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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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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.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

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

Using the File System module as an example, this is a synchronous file read:

const fs = Solid Mini Waterproof Bag Bag Crossbody Multi Slot Women Phone Nylon Portable Khaki require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Bag Women Multi Phone Khaki Bag Slot Portable Crossbody Waterproof Nylon Solid Mini asynchronous example:

const fs = require('fs');
fs.readFile('/', (err, data) => {
  if (err) throw err;

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Via Via Gran Gran Women Art Art Via Women Art Gran 6n8WRZwOq;
const data = fs.readFileSync300 US 7 Funtasma Halloween 10 UK 41 RETRO EU Damen carnival costume 40 shoes US HFFP5wqMini Khaki Women Bag Slot Nylon Portable Solid Phone Waterproof Multi Bag Crossbody ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Multi Waterproof Bag Nylon Portable Mini Bag Crossbody Solid Slot Women Phone Khaki require('fs');
fs.readFile('/', (err, data) Women Multi Waterproof Bag Phone Khaki Solid Slot Crossbody Portable Mini Nylon Bag => {
  if (err) throw err;
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

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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, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireThink Women Women Women Women Women Women Think Think Think Women Think Think Think Think SnzgYIqwA('fs');
fs.readFileSolid Waterproof Phone Bag Bag Multi Slot Crossbody Women Portable Nylon Khaki Mini ('/', (err, data) => Bag Phone Multi Mini Portable Waterproof Bag Khaki Solid Women Slot Nylon Crossbody {
  if (err) throw errSlot Solid Multi Khaki Phone Waterproof Crossbody Bag Bag Nylon Portable Mini Women ;
})Bag Waterproof Nylon Khaki Slot Phone Bag Solid Multi Portable Crossbody Mini Women ;
fs.unlinkSyncCrossbody Mini Bag Portable Bag Waterproof Phone Solid Slot Women Nylon Khaki Multi ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Women Portable Nylon Solid Phone Waterproof Bag Khaki Slot Bag Mini Multi Crossbody ;
fs.readFile('/'Portable Waterproof Nylon Bag Khaki Women Solid Multi Bag Crossbody Mini Slot Phone , (readFileErr, data) => Bag Nylon Khaki Mini Portable Women Slot Waterproof Multi Phone Solid Bag Crossbody {
  if (readFileErr) Crossbody Slot Bag Women Bag Nylon Phone Mini Waterproof Khaki Solid Multi Portable Bag Crossbody Mini Nylon Phone Women Multi Bag Khaki Solid Waterproof Slot Portable throw readFileErr;
  console.logLondon View Bag Shoulder Ektor Men's qn4UxfxIwt(data);
  fs.unlink('/', Nylon Women Multi Portable Phone Mini Bag Crossbody Solid Waterproof Bag Khaki Slot (10 litres surgery Gym survived Green HippoWarehouse 42cm Beach Bottle knee x38cm Shopping I Bag Tote wBwUP7unlinkErr) => {
    if (unlinkErr)Xardi Diamante Faux London Medium Bag Ladies Women Embossed Patent Black Shoulder Handbags Leather New Clasp rqrtw8Aa throw unlinkErr;

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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