API Design Patterns

Conventions and best practices for designing consistent, developer-friendly REST APIs.

When to Activate

• Designing new API endpoints
• Reviewing existing API contracts
• Adding pagination, filtering, or sorting
• Implementing error handling for APIs
• Planning API versioning strategy
• Building public or partner-facing APIs

Resource Design

URL Structure

# Resources are nouns, plural, lowercase, kebab-case
GET    /api/v1/users
GET    /api/v1/users/:id
POST   /api/v1/users
PUT    /api/v1/users/:id
PATCH  /api/v1/users/:id
DELETE /api/v1/users/:id

# Sub-resources for relationships
GET    /api/v1/users/:id/orders
POST   /api/v1/users/:id/orders

# Actions that don't map to CRUD (use verbs sparingly)
POST   /api/v1/orders/:id/cancel
POST   /api/v1/auth/login
POST   /api/v1/auth/refresh

Naming Rules

# GOOD
/api/v1/team-members          # kebab-case for multi-word resources
/api/v1/orders?status=active  # query params for filtering
/api/v1/users/123/orders      # nested resources for ownership

# BAD
/api/v1/getUsers              # verb in URL
/api/v1/user                  # singular (use plural)
/api/v1/team_members          # snake_case in URLs
/api/v1/users/123/getOrders   # verb in nested resource

HTTP Methods and Status Codes

Method Semantics

*PATCH can be made idempotent with proper implementation

Status Code Reference

# Success
200 OK                    — GET, PUT, PATCH (with response body)
201 Created               — POST (include Location header)
204 No Content            — DELETE, PUT (no response body)

# Client Errors
400 Bad Request           — Validation failure, malformed JSON
401 Unauthorized          — Missing or invalid authentication
403 Forbidden             — Authenticated but not authorized
404 Not Found             — Resource doesn't exist
409 Conflict              — Duplicate entry, state conflict
422 Unprocessable Entity  — Semantically invalid (valid JSON, bad data)
429 Too Many Requests     — Rate limit exceeded

# Server Errors
500 Internal Server Error — Unexpected failure (never expose details)
502 Bad Gateway           — Upstream service failed
503 Service Unavailable   — Temporary overload, include Retry-After

Common Mistakes

# BAD: 200 for everything
{ "status": 200, "success": false, "error": "Not found" }

# GOOD: Use HTTP status codes semantically
HTTP/1.1 404 Not Found
{ "error": { "code": "not_found", "message": "User not found" } }

# BAD: 500 for validation errors
# GOOD: 400 or 422 with field-level details

# BAD: 200 for created resources
# GOOD: 201 with Location header
HTTP/1.1 201 Created
Location: /api/v1/users/abc-123

Response Format

Project envelope first: if the project already defines a unified response wrapper (e.g., BaseResp<T> with code/msg/data fields), use that instead of the ApiResponse structure shown below. The patterns here are reference designs for greenfield APIs.

Success Response

{
  "data": {
    "id": "abc-123",
    "email": "alice@example.com",
    "name": "Alice",
    "created_at": "2025-01-15T10:30:00Z"
  }
}

Collection Response (with Pagination)

{
  "data": [
    { "id": "abc-123", "name": "Alice" },
    { "id": "def-456", "name": "Bob" }
  ],
  "meta": {
    "total": 142,
    "page": 1,
    "per_page": 20,
    "total_pages": 8
  },
  "links": {
    "self": "/api/v1/users?page=1&per_page=20",
    "next": "/api/v1/users?page=2&per_page=20",
    "last": "/api/v1/users?page=8&per_page=20"
  }
}

Error Response

{
  "error": {
    "code": "validation_error",
    "message": "Request validation failed",
    "details": [
      {
        "field": "email",
        "message": "Must be a valid email address",
        "code": "invalid_format"
      },
      {
        "field": "age",
        "message": "Must be between 0 and 150",
        "code": "out_of_range"
      }
    ]
  }
}

Response Envelope Variants

// Option A: Envelope with data wrapper (recommended for public APIs)
// record requires Java 16+. For Java 11–15, use a regular class with Lombok.
public record ApiResponse<T>(T data, PaginationMeta meta, PaginationLinks links) {
    public ApiResponse(T data) { this(data, null, null); }
}

public record ApiError(String code, String message, List<FieldError> details) {}

// Option B: Flat response (simpler, common for internal APIs)
// Success: just return the resource directly
// Error: return error object
// Distinguish by HTTP status code

Pagination

Offset-Based (Simple)

GET /api/v1/users?page=2&per_page=20

# Implementation
SELECT * FROM users
ORDER BY created_at DESC
LIMIT 20 OFFSET 20;

Pros: Easy to implement, supports "jump to page N" Cons: Slow on large offsets (OFFSET 100000), inconsistent with concurrent inserts

Cursor-Based (Scalable)

GET /api/v1/users?cursor=eyJpZCI6MTIzfQ&limit=20

# Implementation
SELECT * FROM users
WHERE id > :cursor_id
ORDER BY id ASC
LIMIT 21;  -- fetch one extra to determine has_next

{
  "data": [...],
  "meta": {
    "has_next": true,
    "next_cursor": "eyJpZCI6MTQzfQ"
  }
}

Pros: Consistent performance regardless of position, stable with concurrent inserts Cons: Cannot jump to arbitrary page, cursor is opaque

When to Use Which

Filtering, Sorting, and Search

Filtering

# Simple equality
GET /api/v1/orders?status=active&customer_id=abc-123

# Comparison operators (use bracket notation)
GET /api/v1/products?price[gte]=10&price[lte]=100
GET /api/v1/orders?created_at[after]=2025-01-01

