title: "Lesson 3: API-First Design" weight: 3 summary: "Design-first API development using OpenAPI and Sruja together."

Lesson 3: API-First Design

Before frontend and backend teams start working, they need to agree on the API. This is where API-First Design comes in.

API-First Design

Instead of writing code and then documenting it, we define the API schema first using OpenAPI. This allows frontend devs to mock the API while backend devs build it.

Sruja's Role: Architecture Modeling

Sruja models which services exist and how they connect. For detailed API schemas (endpoints, request/response structures), use OpenAPI/Swagger.

import { * } from 'sruja.ai/stdlib'


customer = person "Customer"

ecommerce = system "E-Commerce Platform" {
  api = container "Core API" {
    technology "Rust, Axum"
    // API schemas defined in openapi.yaml
  }

  orderDB = database "Order Database" {
    technology "PostgreSQL"
  }

  api -> orderDB "reads and writes to"
}

customer -> ecommerce.api "uses"

view index {
  title "E-Commerce Architecture"
  include *
}

Best Practice: Separation of Concerns

1. Sruja: Models architecture (services, containers, relationships)
2. OpenAPI: Defines API schemas (endpoints, request/response structures)
3. Together: Architecture shows the big picture, OpenAPI shows the details

Why this matters

1. Right tool for the job: Architecture modeling vs. API specification
2. Industry standard: OpenAPI is widely supported by tools and frameworks
3. Code Generation: Generate Rust types, TypeScript interfaces, and client SDKs from OpenAPI