🔑 What is API-First?

The API-first approach means you design and define your APIs before writing any implementation code.

Instead of building a backend and then exposing endpoints, you:

• Start by defining the API contract (using OpenAPI/Swagger, AsyncAPI, GraphQL schema, etc.).

• Share this contract with teams (frontend, backend, QA, external partners).

• Generate mocks, SDKs, and stubs from the definition.

• Build services according to the agreed contract.

⚙️ How It Works (Steps)

1. Design the API

   • Define endpoints, request/response payloads, error models.

   • Use OpenAPI (REST), gRPC proto (RPC), or AsyncAPI (event-driven).

2. Review & Collaboration

   • Teams discuss and agree on the contract.

   • Changes go through versioning and review, just like code.

3. Mock & Test Early

   • Use API mock servers so frontend & QA can start testing before backend exists.

4. Code Generation

   • Generate client SDKs, server stubs, and documentation automatically.

5. Implement & Deploy

   • Backend devs implement the logic behind the API.

   • Frontend/devs already integrate against the agreed API.

📊 Diagram

flowchart TB
    A[API Specification] --> B[Mock Server]
    A --> C[SDK Clients]
    A --> D[Server Stubs]
    B --> Frontend
    C --> Frontend
    D --> Backend
    Backend --> API

🎯 Benefits

• 🔄 Parallel Development – frontend, backend, QA work at the same time.

• 📖 Single Source of Truth – the API spec is the contract.

• 🧪 Better Testing – mock servers & contract testing catch issues early.

• 🚀 Faster Delivery – code generators speed up SDK/server creation.

• 🔐 Consistency – shared design standards across services.

✅ Example (OpenAPI snippet)

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openapi: 3.0.0
info:
  title: Orders API
  version: 1.0.0
paths:
  /orders:
    get:
      summary: List orders
      responses:
        "200":
          description: List of orders
          content:
            application/json:
              schema:
                type: array
                items:
                  $ref: "#/components/schemas/Order"
components:
  schemas:
    Order:
      type: object
      properties:
        id:
          type: string
        status:
          type: string

From this spec, you can generate:

• API documentation

• Mock server

• Go/Python/TypeScript SDKs

• Server skeletons

🧰 Common Tools

• OpenAPI / Swagger – REST APIs

• gRPC + Protocol Buffers – RPC style

• AsyncAPI – Event-driven APIs (Kafka, MQTT, etc.)

• Stoplight, Postman, SwaggerHub – API design & mocking platforms

• openapi-generator / Swagger Codegen – generate SDKs & servers

👉 In short:

• API-first = contract first.

• The API spec becomes the blueprint that all teams build against, ensuring consistency, speed, and parallel work.

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