on dev machine
57. The five main benefits Photo de LUCHI CHENG sur Unsplash
58. Trust in domain tests 1st Benefit Photo de Cytonn Photography sur Unsplash
59. Relevant infrastructure tests 2nd benefit
60. Safer refactoring 3rd Benefit Photo de Xavi Cabrera sur Unsplash
61. Easy dependence migration 4th benefit Photo de Todd Trapani sur Unsplash
62. Complexity isolation 5th benefit Photo de Todd Trapani sur Unsplash
63. Con fi ance dans les tests du domaine Tests de l’infra pertinents Migration de dépendance facile Refactorabilité ++ Isolation de la complexité Abstraction forte The 5 bene fi ts
64. Feedbacks are welcomed Thanks for your participation 👉https://mobile.devoxx.com/events/devoxxfr2025/rate-talk/52509
65. References • Contract Test - Martin Fowler • Testing Without Mocks - James Shore • Architecture Hexagonale : gestion de la complexité logicielle et de la dette technique - Julien Topçu • Outside-In Diamond TDD - Thomas Pierrain
66. Annexe Examples and feedbacks
67. Orange Restricted Use contract test for production metrics • Use exactly same test to generate metrics on production • Useful when traffic still small • Better diagnostic • OPS can monitor them
68. Example: gmail reply detection Gmail has no native way to check if an email has been replied to It can be implemented through threads Need to expose this to the domain 1. Discovered how thread works through contract tests 2. Use these tests to write a simulator like gmail 3. Very productive and con fi dent in implementing the domain logic
69. Example : api change Both simulators and contract test shave been written to consume orange mails The application was tested with simulators Need to change end point used to read mails in the adapter (QOS issue) 1. Write the adapter with the new end point 2. Check the behavior still works as the old one and the simulator 3. Integrate the new end point safely without change neither domain code nor domain tests
70. Easy DB Migration
72. Example: remote fi le system (OneDrive) • Very slow, in particular for listing nested folders • There were around 15 methods on it • Having the fake made it very easy to implement a good caching system • As a result it was easy to implement both a persistent and an ephemeral caching system Storage OneDrive Slow
73. Example: public company registry • Getting info about companies • Problem: not really. Read-only but stable • Solution: keep the exact assertions • In retrospect: They changed sometimes, but not often. Our app Data.gouv
74. Example: data.entreprise.gouv • Getting (more) info about companies • Problem: Heteregenous data • Solution: Try ingesting big amounts of data as a test • Result: We discovered rare problems in dev as opposed to production Our app Data.gouv Heteregenous 
 data
75. Example: Android system • A js-app running on android, it has a chatty api with a native app. • Problem: complex and wrong mocking • Solution: build a simulator (without contract testing) • Gain: Reliable tests of the isolated js-app js app native app Complex 
 interactions
76. Example: Run tests on DB production • Warning : after the training • Running contract test in isolation as much as possible