How Node.js & Express Handle Multiple Requests: Behind the Scenes

Node.js handles thousands of concurrent connections with a single thread — but how? The secret is the event loop and non-blocking I/O. When a request arrives, Node.js doesn't create a new thread. Instead, it registers callbacks and delegates I/O operations (file reads, database queries, network calls) to the libuv thread pool or OS-level async APIs.

The event loop continuously cycles through phases: timers → pending callbacks → poll (I/O) → check → close. When an I/O operation completes, its callback enters the queue and executes on the main thread. This means Node.js excels at I/O-bound work but struggles with CPU-intensive tasks (they block the single thread, stalling all other requests).

Solutions for CPU work: worker threads, child processes, or offloading to separate services. Understanding this model is essential for writing performant Node.js applications that don't accidentally block.