# Multiple values (comma-separated)
GET /api/v1/products?category=electronics,clothing

# Nested fields (dot notation)
GET /api/v1/orders?customer.country=US

Sorting

# Single field (prefix - for descending)
GET /api/v1/products?sort=-created_at

# Multiple fields (comma-separated)
GET /api/v1/products?sort=-featured,price,-created_at

Full-Text Search

# Search query parameter
GET /api/v1/products?q=wireless+headphones

# Field-specific search
GET /api/v1/users?email=alice

Sparse Fieldsets

# Return only specified fields (reduces payload)
GET /api/v1/users?fields=id,name,email
GET /api/v1/orders?fields=id,total,status&include=customer.name

Authentication and Authorization

Token-Based Auth

# Bearer token in Authorization header
GET /api/v1/users
Authorization: Bearer eyJhbGciOiJIUzI1NiIs...

# API key (for server-to-server)
GET /api/v1/data
X-API-Key: sk_live_abc123

Authorization Patterns

// Resource-level: check ownership
@GetMapping("/api/v1/orders/{id}")
public ResponseEntity<ApiResponse<OrderResponse>> getOrder(
        @PathVariable String id,
        @AuthenticationPrincipal UserDetails user) {
    Order order = orderService.findById(id)
        .orElseThrow(() -> new ResourceNotFoundException("Order not found"));
    if (!order.getUserId().equals(user.getUsername())) {
        throw new AccessDeniedException("Forbidden");
    }
    return ResponseEntity.ok(new ApiResponse<>(orderMapper.toResponse(order)));
}

// Role-based: check permissions
@PreAuthorize("hasRole('ADMIN')")
@DeleteMapping("/api/v1/users/{id}")
public ResponseEntity<Void> deleteUser(@PathVariable String id) {
    userService.delete(id);
    return ResponseEntity.noContent().build();
}

Rate Limiting

Headers

HTTP/1.1 200 OK
X-RateLimit-Limit: 100
X-RateLimit-Remaining: 95
X-RateLimit-Reset: 1640000000

# When exceeded
HTTP/1.1 429 Too Many Requests
Retry-After: 60
{
  "error": {
    "code": "rate_limit_exceeded",
    "message": "Rate limit exceeded. Try again in 60 seconds."
  }
}

Rate Limit Tiers

Versioning

URL Path Versioning (Recommended)

/api/v1/users
/api/v2/users

Pros: Explicit, easy to route, cacheable Cons: URL changes between versions

Header Versioning

GET /api/users
Accept: application/vnd.myapp.v2+json

Pros: Clean URLs Cons: Harder to test, easy to forget

Versioning Strategy

1. Start with /api/v1/ — don't version until you need to
2. Maintain at most 2 active versions (current + previous)
3. Deprecation timeline:
   - Announce deprecation (6 months notice for public APIs)
   - Add Sunset header: Sunset: Sat, 01 Jan 2026 00:00:00 GMT
   - Return 410 Gone after sunset date
4. Non-breaking changes don't need a new version:
   - Adding new fields to responses
   - Adding new optional query parameters
   - Adding new endpoints
5. Breaking changes require a new version:
   - Removing or renaming fields
   - Changing field types
   - Changing URL structure
   - Changing authentication method

Implementation Patterns

Java (Spring Boot)

// Request DTO with Bean Validation
// Note: record requires Java 16+. For Java 11–15, use @Data @Builder @NoArgsConstructor @AllArgsConstructor class instead.
public record CreateUserRequest(
    @NotBlank @Email String email,
    @NotBlank @Size(min = 1, max = 100) String name
) {}

// Response DTO (Java 16+; see note above)
public record UserResponse(String id, String email, String name, Instant createdAt) {}

// Controller
@RestController
@RequestMapping("/api/v1/users")
@RequiredArgsConstructor
public class UserController {

    private final UserService userService;

    @PostMapping
    public ResponseEntity<ApiResponse<UserResponse>> createUser(
            @RequestBody @Valid CreateUserRequest request) {
        UserResponse user = userService.create(request);
        URI location = URI.create("/api/v1/users/" + user.id());
        return ResponseEntity.created(location).body(new ApiResponse<>(user));
    }

    @GetMapping("/{id}")
    public ResponseEntity<ApiResponse<UserResponse>> getUser(@PathVariable String id) {
        return userService.findById(id)
            .map(user -> ResponseEntity.ok(new ApiResponse<>(user)))
            .orElseThrow(() -> new ResourceNotFoundException("User not found"));
    }
}

// Global exception handler
@RestControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<ApiError> handleValidation(MethodArgumentNotValidException ex) {
        List<FieldError> details = ex.getBindingResult().getFieldErrors().stream()
            .map(e -> new FieldError(e.getField(), e.getDefaultMessage(), "invalid"))
            .toList();
        return ResponseEntity.status(422).body(
            new ApiError("validation_error", "Request validation failed", details));
    }

    @ExceptionHandler(ResourceNotFoundException.class)
    public ResponseEntity<ApiError> handleNotFound(ResourceNotFoundException ex) {
        return ResponseEntity.status(404).body(
            new ApiError("not_found", ex.getMessage(), null));
    }
}

API Design Checklist

Before shipping a new endpoint:

• Resource URL follows naming conventions (plural, kebab-case, no verbs)
• Correct HTTP method used (GET for reads, POST for creates, etc.)
• Appropriate status codes returned (not 200 for everything)
• Input validated with Bean Validation (@Valid, @NotBlank, etc.)
• Error responses follow standard format with codes and messages
• Pagination implemented for list endpoints (cursor or offset)
• Authentication required (or explicitly marked as public)
• Authorization checked (user can only access their own resources)
• Rate limiting configured
• Response does not leak internal details (stack traces, SQL errors)
• Consistent naming with existing endpoints (camelCase vs snake_case)
• Documented (OpenAPI/Swagger spec updated)