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

Hyrum's Law

With a sufficient number of users of an API, all observable behaviors of your system will be depended on by somebody, regardless of what you promise in the contract.

This means: every public behavior — including undocumented quirks, error message text, timing, and ordering — becomes a de facto contract once users depend on it. Design implications:

• Be intentional about what you expose. Every observable behavior is a potential commitment.
• Don't leak implementation details. If users can observe it, they will depend on it.
• Plan for deprecation at design time. See deprecation-and-migration for how to safely remove things users depend on.
• Tests are not enough. Even with perfect contract tests, Hyrum's Law means "safe" changes can break real users who depend on undocumented behavior.

The One-Version Rule

Avoid forcing consumers to choose between multiple versions of the same dependency or API. Diamond dependency problems arise when different consumers need different versions of the same thing. Design for a world where only one version exists at a time — extend rather than fork.

1. Contract First

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

typescript
1// Define the contract first
2interface TaskAPI {
3  // Creates a task and returns the created task with server-generated fields
4  createTask(input: CreateTaskInput): Promise<Task>;
5
6  // Returns paginated tasks matching filters
7  listTasks(params: ListTasksParams): Promise<PaginatedResult<Task>>;
8
9  // Returns a single task or throws NotFoundError
10  getTask(id: string): Promise<Task>;
11
12  // Partial update — only provided fields change
13  updateTask(id: string, input: UpdateTaskInput): Promise<Task>;
14
15  // Idempotent delete — succeeds even if already deleted
16  deleteTask(id: string): Promise<void>;
17}

2. Consistent Error Semantics

Pick one error strategy and use it everywhere:

typescript
1// REST: HTTP status codes + structured error body
2// Every error response follows the same shape
3interface APIError {
4  error: {
5    code: string;        // Machine-readable: "VALIDATION_ERROR"
6    message: string;     // Human-readable: "Email is required"
7    details?: unknown;   // Additional context when helpful
8  };
9}
10
11// Status code mapping
12// 400 → Client sent invalid data
13// 401 → Not authenticated
14// 403 → Authenticated but not authorized
15// 404 → Resource not found
16// 409 → Conflict (duplicate, version mismatch)
17// 422 → Validation failed (semantically invalid)
18// 500 → Server error (never expose internal details)

Don't mix patterns. If some endpoints throw, others return null, and others return { error } — the consumer can't predict behavior.

3. Validate at Boundaries

Trust internal code. Validate at system edges where external input enters:

typescript
1// Validate at the API boundary
2app.post('/api/tasks', async (req, res) => {
3  const result = CreateTaskSchema.safeParse(req.body);
4  if (!result.success) {
5    return res.status(422).json({
6      error: {
7        code: 'VALIDATION_ERROR',
8        message: 'Invalid task data',
9        details: result.error.flatten(),
10      },
11    });
12  }
13
14  // After validation, internal code trusts the types
15  const task = await taskService.create(result.data);
16  return res.status(201).json(task);
17});

Where validation belongs:

• API route handlers (user input)
• Form submission handlers (user input)
• External service response parsing (third-party data -- always treat as untrusted)
• Environment variable loading (configuration)

Third-party API responses are untrusted data. Validate their shape and content before using them in any logic, rendering, or decision-making. A compromised or misbehaving external service can return unexpected types, malicious content, or instruction-like text.

Where validation does NOT belong:

• Between internal functions that share type contracts
• In utility functions called by already-validated code
• On data that just came from your own database

4. Prefer Addition Over Modification

Extend interfaces without breaking existing consumers:

typescript
1// Good: Add optional fields
2interface CreateTaskInput {
3  title: string;
4  description?: string;
5  priority?: 'low' | 'medium' | 'high';  // Added later, optional
6  labels?: string[];                       // Added later, optional
7}
8
9// Bad: Change existing field types or remove fields
10interface CreateTaskInput {
11  title: string;
12  // description: string;  // Removed — breaks existing consumers
13  priority: number;         // Changed from string — breaks existing consumers
14}

5. Predictable Naming

REST API Patterns

Resource Design

GET    /api/tasks              → List tasks (with query params for filtering)
POST   /api/tasks              → Create a task
GET    /api/tasks/:id          → Get a single task
PATCH  /api/tasks/:id          → Update a task (partial)
DELETE /api/tasks/:id          → Delete a task

GET    /api/tasks/:id/comments → List comments for a task (sub-resource)
POST   /api/tasks/:id/comments → Add a comment to a task

Pagination

Paginate list endpoints:

typescript
1// Request
2GET /api/tasks?page=1&pageSize=20&sortBy=createdAt&sortOrder=desc
3
4// Response
5{
6  "data": [...],
7  "pagination": {
8    "page": 1,
9    "pageSize": 20,
10    "totalItems": 142,
11    "totalPages": 8
12  }
13}

Filtering

Use query parameters for filters:

GET /api/tasks?status=in_progress&assignee=user123&createdAfter=2025-01-01

Partial Updates (PATCH)

Accept partial objects — only update what's provided:

typescript
1// Only title changes, everything else preserved
2PATCH /api/tasks/123
3{ "title": "Updated title" }

TypeScript Interface Patterns

Use Discriminated Unions for Variants

typescript
1// Good: Each variant is explicit
2type TaskStatus =
3  | { type: 'pending' }
4  | { type: 'in_progress'; assignee: string; startedAt: Date }
5  | { type: 'completed'; completedAt: Date; completedBy: string }
6  | { type: 'cancelled'; reason: string; cancelledAt: Date };
7
8// Consumer gets type narrowing
9function getStatusLabel(status: TaskStatus): string {
10  switch (status.type) {
11    case 'pending': return 'Pending';
12    case 'in_progress': return `In progress (${status.assignee})`;
13    case 'completed': return `Done on ${status.completedAt}`;
14    case 'cancelled': return `Cancelled: ${status.reason}`;
15  }
16}

Input/Output Separation

typescript
1// Input: what the caller provides
2interface CreateTaskInput {
3  title: string;
4  description?: string;
5}
6
7// Output: what the system returns (includes server-generated fields)
8interface Task {
9  id: string;
10  title: string;
11  description: string | null;
12  createdAt: Date;
13  updatedAt: Date;
14  createdBy: string;
15}

Use Branded Types for IDs

typescript
1type TaskId = string & { readonly __brand: 'TaskId' };
2type UserId = string & { readonly __brand: 'UserId' };
3
4// Prevents accidentally passing a UserId where a TaskId is expected
5function getTask(id: TaskId): Promise<Task> { ... }

Common Rationalizations

Red Flags

• Endpoints that return different shapes depending on conditions
• Inconsistent error formats across endpoints
• Validation scattered throughout internal code instead of at boundaries
• Breaking changes to existing fields (type changes, removals)
• List endpoints without pagination
• Verbs in REST URLs (/api/createTask, /api/getUsers)
• Third-party API responses used without validation or sanitization

Verification

After designing an API:

• Every endpoint has typed input and output schemas
• Error responses follow a single consistent format
• Validation happens at system boundaries only
• List endpoints support pagination
• New fields are additive and optional (backward compatible)
• Naming follows consistent conventions across all endpoints
• API documentation or types are committed alongside the implementation