API Design Guidelines

This document outlines the recommended guidelines for designing APIs that are consistent, predictable, and easy for developers to integrate with. Following these principles ensures a high-quality developer experience.

1. RESTful Principles

Our APIs are designed following RESTful architectural constraints:

• Client-Server Architecture: Separation of concerns between client and server.
• Statelessness: 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.
• Cacheability: Responses should indicate whether they are cacheable to improve performance.
• Uniform Interface: This is the core of REST. It includes:
  • Identification of resources through URIs.
  • Manipulation of resources through representations.
  • Self-descriptive messages.
  • HATEOAS (Hypermedia as the Engine of Application State) where applicable.
• Layered System: Clients cannot tell whether they are connected directly to the end server, or to an intermediary along the way.
• Code on Demand (Optional): Servers can temporarily extend or customize the functionality of a client by transferring executable code.

2. Resource Naming and URIs

Resources should be named using nouns, not verbs, and should follow a hierarchical structure.

• Use plural nouns for collections of resources (e.g., /users, /products).
• Use specific identifiers for individual resources within a collection (e.g., /users/{userId}, /products/{productId}).
• Use nested resources for related entities (e.g., /users/{userId}/orders).
• URIs should be lowercase and use hyphens to separate words (e.g., /api-guidelines).
• Avoid overly deep nesting.

3. HTTP Methods

Use standard HTTP methods to perform operations on resources:

• GET: Retrieve a representation of a resource or a collection of resources. Should be safe and idempotent.
• POST: Create a new resource within a collection, or perform an action that doesn't fit other methods. Not idempotent.
• PUT: Update or replace an existing resource entirely. Idempotent.
• PATCH: Partially update an existing resource. Not necessarily idempotent.
• DELETE: Remove a resource. Idempotent.

4. Request and Response Formats

APIs should consistently use JSON for request and response bodies.

• Content-Type: Use application/json for all JSON payloads.
• Accept: Clients should specify Accept: application/json to indicate their preference.
• Data Structure: Use clear, descriptive keys for JSON objects. Avoid abbreviations.

Example: Fetching User Data

GET /users/123 HTTP/1.1
Host: api.example.com
Accept: application/json

{
  "id": "123",
  "firstName": "Jane",
  "lastName": "Doe",
  "email": "jane.doe@example.com",
  "registeredAt": "2023-10-27T10:00:00Z"
}

5. Status Codes

Use standard HTTP status codes to indicate the outcome of a request.

• 2xx Success:
  • 200 OK: Request succeeded.
  • 201 Created: Resource successfully created.
  • 204 No Content: Request succeeded, but there's no response body.
• 3xx Redirection: (Less common in typical API interactions)
• 4xx Client Errors:
  • 400 Bad Request: The request was malformed or invalid.
  • 401 Unauthorized: Authentication is required and has failed or not been provided.
  • 403 Forbidden: The authenticated user does not have permission to access the resource.
  • 404 Not Found: The requested resource could not be found.
  • 405 Method Not Allowed: The HTTP method used is not supported for this resource.
  • 409 Conflict: The request could not be completed due to a conflict with the current state of the resource.
  • 422 Unprocessable Entity: The request is well-formed but contains semantic errors.
• 5xx Server Errors:
  • 500 Internal Server Error: A generic error occurred on the server.
  • 503 Service Unavailable: The server is temporarily unable to handle the request.

{
  "error": {
    "code": "invalid_input",
    "message": "The provided email address is not valid."
  }
}

6. Pagination

For collections that may return many items, implement pagination to avoid overwhelming the client and server.

• Use query parameters like page, limit, offset, or pageSize.
• Include metadata in the response about pagination, such as total count, next/previous page links.

Example: Paginated Users

GET /users?page=2&limit=50 HTTP/1.1
Host: api.example.com
Accept: application/json

{
  "totalCount": 1234,
  "pageSize": 50,
  "currentPage": 2,
  "nextPage": "/users?page=3&limit=50",
  "previousPage": "/users?page=1&limit=50",
  "data": [
    // ... 50 user objects ...
  ]
}

7. Filtering, Sorting, and Searching

Provide mechanisms for clients to filter, sort, and search collections.

• Use query parameters for these operations (e.g., /products?category=electronics&sortBy=price:desc).
• Document the available filter fields, sortable fields, and search capabilities clearly.

8. Security Considerations

• Implement robust authentication mechanisms (e.g., OAuth 2.0, API Keys).
• Enforce authorization based on user roles and permissions.
• Validate all incoming data to prevent injection attacks.
• Do not expose sensitive information in URIs or query parameters.

9. Versioning

Plan for API evolution by implementing a clear versioning strategy.

• Common strategies include URI versioning (e.g., /v1/users) or header versioning.
• Clearly document your versioning strategy and provide a migration path for older versions.

10. Documentation and Examples

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

• Provide clear descriptions of all resources, endpoints, parameters, and response structures.
• Include practical code examples in various popular programming languages.
• Use tools like OpenAPI (Swagger) to generate interactive documentation.

By adhering to these guidelines, we aim to build APIs that are robust, scalable, and a pleasure for developers to work with.