Do Node.js API functions execute in separate threads

Question

Javascript is single threaded. So every piece of code which I write is executed in a single thread of javascript runtime provided by Node. So when I execute following code:

var fs = require("fs");

fs.readFile('input.txt', function (err, data) {
   if (err) {
       return console.error(err);
   }
   console.log("Asynchronous read: " + data.toString());
});

// Synchronous read
var data = fs.readFileSync('input.txt');

Following is my understanding of workflow, please correct me if I am wrong:

fs.readFile is popped off the call stack by manager in Node immediately and moved to some other thread where it starts the reading stuff.

When reading is complete, manager in Node:

puts the callback function of fs.readFile in the queue stack
which event loop eventually puts in the call stack and executes

fs.readFileSync is not popped off the stack but is executed in the runtime thread itself like any other function.

So, who is this manager in node which executes Node API I/O functions in other threads and manages their call stacks.

So does Node use multi-cores to execute it's API functions in the background?

There's no need for separate threads. The underlying operating system essentially functions as the "other" thread, and signals the Node process when I/O operations progress/complete. — Pointy
– Pointy, Commented Oct 3, 2015 at 12:09
@Pointy what pops fs.readFile from the call stack so that other functions can execute, is their a separate call stack for Node API functions where they are executed? — adnan kamili
– adnan kamili, Commented Oct 3, 2015 at 12:18
Nothing "pops" it from the call stack other than the fact that it simply returns immediately after initiating the I/O operation. (It uses an operating system asynchronous I/O call to do its work.) — Pointy
– Pointy, Commented Oct 3, 2015 at 12:55
@Pointy It makes sense, but what puts its callback in to the queue stack, and what about SetTimeOut() — adnan kamili
– adnan kamili, Commented Oct 3, 2015 at 13:03
The Node runtime manages callbacks etc. At the heart of it is a (probably fairly involved) loop that uses OS polling facilities, timers, etc. When something happens at a low level, the runtime has to map that to some piece of user code and invoke it. — Pointy
– Pointy, Commented Oct 3, 2015 at 13:18

Eugene Obrezkov · Accepted Answer · 2015-10-03 14:52:51Z

3

NodeJS is using event loop for asynchronous operations.

If you know, there is a library exists called libuv. libuv is responsible for asynchronous code execution. Here is a little pseudo-code how it managing this:

while there are still events to process:
  e = get the next event
  if there is a callback associated with e:
    call the callback

You can read more about libuv here - http://nikhilm.github.io/uvbook/introduction.html

So, basically:

Do Node.js API functions execute in separate threads?

Both, yes and no. If function can move I/O operation to separate thread in thread-pool then yes. Otherwise it executes in the same thread.

So, who is this manager in node which executes Node API I/O functions in other threads and manages their call stacks?

libuv - https://github.com/libuv/libuv

So does Node use multi-cores to execute it's API functions in the background?

I don't know for sure, anyone can correct me if I'm wrong, but Node is executes within single-core. If you want to run with multi-core support, you need run it in cluster-mode.

UPD: Here is a little diagram that can help you to understand.

answered Oct 3, 2015 at 14:52

Eugene Obrezkov

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2 Comments

adnan kamili Over a year ago

It's a nice answer. In cluster mode, node actually works by creating multiple processes to use multiple cores - not threads.

Eugene Obrezkov Over a year ago

Yeah, so if you want to run NodeJS in threads (not processes) you can take a look at fork of NodeJS - JXCore - jxcore.com/home. JXCore exposes API for running your functions in separate threads and implements messaging between them.

piemonkey · Accepted Answer · 2015-10-03 12:41:34Z

0

While looking for resources to quote on answering this I couldn't actually find any that go into much detail. There's a nice intro here and some details on how the event loop works on MDN

It seems that there is another thread internal to the V8 engine which manages the event loop. This will take responses from things like operating system I/O and add the data and relevant callback to the queue. Every time the event loop finishes what it's doing it picks the next thing from the queue.

answered Oct 3, 2015 at 12:41

piemonkey

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