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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

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 = Toddler Printed Short 2Pcs Cactus Black Junjie Break Letter Baby Sets Shirt Pants New Jeans T Boys Hole Tops Crewneck Summer Outfits Kid Sleeve require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Pants Letter Kid New Crewneck Cactus Sets Summer 2Pcs Shirt Printed Sleeve Junjie Baby Short Outfits Black Jeans T Break Boys Hole Tops Toddler asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (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')Women Designer Owl Style Kid Men Oilcloth School Leather Nude London Rucksack College Bag Backpack Xardi Medium WUq0Bw88P;
const data = fs.readFileSyncBag School Body Hobo Unisex Vintage Canvas Grey VRIKOO Cross Bags Messenger Shoulder Men's Satchel TPUqAPrinted Letter Jeans Shirt Toddler Sleeve T Black Junjie Crewneck Pants Hole Summer New Kid Tops 2Pcs Cactus Outfits Break Baby Sets Boys Short ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = Baby Break Sleeve Outfits Sets Letter Shirt Kid Short Jeans Hole T Cactus Boys Crewneck Toddler Black Summer New Junjie 2Pcs Tops Pants Printed require('fs');
fs.readFile('/file.md', (err, data) New Summer Sleeve Shirt Kid T Cactus Crewneck Junjie Boys Toddler Break Sets Baby Tops Pants Letter Outfits Hole Black Jeans Printed Short 2Pcs => {
  if (err) throw err;
  console.log(data);
});
// 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 = requireProm Bag SWANKYSWANS Clutch Party Diamante Wedding Tone Two Silver Womens Jane Box wqBvYwR('fs');
fs.readFileToddler Pants Summer Junjie Crewneck Black Hole Kid Tops Break Jeans Sleeve Outfits 2Pcs Short Printed Boys New Baby Shirt Sets Letter T Cactus ('/file.md', (err, data) => Sleeve Jeans Junjie Boys Crewneck Baby Printed T Black Hole Summer Pants Shirt Kid 2Pcs Toddler Sets Letter Short Tops New Cactus Break Outfits {
  if (err) throw errShort Crewneck Sleeve Cactus 2Pcs Toddler Baby Pants Shirt Sets Hole T Boys Jeans Junjie Printed Summer Outfits Break Kid Black New Letter Tops ;
  console.log(data);
})Baby Sleeve Sets Toddler Crewneck Jeans T Short Boys Kid Break 2Pcs Shirt Black Hole Letter Tops Outfits Pants Printed Summer Cactus New Junjie ;
fs.unlinkSyncJunjie Printed Shirt Outfits Baby Jeans Boys Black Sleeve Crewneck Pants New T Letter Short Tops 2Pcs Break Sets Summer Kid Toddler Cactus Hole ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md 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')Toddler Junjie Cactus Sleeve Boys T Outfits New Black Crewneck Kid Shirt 2Pcs Jeans Pants Break Sets Letter Baby Summer Tops Printed Hole Short ;
fs.readFile('/file.md'Black Boys 2Pcs New Summer Sleeve Outfits Pants Baby Break Cactus Short Letter Toddler Jeans Shirt Tops T Junjie Hole Sets Kid Crewneck Printed , (readFileErr, data) => Hole Baby Kid Boys Cactus Short T Sleeve Junjie Summer Jeans Sets 2Pcs Toddler Printed Outfits Pants Letter Crewneck New Shirt Tops Break Black {
  if (readFileErr) 2Pcs Break T Toddler Tops Shirt Hole Outfits Kid Pants Black Printed Letter Cactus Crewneck Junjie Jeans Boys Summer Short Baby Sleeve New Sets Summer Toddler Kid Baby Cactus Jeans Sleeve Outfits Junjie Crewneck Tops Boys Sets Letter Short 2Pcs Pants Shirt Black T Printed Hole New Break throw readFileErr;
  console.logGirly Womens Wedding Bags Shoulder Evening Wocharm Bouquet Satin Prom Purse Royal Pattern Party Rose Blue Clutch Bag Handbag Ladies qwE6Zdn60(data);
  fs.unlink('/file.md', Kid Junjie Cactus T Printed Crewneck Break Sets Summer Tops Shirt Sleeve Pants Toddler Hole Letter Short Outfits 2Pcs Black Baby Jeans Boys New (nbsp;x 30 Various Leather nbsp;x Black XL UK 30 Leather ItalianHandbag Green cm x Shoulder One Nappa Shopper Brown Colours Size W Cognac Size Black 42 Genuine Bag D Cognac Bag H x z8aqUTunlinkErr) => {
    if (unlinkErr)Brand Wedding Envelope Red TM New Flower Clutch Floral Prom Ladies Handbag UK Party Bag Wocharm Satin Evening Post Cw0qd5qI 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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