How does node.js know if a call has to be executed

Question

I've read that node.js uses both treads and an event loop.

I'm curious to know how does it know how to treat a call back... Is it specified by the EventEmitter (and does the engineer know if it is going to be blocking or not)?

Or is the core itself that chooses it at runtime? If it's this one how does it detect if it has to be run async or threaded?

I've already read a lot of resources but i didn't find about this. Im reading the source code but it's hard since it is a lot of time since the last time i coded with C++.

thanks

Answer 1

Your JavaScript code always runs in a single thread. That's because the V8 JavaScript engine is not threadsafe.

However, as an implementation detail of some of the C++ code, there may be threads. For example, suppose you write some JavaScript code that connects to a database. Your JavaScript code will of course be async, like any good Node code. But async coding is very uncommon in the C/C++ world, so the database vendor probably didn't write an async C/C++ API.

So when someone is writing a Node package for database access, they have to write a shim that adapts between the "blocking" C++ behavior and the "non-blocking, event-driven" Node behavior. When you call, say, a "connect" method, that goes to C++ code that spawns a new thread, and that thread issues a (blocking) "connect" call to the database, which blocks the thread until the connection is done. Then the C++ code will communicate the "connection done" back to the event queue, and the next time the main (JavaScript) thread polls the event queue, your callback will fire.

So yes, there are threads, but their use should be completely transparent to you. When you're writing Node.js code in JavaScript, you don't need to worry about threads -- you just care that things happen when they're supposed to. Package authors may use threads, but that's purely an implementation detail and you should never have to worry about it. Your JavaScript code never explicitly uses threads.