Async

One synchronous, blocking operation in your Node.js server blocks EVERY concurrent request. JSON.parse on a 10MB payload, a for-loop over 100k items, or a synchronous file read — all of them freeze your event loop and make your entire server unresponsive. Here's how to find and eliminate blocking I/O.

You wrote perfectly async Node.js code — no blocking I/O, no synchronous loops. Yet under load, responses stall and CPU pegs. The culprit is Node.js's hidden libuv thread pool being exhausted by crypto, file system, and DNS operations. Here's what's really happening.