What happens in an API request lifecycle?

1. Client sends HTTP request
2. DNS resolution
3. TCP connection (handshake)
4. TLS handshake (HTTPS)
5. Request reaches server
6. Server processes request
7. Response sent back
8. Connection closed/kept alive

// Order of execution
app.use(morgan('dev'));           // 1. Logging
app.use(cors());                  // 2. CORS headers
app.use(helmet());                // 3. Security headers
app.use(express.json());          // 4. Parse JSON body
app.use(authenticate);            // 5. Authentication
app.use(rateLimit());             // 6. Rate limiting

// Route handlers
app.get('/users', (req, res) => { // 7. Route logic
  res.json(users);
});

// Error handler (last)
app.use((err, req, res, next) => { // 8. Error handling
  res.status(500).json({ error: err.message });
});

app.post('/users', (req, res) => {
  req.method;      // 'POST'
  req.url;         // '/users'
  req.headers;     // { 'content-type': 'application/json', ... }
  req.query;       // { page: '1' } from ?page=1
  req.params;      // { id: '123' } from /users/:id
  req.body;        // Parsed JSON body
  req.cookies;     // Parsed cookies
});

app.get('/users', (req, res) => {
  res.status(200);                    // Set status
  res.set('X-Custom', 'value');       // Set header
  res.cookie('token', 'abc');         // Set cookie
  res.json({ data: users });          // Send JSON
  // or
  res.send('Hello');                  // Send text
  res.redirect('/login');             // Redirect
});