TDDing http4k

This post is a guide to how we build http4k applications test first to provide excellent test coverage driven by decoupled tests.

Application Design

For this example, we will use an example of a Maths app with the following requirements:

• The app must add 2 numbers together via an HTTP call
• The app must multiply 2 numbers together via an HTTP call
• Answers generated by the service will be logged (via HTTP POST) to another server - the Recorder.

Apps can generally be split into 3 tiers:

1. Endpoint: HttpHandlers are constructed individually, by providing a builder function which takes the business-level dependencies.
2. Application: Builder function which takes the transport-level dependencies, and converts them into business-level dependencies. All routes are constructed and collected in this tier.
3. Server: Builder function which takes the configuration for environmental concerns such as ports and downstream urls.

The tutorial is split into 4 sections:

• Part 1: Building a walking skeleton
• Part 2: Adding an endpoint
• Part 3: Adding another endpoint
• Part 4: Adding an external dependency