It is always tough to test a complex API comprehensively. The additional level of complexity brings us to the question “How can we validate that our API is working as intended?” In this talk I will explain how to use test driven development for APIs to solve this problem and even further how TDD can drive an API Design towards a more usable design. I will outline my practical approach with an implementation example based on django. And finally I will give you a brief summary of my lessons learned using this approach in customer projects.

1. TDD for APIs
Michael Kuehne

2. Michael Kuehne
• Hamburg, Germany
• Independent Software
Engineer
• Likes Python & APIs
• Clients projects mostly API-
related

3. "How can we validate that our API is
working as intended?“ - Steve Klabnik

4. TDD Cycle
Inside-Out
Write a
failing
Unit Test
Make
the Test
pass
Refactor

5. API
Business
Logic
Task
Management
ORM
Message
Queue
No9fica9on
System
Mobile
Device
Single
Page
App
Service
API
Models
Database
API
Example Architecture
Clients
External
Services

6. Defined
Input
POST /mailboxes HTTP/1.1
Host: http://api.mkuehne.com/
Content-Type: application/json
{
‘username’: ‘michael’
‘mail_address’: ‘michael@mkuehne.com’
}
Defined
Output
HTTP/1.1 201 CREATED
Date: Wed, 22 Jul 2015 11:45
Content-Type: application/hal
+json
Location: /mailboxes/1a3b3s24bd

7.
„Unit Tests confirm we have written the
code right“ – Unknown
„Acceptance Tests confirm that we
built the right software.“ – Ian Cooper

9. Acceptance Tests on all Levels

10. BDD Cycle
Outside Inside
Write failing
Acceptance
Tests
Write
failing
Unit Tests
Make
Tests
pass
Refactor

11. Numbers from Clients Project
• ~3.000 Tests
• ~2.000 Unit Tests
• ~1.000 Acceptance Tests
• > 300 PyTest Fixtures

12. Test Architecture
• Integration tests => different test project
• Use your own client sdk
• Functional & Unit tests => main test project
• Debian packages
• Mock only external services

13. How to design API tests?
• Always test to the end
• F.I.R.S.T. Principle
• Reusable helper classes
• Deterministic test environment
• PyTest: Fixtures, fixtures, fixtures
§ Clean database & ORM
§ Test data
§ Versioned Endpoints

14. Testing Versioned endpoints with Django
Rest Framework and PyTest

15. API
Business
Logic
Task
Management
ORM
Message
Queue
No9fica9on
System
Service
API
Models
Database
API
Asynchronous Architecture
Clients
External
Services

16. Tests for asynchronous behaviors
• Celery (task management) caches!
• Validate the input parameters synchronous
• Test Deltas (validation <delta> processing)
• Make aync results accessable
• Wait for all async tasks in each test

17. Wait for all task in celery_taskstate

18. Async Testing: Outside to Inside
API
Business
Logic
Celery
Task
Task
without
Celery

19. "TDD doesn't drive good design.
TDD gives you immediate feedback
about what is likely to be bad
design."
- Kent Beck

20. How can TDD help improve API design or
avoid bad design?
• Inverse Conway Maneuver
• API design evolves from the outside to inside
• Refactoring without changing behavior
• Endpoint Versioning with confidence

21. Lessons learned
• Great API specifications required
• Best API design come from outside the team
• Don‘t forget your versioned Endpoints
• Don‘t underestimated the test effort for API
with asynchronous behaviors
• Complexity vs. deterministic environments

22. Thank you
and use TDD for your APIs
Michael Kuehne
michael@mkuehne.com