CODEWITHSUNDEEP
javascriptnode.js
confusedWarrior
confusedWarrior

Reputation: 1390

Does event loops polls for event completion or kernel/os notifies back?

When Node.js starts, it initializes the event loop, processes the provided input script which may make async API calls, schedule timers, or call process.nextTick(), then begins processing the event loop.

There are seven phases and each phase has its own event queue which is based on FIFO.

So application makes a request event, event demultiplexer gathers those requests and pushes to respective event queues.

For example, If my code makes two reqeusts one is setTimeOut() and another is some API Call, demultiplexer will push the first one in timer queue and other in poll queue.

But events are there, and loop watches over those queues and events, on completion in pushes the registered callback to the callstack where it is processed.

My question is,

1). Who handles events in event queue to OS?

2). Does event loop polls for event completion in each event queue or does OS notifies back?

3). Where and who decides whether to call native asyncrhonous API or handle over to a thread pool?

I am very verge of understanding this, I have been strugling a lot to grasp the concepts. There are a lot of false information about node.js event loop and how it handles asynchronous calls using one thread.

Please answer this questions if possible. Below are the references where I could get some better insight from.

https://github.com/nodejs/nodejs.org/blob/master/locale/en/docs/guides/event-loop-timers-and-nexttick.md

https://dev.to/lunaticmonk/understanding-the-node-js-event-loop-phases-and-how-it-executes-the-javascript-code-1j9

how does reactor pattern work in Node.js?

https://www.youtube.com/watch?v=PNa9OMajw9w&t=3s

Upvotes: 2

Views: 511

Answers (1)

jfriend00
jfriend00

Reputation: 707786

Who handles events in event queue to OS?

How OS events work depends upon the specific type of event. Disk I/O works one way and Networking works a different way. So, you can't ask about OS events generically - you need to ask about a specific type of event.

Does event loop polls for event completion in each event queue or does OS notifies back?

It depends. Timers for example are built into the event loop and the head of the timer list is checked to see if it's time has come in each timer through the event loop. File I/O is handled by a thread pool and when a disk operation completes, the thread inserts a completion event into the appropriate queue directly so the event loop will just find it there the next time through the event loop.

Where and who decides whether to call native asynchronous API or handle over to a thread pool?

This was up to the designers of nodejs and libuv and varies for each type of operation. The design is baked into nodejs and you can't yourself change it. Nodejs generally uses libuv for cross platform OS access so, in most cases, it's up to the libuv design for how it handles different types of OS calls. In general, if all the OSes that nodejs runs on offer a non-blocking, asynchronous mechanism, then libuv and nodejs will use it (like for networking). If they don't (or it's problematic to make them all work similarly), then libuv will build their own abstraction (as with file I/O and a thread pool).

You do not need to know the details of how this works to program asynchronously in nodejs. You make a call and get a callback (or resolved promise) when its done, regardless of how it works internally. For example, nodejs offers some asynchronous crypto APIs. They happen to be implemented using a thread pool, but you don't need to know that in order to use them.

Upvotes: 1

Related Questions