Is API design first or code first?

The choice between API Design-First and Code-First approaches depends on the project’s requirements, collaboration needs, and the development environment. Here's an overview of both approaches:

1. API Design-First Approach

In the design-first approach, the API is fully designed before any code is written. This method focuses on creating a comprehensive API specification using tools like OpenAPI (formerly Swagger), RAML, or Blueprint.

Benefits:

• Collaboration: Teams (developers, product managers, stakeholders) can review the API's design early in the process, ensuring alignment on functionality before development starts.
• Clear API Contract: The API contract (inputs, outputs, error messages) is defined upfront, reducing misunderstandings and changes later in the development cycle.
• Documentation-First: Since the design is created early, detailed documentation can be automatically generated from the API specification, helping both internal and external users understand how to work with the API.
• Mock APIs: Mock servers can be set up based on the API design to allow frontend teams or clients to start working before the backend is fully implemented.

Use Cases:

• When multiple teams need to work together, ensuring consistency and clarity.
• When an external client will be using the API, and its structure needs to be agreed upon in advance.

Sources:

• API First Design Best Practices

2. API Code-First Approach

In the code-first approach (also known as implementation-first), the API is developed alongside the application, and the API specification is generated after the code is written. The focus is on coding the API functionality first and documenting it afterward.

Benefits:

• Rapid Prototyping: Developers can quickly get started on coding and test ideas or features without spending too much time on upfront design.
• Easier for Small Teams: If the same developers are building and consuming the API, and there are minimal external dependencies, the code-first approach can streamline the process.
• Flexibility: Changes can be made on the fly, which is beneficial for fast-moving projects or when requirements are unclear at the start.

Drawbacks:

• Lack of Upfront Documentation: Documentation often becomes an afterthought, leading to incomplete or missing documentation, which can be a problem for API users.
• Potential for Misalignment: Without a well-defined API contract, different teams or developers may interpret functionality differently, leading to integration issues.

Sources:

• Code-First API Development

Conclusion: Which is Better?

• API Design-First is better when collaboration, consistency, and clear contracts are important, especially for public or partner APIs. It also works well for large teams where different groups are involved in different parts of the API development process.
• API Code-First is suitable for internal APIs, rapid development cycles, or smaller teams where flexibility and speed are more important than upfront documentation.

Each approach has its pros and cons, so the choice depends on the project's specific needs.