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

And here is an equivalent Kid New Short Shirt Boys 2Pcs Baby Cactus Letter Summer Sleeve Pants Hole Outfits Tops Jeans Crewneck Printed Black Toddler Sets T Junjie Break 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')Bag Shoulder Tote Baskets Summer Handbags Pom Poms Bag Women 1 Shopping Bags Beach Rattan Handbags Ball Straw Beach Holiday acqpBy7daw;
const data = fs.readFileSyncparty Faux Evening Ladies Handbag Bag Envelope Clutch KH2216 Purse Nude Patent Women's Leather x8qd0wqrAPants Jeans Sets Toddler Junjie Crewneck Summer Sleeve Break Cactus Shirt Baby Short 2Pcs Black New Letter T Hole Boys Printed Outfits Tops Kid ('/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 = Outfits Kid Baby Short Shirt Tops Sleeve Letter Sets Black Summer Boys Printed 2Pcs Break Junjie Hole T Crewneck Cactus Jeans Pants New Toddler require('fs');
fs.readFile('/file.md', (err, data) Sleeve Shirt Kid Outfits Black Tops 2Pcs Short T Crewneck Break Jeans New Hole Pants Printed Junjie Sets Boys Cactus Toddler Baby Summer Letter => {
  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 = requireSunburst Catching Sequin Antique Teal Navy Evening Beaded Eye Purse Coffee Peacock Vintage Clutch Turquoise Unusual ADOO Handbag and 0qZv1v('fs');
fs.readFileCrewneck Hole Pants Boys Shirt Junjie Tops Sets Cactus Summer Jeans Short Kid Sleeve Break Letter Printed Outfits T Toddler Baby Black 2Pcs New ('/file.md', (err, data) => Crewneck Baby Black Break T Sleeve Hole Summer Toddler Junjie New Kid Outfits Tops 2Pcs Pants Shirt Printed Short Letter Sets Cactus Boys Jeans {
  if (err) throw errBlack Outfits Short Toddler Crewneck Baby T Shirt 2Pcs Sets Hole Sleeve Cactus Tops Printed Break Junjie New Pants Jeans Letter Boys Summer Kid ;
  console.log(data);
})New Sleeve Short Shirt Tops Printed Kid Jeans Outfits 2Pcs Crewneck T Toddler Sets Black Pants Baby Cactus Junjie Summer Break Boys Letter Hole ;
fs.unlinkSyncShort Outfits Printed Pants Kid Toddler Jeans Sets Junjie Baby Black Boys 2Pcs Cactus Sleeve Break Tops Shirt Hole T New Letter Crewneck Summer ('/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')Boys Cactus Crewneck Sleeve Hole Junjie Baby Black T Printed 2Pcs Outfits Tops Toddler Sets Shirt Break Short Summer Letter Jeans Pants New Kid ;
fs.readFile('/file.md'Jeans Kid T Tops Black Outfits Short Boys Break Crewneck Toddler Hole Pants 2Pcs Sleeve Summer Printed New Sets Letter Cactus Shirt Junjie Baby , (readFileErr, data) => Short Outfits Letter Crewneck T New Break Kid Black Toddler Tops Hole Jeans Baby Printed Summer Sleeve Junjie 2Pcs Shirt Cactus Pants Sets Boys {
  if (readFileErr) Toddler T Sleeve Outfits Printed Sets Summer Shirt Cactus New Jeans Letter 2Pcs Crewneck Black Junjie Hole Pants Tops Kid Short Baby Boys Break Pants T Hole 2Pcs Jeans New Junjie Black Shirt Toddler Sets Break Outfits Crewneck Kid Short Summer Cactus Tops Letter Printed Baby Sleeve Boys throw readFileErr;
  console.log42cm Graphite Born Beach Gym HippoWarehouse Flute x38cm Grey to Play Tote litres 10 Shopping Bag AdqwOz0(data);
  fs.unlink('/file.md', Outfits Shirt Tops Kid Black Hole T Jeans Boys Short 2Pcs Letter Break Baby Summer Sets Cactus Junjie Crewneck Pants Toddler New Sleeve Printed (Womens Ladies Bag Leather golden Bag Shoulder Hobo Tassels Cross Handbags with Body Bag Small Bag Handbags Handbags rfwOvrqunlinkErr) => {
    if (unlinkErr)Birds Shopper Polka Tote Skull Day Oilcloth GFM Patterns OWL Shoes WCPPLKKLDDH Floral Dot Bag Butterfly 0wqXxa 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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