• Prevents too many requests from overwhelming system.
• Only allow X requests per user per time window.

const requests = {};

function rateLimit(userId) {
  const now = Date.now();
  const window = 60000; // 1 minute
  const limit = 5;

  if (!requests[userId]) requests[userId] = [];

  // keep only recent requests
  requests[userId] = requests[userId].filter(t => now - t < window);

  if (**requests[userId].length >= limit**) {
    throw new Error("Too many requests");
  }

  requests[userId].push(now);
}

3. Semaphore (rate limiting / concurrency control)