# REST API Design

## HTTP Methods

| Method | Purpose | Example |
|--------|---------|---------|
| GET | Retrieve data | `GET /users` |
| POST | Create new resource | `POST /users` |
| PUT | Replace entire resource | `PUT /users/1` |
| PATCH | Partial update | `PATCH /users/1` |
| DELETE | Remove resource | `DELETE /users/1` |

## URL Structure

```
https://api.example.com/v1/users/123/posts?page=1&limit=10
│                         │    │      │        │
│                         │    │      │        └── Query Parameters
│                         │    │      └── Resource ID
│                         │    └── Collection
│                         └── Version
└── Base URL
```

## Status Codes

### Success
- `200 OK` - Request succeeded
- `201 Created` - Resource created
- `204 No Content` - Success, no body (DELETE)

### Client Errors
- `400 Bad Request` - Invalid input
- `401 Unauthorized` - Authentication required
- `403 Forbidden` - Authenticated but not permitted
- `404 Not Found` - Resource doesn't exist
- `422 Unprocessable Entity` - Validation error

### Server Errors
- `500 Internal Server Error` - Server error
- `502 Bad Gateway` - Gateway error
- `503 Service Unavailable` - Server down

## Best Practices

### Request
```http
GET /api/v1/users?status=active&sort=-created_at
Accept: application/json
Authorization: Bearer <token>
```

### Response
```json
{
  "data": [
    {
      "id": 1,
      "name": "John",
      "email": "john@example.com"
    }
  ],
  "meta": {
    "page": 1,
    "per_page": 20,
    "total": 100
  },
  "links": {
    "self": "/api/v1/users?page=1",
    "next": "/api/v1/users?page=2"
  }
}
```

### Error Response
```json
{
  "error": {
    "code": "VALIDATION_ERROR",
    "message": "Invalid input data",
    "details": [
      {
        "field": "email",
        "message": "Invalid email format"
      }
    ]
  }
}
```

## REST API Example (Node.js/Express)

### Basic Setup
```javascript
const express = require('express');
const app = express();

app.use(express.json());

// GET all users
app.get('/api/v1/users', (req, res) => {
    res.json({ data: users });
});

// GET single user
app.get('/api/v1/users/:id', (req, res) => {
    const user = users.find(u => u.id === parseInt(req.params.id));
    if (!user) return res.status(404).json({ error: 'Not found' });
    res.json({ data: user });
});

// POST create user
app.post('/api/v1/users', (req, res) => {
    const { name, email } = req.body;
    // Validation
    if (!name || !email) {
        return res.status(400).json({ error: 'Missing fields' });
    }
    // Create
    const newUser = { id: users.length + 1, name, email };
    users.push(newUser);
    res.status(201).json({ data: newUser });
});

// PUT update user
app.put('/api/v1/users/:id', (req, res) => {
    const user = users.find(u => u.id === parseInt(req.params.id));
    if (!user) return res.status(404).json({ error: 'Not found' });
    
    const { name, email } = req.body;
    user.name = name || user.name;
    user.email = email || user.email;
    res.json({ data: user });
});

// DELETE user
app.delete('/api/v1/users/:id', (req, res) => {
    const index = users.findIndex(u => u.id === parseInt(req.params.id));
    if (index === -1) return res.status(404).json({ error: 'Not found' });
    
    users.splice(index, 1);
    res.status(204).send();
});
```