Debug Memory Leak in Express Middleware

Problem

Your Express.js application's memory usage grows steadily over time. Starting at 150 MB after deployment, it reaches 1.2 GB within 6 hours and eventually gets OOM-killed by the container orchestrator. Restarting the process temporarily fixes the issue, but memory climbs again.

The Scenario

The application has several middleware layers and handles around 1,000 requests per minute. Memory profiling shows that the heap grows by approximately 2 MB every minute, but garbage collection runs regularly and reclaims some memory. The leak is gradual, making it hard to pinpoint.

You have narrowed the issue to the following suspicious patterns in the codebase:

Pattern 1: Request Logger Middleware

const requestLogs: any[] = [];


app.use((req, res, next) => {
  requestLogs.push({
    method: req.method,
    url: req.url,
    timestamp: Date.now(),
    headers: req.headers,
  });
  next();
});

Pattern 2: Cache Without Bounds

const cache = new Map();


function getCachedUser(id: string) {
  if (!cache.has(id)) {
    const user = db.getUserById(id);
    cache.set(id, user);
  }
  return cache.get(id);
}

Pattern 3: Event Listener Accumulation

app.use((req, res, next) => {
  const onData = (chunk: Buffer) => {
    // process chunk with access to req via closure
    processChunk(chunk, req);
  };
  someEventEmitter.on('data', onData);
  next();
  // Listener is never removed
});

Your Task

1. Explain why each pattern causes a memory leak.
2. Fix each pattern with production-ready code.
3. Describe how to detect and diagnose memory leaks in Node.js applications.
4. Propose architectural patterns that prevent memory leaks from reaching production.

Constraints

• Fixes must not significantly alter application behavior.
• Consider the impact on garbage collection and heap snapshots.
• Solutions should work in a production containerized environment.