API-first design: Why the software of the future starts with the API

Introduction

Imagine building a house by first designing the facade and then trying to get plumbing, electrical work, and the foundation to fit. Sounds absurd, right? But that's how many still build software: they design the user interface first and consider the API as an afterthought.

At Hyperbolic, we have taken a different approach at heart: API-first design. We always start by designing the API before a single pixel of the UI is drawn. In this article, we delve into why this approach results in better software and happier teams.

What is API-first design?

API-first means that designing the API is the first thing you do when starting a new project. Before UI, before database design, before implementation, you define the API contract.

This is fundamentally different from the traditional approach, where the API was often created as a wrapper around existing functionality.

Why is API-first important?

Multi-platform reality Modern software needs to run everywhere: web, iOS, Android, wearables, maybe even smart TVs and voice assistants. If you start building a web app and later need to add a mobile app, you often end up having to refactor the entire backend.

With API-first, you build one backend that can serve all platforms. The web app, mobile app, and future platforms all consume the same API.

Parallel development Once the API contract is defined, frontend and backend teams can work in parallel. The frontend can build against mock data based on the API specification while the backend implements the actual logic. This significantly accelerates development.

Better developer experience A well-designed API is a joy to work with. It is intuitive, consistent, and well-documented. This makes both internal teams and external developers more productive.

Future-proofing Business requirements change. Your data may need to be used by partners, third-party apps, or an entirely new product in the future. A well-designed API makes it easy to extend without having to tear everything down and start over.

This is how we design APIs at Hyperbolic

1. Start with user journeys

Even before we design the API, we understand the underlying user journeys. What should users be able to do? What data do they need and when?

This gives us insight into what endpoints we need and how data should be structured.

2. Design the API contract

We use the OpenAPI Specification (formerly Swagger) to define our APIs. This gives us:

• A machine-readable contract

• Automatically generated documentation

• Ability to generate client code

• Validation of requests and responses

A typical OpenAPI spec defines:

• Endpoints (URLs)

• HTTP methods (GET, POST, PUT, DELETE)

• Request parameters and bodies

• Response formats

• Error responses

• Authentication

3. Review and iteration

Before a line of code is written, we review the API design with both frontend teams, other backend developers, and often with the client.

We ask questions like:

• Is the naming intuitive?

• Is the structure consistent?

• Can this API be easily extended in the future?

• Is it efficient? Do we require too many requests?

4. Mock first

We often set up a mock server based on the OpenAPI specification. This allows the frontend to start development immediately, even if the backend has not yet been implemented.

5. Implement with the contract as a guide

When we implement the backend, the API specification serves as a contract. We automatically validate that our implementation matches the specification.

Principles of good API design

Consistency is king Use consistent naming, structure, and error handling across the entire API. If users are called /users/{id}, then projects should not be named /project/{projectId}.

RESTful when it makes sense We generally follow REST principles:

• Use HTTP verbs correctly (GET for reading, POST for creating, etc.)

• Resource-based URLs

• Stateless communication

• Use HTTP status codes correctly (200, 201, 400, 401, 404, 500, etc.)

But we are not dogmatic. If GraphQL makes more sense for your use case, we use it.

Versioning from day one Even if it's your first version, version the API. It is much easier to version from the start than to have to introduce it later.

We often use URL-based versioning: /api/v1/users

Comprehensive error handling Errors happen. A good API provides clear, actionable error messages:

Bad:

{

  "error": "Invalid input"

}

Good:

{

  "error": {

    "code": "VALIDATION_ERROR",

    "message": "Email address is required",

    "field": "email",

    "suggestion": "Please provide a valid email address"

  }

}

Pagination and filtering Design from the start for large datasets. All list endpoints should support pagination.

We often use:

• ?page=1&limit=20 for pagination

• ?filter[status]=active for filtering

• ?sort=-created_at for sorting

Documentation is not optional

Good documentation is the difference between an API that people can use and one that people will use.

At Hyperbolic, our API documentation always includes:

• Overview of all endpoints

• Detailed descriptions of parameters

• Example requests and responses

• Authentication guide

• Rate limiting info

• Code examples in multiple languages

We often generate the documentation automatically from our OpenAPI spec, ensuring that documentation and implementation are always synchronized.

Security from the design phase

Security should not be an afterthought. From the design phase, we consider:

Authentication and authorization Who has access to what? We often design with multiple levels:

• Public endpoints (no auth)

• Authenticated endpoints (requires login)

• Role-based endpoints (only admins can delete, for example)

Rate limiting Protect your API from abuse by implementing rate limiting from the start.

Input validation Never trust client input. Validate everything on the server side.

HTTPS everywhere All API communication must occur over HTTPS. No exceptions.

Performance considerations

N+1 problems A classic problem arises when an endpoint requires multiple database queries. We design to avoid this from the start, often by using eager loading or offering batch endpoints.

Caching strategy Design the API with caching in mind. Use HTTP cache headers correctly:

• Cache-Control

• ETag

• Last-Modified

Data volume Avoid sending unnecessary data. Consider offering:

• Field selection: ?fields=id,name,email

• Sparse fieldsets in JSON:API standard

• GraphQL for ultimate flexibility

API-first in practice: A case

We worked with a fintech startup that needed to build a loan comparison platform. They needed web, iOS, and Android from day one.

Our approach:

1. Two weeks on API design with all teams involved

2. OpenAPI spec approved by all stakeholders

3. Mock server deployed so the frontend could start

4. Backend and frontend teams worked in parallel

5. Integration was nearly seamless as everyone worked towards the same contract

The outcome:

• Launch on all three platforms simultaneously

• Backend could be reused 100% across platforms

• Later addition of partner API was trivial as the infrastructure was there

Tools we use

At Hyperbolic, we have a standard toolkit for API development:

Design and documentation:

• OpenAPI/Swagger for spec

• Postman for testing and documentation

• Stoplight for API design collaboration

Development:

• Express.js or FastAPI depending on the language

• Automatic validation based on OpenAPI spec

• API gateways like Kong or AWS API Gateway for larger projects

Testing:

• Contract testing with Pact

• Automated API tests in CI/CD

• Load testing with k6 or Artillery

Monitoring:

• Logging all requests

• Performance metrics

• Error tracking

GraphQL as an alternative

While we often use REST, GraphQL is great for certain use cases:

Advantages of GraphQL:

• The client specifies exactly what data they want

• A request can fetch data from many resources

• Powerful for frontend with complex data needs

Disadvantages:

• More complex to implement

• Harder to cache

• Potential over-fetching on the server side

We choose GraphQL when:

• The client has highly variable data needs

• Performance of many small requests is an issue

• Frontend teams need maximum flexibility

Conclusion

API-first design is not about following a trend. It is about building software that is flexible, scalable, and future-proof.

At Hyperbolic, we have seen time and again how API-first accelerates development, improves code quality, and makes it easier to extend products over time.

Projects that start with a well-designed API have a significant head start. They can easily add new platforms, integrate with partners, and pivot when the business requires it.

Next time you start a new project, start with the API. Design the contract, gather feedback from all teams, and document it thoroughly. Your future self will thank you.

Do you need help designing a robust API for your project? Contact us at Hyperbolic for an API design workshop.