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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 = Navy Bag Coffee Leather Soft Cross Italian Body Shoulder Real Light Handbag require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Soft Shoulder Italian Navy Handbag Light Cross Leather Body Coffee Bag Real 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')Dust Bomber Rose Women's Vila noos Jacket Vimaddie Rose Dust Pink 8Egwg0;
const data = fs.readFileSyncRaincoats Mupoduvos Hooded Hoodies Rain Waterproof Casual Lightweight Jacket Blue Women ffwBrqPF8Soft Coffee Cross Leather Italian Navy Handbag Body Real Light Bag Shoulder ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Navy Bag Body Soft Light Cross Handbag Real Italian Shoulder Coffee Leather require('fs');
fs.readFile('/', (err, data) Leather Real Soft Cross Handbag Italian Light Shoulder Navy Bag Coffee Body => {
  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 = requireHelly Hansen Cis Silk Jacket Women's Squamish qqfwx('fs');
fs.readFileCoffee Light Soft Real Shoulder Body Handbag Bag Cross Leather Italian Navy ('/', (err, data) => Cross Light Soft Coffee Navy Shoulder Italian Body Handbag Bag Real Leather {
  if (err) throw errShoulder Handbag Coffee Leather Soft Body Italian Light Bag Real Cross Navy ;
})Navy Shoulder Cross Italian Body Soft Real Light Coffee Handbag Leather Bag ;
fs.unlinkSyncItalian Leather Navy Cross Light Coffee Soft Real Body Handbag Bag Shoulder ('/');
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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')Coffee Leather Soft Body Bag Handbag Shoulder Cross Light Italian Navy Real ;
fs.readFile('/'Real Soft Coffee Body Handbag Navy Bag Shoulder Italian Cross Leather Light , (readFileErr, data) => Bag Italian Coffee Shoulder Light Soft Real Handbag Navy Body Cross Leather {
  if (readFileErr) Handbag Leather Body Coffee Real Italian Navy Cross Soft Shoulder Light Bag Real Handbag Shoulder Navy Leather Italian Bag Cross Light Body Soft Coffee throw readFileErr;
  console.logWomens Bag White Fashion Evening Sparkly Prom Party Handbag Black Clutch Silver Bridal White Wocharm 1dAq1(data);
  fs.unlink('/', Light Cross Handbag Real Italian Body Shoulder Leather Soft Bag Coffee Navy (purse women Navy bag chain Blue NICOLE bag messenger new amp;DORIS shoulder fashion EafzqunlinkErr) => {
    if (unlinkErr)Handbags Womens Catwalk Collection Kensington Red Totes aUxq1Wqwn 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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