API-First development (also known as Contract-driven development) starts by designing and documenting the contracts between the components that build your system (services, user interfaces, third party providers, etc.) before writing the source code.

There are many benefits to this approach. Some of the main ones include:

• Central Source of Truth: The API specification serves as the definitive guide for component behavior, simplifying testing and validation.
• Parallel Development: Provider and consumer components can be developed simultaneously.
• Automation: Shared artifacts of the system can be generated directly from the specification documents.

An API specification document describes two things: component behavior and shared data schema. For component behavior we use specifications like OpenAPI for REST APIs, or AsyncAPI for Event-driven Architectures. These specifications allow us to define how our components interact, which endpoints they expose, which messages they publish and consume.