api-and-interface-design

API and Interface Design

Overview

Design stable, well-documented interfaces that are hard to misuse. Good interfaces make the right thing easy and the wrong thing hard. This applies to REST APIs, GraphQL schemas, module boundaries, component props, and any surface where one piece of code talks to another.

When to Use

• Designing new API endpoints
• Defining module boundaries or contracts between teams
• Creating component prop interfaces
• Establishing database schema that informs API shape
• Changing existing public interfaces

Core Principles

1. Contract First

Define the interface before implementing it. The contract is the spec — implementation follows.

// Define the contract first
interface TaskAPI {
  // Creates a task and returns the created task with server-generated fields
  createTask(input: CreateTaskInput): Promise<Task>;

  // Returns paginated tasks matching filters
  listTasks(params: ListTasksParams): Promise<PaginatedResult<Task>>;

  // Returns a single task or throws NotFoundError
  getTask(id: string): Promise<Task>;

  // Partial update — only provided fields change
  updateTask(id: string, input: UpdateTaskInput): Promise<Task>;

  // Idempotent delete — succeeds even if already deleted
  deleteTask(id: string): Promise<void>;
}

2. Consistent Error Semantics

Pick one error strategy and use it everywhere:

// REST: HTTP status codes + structured error body
// Every error response follows the same shape
interface APIError {
  error: {
    code: string;        // Machine-readable: "VALIDATION_ERROR"
    message: string;     // Human-readable: "Email is required"
    details?: unknown;   // Additional context when helpful
  };
}

// Status code mapping
// 400 → Client sent invalid data
// 401 → Not authenticated
// 403 → Authenticated but not authorized
// 404 → Resource not found
// 409 → Conflict (duplicate, version mismatch)
// 422 → Validation failed (semantically invalid)
// 500 → Server error (never expose internal details)