Simple Rate Limiter for Outgoing API Requests

// A simple in-memory queue and rate limiter
class ApiRateLimiter {
  constructor(requestsPerInterval, intervalMs) {
    this.requestsPerInterval = requestsPerInterval;
    this.intervalMs = intervalMs;
    this.queue = [];
    this.timestamps = []; // Stores timestamps of recent requests
    this.isProcessing = false;

    console.log(`Rate Limiter initialized: ${requestsPerInterval} requests/${intervalMs / 1000}s`);
  }

  // Add a request function to the queue
  addRequest(requestFn) {
    return new Promise((resolve, reject) => {
      this.queue.push({ requestFn, resolve, reject });
      this.processQueue();
    });
  }

  async processQueue() {
    if (this.isProcessing) {
      return;
    }
    this.isProcessing = true;

    while (this.queue.length > 0) {
      // Clean up old timestamps
      const now = Date.now();
      this.timestamps = this.timestamps.filter(ts => ts > now - this.intervalMs);

      if (this.timestamps.length < this.requestsPerInterval) {
        const { requestFn, resolve, reject } = this.queue.shift(); // Get next request
        this.timestamps.push(now); // Mark this request as sent

        try {
          const result = await requestFn(); // Execute the actual API call
          resolve(result);
        } catch (error) {
          reject(error);
        }
      } else {
        // Rate limit hit, wait until next slot is available
        const timeToWait = this.intervalMs - (now - this.timestamps[0]);
        console.log(`Rate limit hit. Waiting for ${timeToWait}ms...`);
        await new Promise(res => setTimeout(res, timeToWait + 50)); // Add a small buffer
      }
    }
    this.isProcessing = false;
  }
}

// Example Usage:
const myRateLimiter = new ApiRateLimiter(5, 1000); // 5 requests per second

async function makeApiCall(id) {
  return myRateLimiter.addRequest(async () => {
    console.log(`[${new Date().toLocaleTimeString()}] Making API call ${id}...`);
    // Simulate an API call
    return new Promise(resolve => setTimeout(() => {
      console.log(`[${new Date().toLocaleTimeString()}] API call ${id} complete.`);
      resolve(`Response for call ${id}`);
    }, Math.random() * 500)); // Simulate variable API response time
  });
}

// Make some requests quickly to test the limiter
// for (let i = 1; i <= 15; i++) {
//   makeApiCall(i).then(result => console.log(result)).catch(console.error);
// }

// After a delay, make more requests
// setTimeout(() => {
//   for (let i = 16; i <= 20; i++) {
//     makeApiCall(i).then(result => console.log(result)).catch(console.error);
//   }
// }, 3000);