Node.js Promise Concurrency Control

What is Promise Concurrency Control in Node.js?

Firing a thousand requests at once with Promise.all will exhaust sockets, hit rate limits and may crash the process. Concurrency control processes a large list with a fixed number of operations in flight.

Promise Concurrency Control example

async function mapWithLimit(items, limit, fn) {
  const results = new Array(items.length);
  let next = 0;

  const workers = Array.from({ length: Math.min(limit, items.length) }, async () => {
    while (next < items.length) {
      const i = next++;
      results[i] = await fn(items[i], i);
    }
  });

  await Promise.all(workers);
  return results;
}

// 5,000 items, never more than 10 requests in flight
const enriched = await mapWithLimit(jobIds, 10, id => fetchJob(id));

How this works

A fixed number of workers pull from a shared cursor, so exactly `limit` operations run at any moment regardless of how many items there are.

Key points to remember

• Ten to fifty concurrent operations is a sensible starting range for HTTP work.
• Add a small delay between batches when calling a rate-limited API.
• The p-limit package does this in one line if you prefer a dependency.

Common mistakes with Promise Concurrency Control

• Promise.all over a huge array, which opens every connection at once.
• Building the entire result array in memory when streaming would do.