Is NodeJS really Single-Threaded?

Question

Node.js solves "One Thread per Connection Problem" by putting the event-based model at its core, using an event loop instead of threads. All the expensive I/O operations are always executed asynchronously with a callback that gets executed when the initiated operation completes.

The Observation IF any Operation occurs is handled by multiplexing mechanisms like epoll().

My question is now:

• Why doesn't NodeJS block while using the blocking Systemcalls select/epoll/kqueue?

• Or isn't NodeJS single threaded at all, so that a second Thread is
  necessary to observe all the I/O-Operations with select/epoll/kqueue?

Answer 1

NodeJS is evented (2nd line from the website), not single-threaded. It internally handles threading needed to do select/epoll/kqueue handling without the user explicitly having to manage that, but that doesn't mean there is no thread usage within it.

Answer 2

No.

When I/O operations are initiated they are delegated to libuv, which manages the request using its own (multi-threaded, asynchronous) environment. libuv announces the completion of I/O operations, allowing any callbacks waiting on this event to be re-introduced to the main V8 thread for execution.

V8 -> Delegate I/O (libuv) -> Thread pool -> Multi threaded async