API Design Principles

This document outlines the core principles and best practices for designing robust, scalable, and user-friendly Application Programming Interfaces (APIs) within the Microsoft ecosystem.

1. Consistency is Key

A consistent API is easier to learn, use, and maintain. Adhere to established patterns and conventions:

• Naming Conventions: Use clear, descriptive names for resources, endpoints, and parameters. Follow standard casing conventions (e.g., camelCase for JSON properties, kebab-case for URL paths if applicable).
• HTTP Methods: Utilize standard HTTP methods (GET, POST, PUT, DELETE, PATCH) appropriately to represent the intended action on a resource.
• Status Codes: Employ standard HTTP status codes (200 OK, 201 Created, 400 Bad Request, 401 Unauthorized, 404 Not Found, 500 Internal Server Error, etc.) to communicate the outcome of an API request.
• Error Handling: Provide consistent and informative error responses, typically in JSON format, including an error code, a human-readable message, and potentially details for developers.

Example Error Response

{
    "error": {
        "code": "InvalidInputError",
        "message": "The provided 'userId' is not a valid UUID.",
        "details": {
            "field": "userId",
            "value": "invalid-id"
        }
    }
}

2. Resource-Oriented Design

Model your API around resources—the objects or data that your API exposes. Use nouns for resource names and verbs (via HTTP methods) for actions.

• Clear Resource Identification: Each resource should have a unique identifier, typically a URL path.
• Relationships: Expose relationships between resources where relevant, often through nested URLs or hypermedia links (e.g., HATEOAS).

Example Resource Structure

GET /users/{userId}
GET /users/{userId}/orders
POST /users/{userId}/orders

3. Statelessness

APIs should be stateless, meaning each request from a client to the server must contain all the information necessary to understand and complete the request. The server should not store any client context between requests.

• Authentication/Authorization: Handle authentication and authorization on a per-request basis, often using tokens (e.g., JWT) passed in headers.

4. Versioning

API evolution is inevitable. Implement a clear versioning strategy to manage changes without breaking existing clients.

• URL Versioning: Include the version number in the URL path (e.g., /v1/users, /v2/users).
• Header Versioning: Use custom request headers (e.g., X-Api-Version: 2).
• Deprecation Policy: Clearly communicate deprecation timelines and support for older versions.

5. Security Considerations

Security should be a primary concern from the outset of API design.

• Authentication: Implement robust authentication mechanisms (e.g., OAuth 2.0, API Keys).
• Authorization: Enforce granular permissions to ensure clients only access what they are authorized to.
• Input Validation: Sanitize and validate all incoming data to prevent common vulnerabilities like SQL injection or cross-site scripting (XSS).
• HTTPS: Always use HTTPS to encrypt data in transit.

6. Performance and Scalability

Design APIs with performance and scalability in mind.

• Pagination: For collections, implement pagination to avoid returning massive datasets.
• Filtering and Sorting: Allow clients to filter and sort resources to retrieve only necessary data.
• Rate Limiting: Protect your API from abuse and ensure fair usage by implementing rate limiting.
• Caching: Utilize HTTP caching mechanisms (e.g., ETag, Last-Modified headers) where appropriate.

7. Documentation

Comprehensive and up-to-date documentation is crucial for API adoption and success.

• Clarity: Use clear language and provide practical examples.
• Tools: Consider using standards like OpenAPI (Swagger) for generating interactive documentation.