Node.js Timeouts and Memory Leaks

- Memory Leak Issue: Node.js timeouts can easily create memory leaks.

• Timeout API: Unlike browsers, where setTimeout returns a number, Node.js returns a Timeout object.
• Timeout Object: This object includes several properties and can retain references, causing memory issues.

Example of Memory Leak

class MyThing {
  constructor() {
    this.timeout = setTimeout(() => { ... }, INTERVAL);
  }

  clearTimeout() {
    clearTimeout(this.timeout);
  }
}

• Problem: The Timeout object remains even after clearTimeout or timeout completion, holding references that prevent garbage collection.

Impact of AsyncLocalStorage

• AsyncLocalStorage: Attaches state to timeouts, making leaks worse.
• Example:

const { AsyncLocalStorage } = require('node:async_hooks');
const als = new AsyncLocalStorage();

let t;
als.run([...Array(10000)], () => {
  t = setTimeout(() => {
    const theArray = als.getStore();
  }, 100);
});

• Result: Timeout holds a reference to a large array, exacerbating the memory issue.

Suggested Fix

• Use ID Instead of Object:

  class MyThing {
    constructor() {
      this.timeout = +setTimeout(() => { ... }, INTERVAL);
    }

    clearTimeout() {
      clearTimeout(this.timeout);
    }
  }

• Problem: Current Node.js bug makes this approach cause an unrecoverable memory leak.

Workaround

• Aggressive Nullification:

  class MyThing {
    constructor() {
      this.timeout = setTimeout(() => {
        this.timeout = null;
      }, INTERVAL);
    }

    clearTimeout() {
      if (this.timeout) {
        clearTimeout(this.timeout);
        this.timeout = null;
      }
    }
  }

Broader Implications

• Widespread Use: Many applications use timeouts and intervals, leading to potential memory issues.
• Next.js: Patches setTimeout and setInterval to clear intervals, though it can encounter related bugs.

Long-Term Considerations

• Better API Design: Node.js could improve by returning a lightweight proxy object instead of the full Timeout object.
• AsyncLocalStorage Management: Need for APIs to prevent propagation of unnecessary state to avoid leaks.

Conclusion

• Memory Management: Developers need to be cautious with timeouts and AsyncLocalStorage to avoid memory leaks.
• Awaiting Fix: A fix for the underlying Node.js bug is crucial for long-term resolution.