Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

What is Node.js?

Why does Node.js exists?

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O.

Node is similar in design to, and influenced by, systems like Ruby's Event Machine or Python's Twisted. Node takes the event model a bit further. It presents an event loop as a runtime construct instead of as a library.

In Node there is no such start-the-event-loop call. Node simply enters the event loop after executing the input script. Node exits the event loop when there are no more callbacks to perform. This behavior is like browser JavaScript

The event loop is what allows Node.js to perform non-blocking I/O operations — despite the fact that JavaScript is single-threaded — by offloading operations to the system kernel whenever possible.

Since most modern kernels are multi-threaded, they can handle multiple operations executing in the background. When one of these operations completes, the kernel tells Node.js so that the appropriate callback may be added to the poll queue to eventually be executed.

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

These are the phases of the event loop. Each phase has his own queue. timers pending callbacks idle, prepare poll <- incoming: connections, data etc. check close callbacks

Each phase has a FIFO queue of callbacks to execute.

While each phase is special in its own way, generally, when the event loop enters a given phase, it will perform any operations specific to that phase, then execute callbacks in that phase's queue until the queue has been exhausted or the maximum number of callbacks has executed

12:33 > This diagram looks basically identical for node, instead of web APIs we have C++ APIs and the threading is being hidden from you by C++

Just because Node is designed without threads, doesn't mean you cannot take advantage of multiple cores in your environment. Child processes can be spawned by using our child_process.fork() API

V8 The V8 library provides Node.js with a JavaScript engine, which Node.js controls via the V8 C++ API. V8 is maintained by Google, for use in Chrome.

V8 is Google’s open source high-performance JavaScript and WebAssembly engine, written in C++. It is used in Google Chrome, the open source browser from Google, and in Node.js, among others.

V8 implements ECMAScript and WebAssembly. Can run standalone, or can be embedded into any C++ application

Libuv, a C library that is used to abstract non-blocking I/O operations to a consistent interface across all supported platforms. It provides mechanisms to handle file system, DNS, network, child processes, pipes, signal handling, polling and streaming.

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