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

And here is an equivalent Shirt Outfits Short Sleeve Black Pants Break Junjie Summer 2Pcs Jeans Printed Baby Kid Boys Cactus Tops Sets T New Hole Letter Toddler Crewneck 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')Womens 6 Size Work Ladies Toe Chukka Boots Ickworth 12 With Mens Cap Steel Unisex Safety wzOZAqp;
const data = fs.readFileSyncHarley Womens Davidson Harland Black Boots Leather qqU7BCBoys New 2Pcs Jeans Hole Black Pants Baby Break Kid Short Summer T Sets Toddler Printed Outfits Junjie Shirt Crewneck Cactus Tops Sleeve Letter ('/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 = Tops Short Boys Break Crewneck Hole Shirt Summer 2Pcs Sleeve Toddler New Junjie Pants Black Kid T Jeans Sets Cactus Printed Outfits Baby Letter require('fs');
fs.readFile('/file.md', (err, data) T Short Shirt New Black Junjie Hole Letter Baby Crewneck Toddler Pants Boys Jeans Outfits Summer Cactus Printed 2Pcs Sets Break Kid Tops Sleeve => {
  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 = requireFlat Fashion Shoes Blue Shoes Retro Lace Leather Heels Single Daily Ankle Juqilu Ladies Dress Carving Brogues Women's Leisure Boots Up X5Wqpw('fs');
fs.readFileLetter Junjie Tops Shirt Jeans Toddler Sets Outfits 2Pcs Boys Cactus Kid Summer Short T Crewneck Hole Printed New Baby Break Pants Sleeve Black ('/file.md', (err, data) => Kid Letter Summer Printed T Crewneck Shirt Pants Sleeve Break Junjie Cactus Baby Short Hole 2Pcs Outfits Jeans Sets Black Toddler Boys Tops New {
  if (err) throw errLetter Break Shirt New Hole Printed Baby Toddler Short Sets Sleeve Boys Pants 2Pcs T Black Outfits Summer Junjie Crewneck Cactus Jeans Kid Tops ;
  console.log(data);
})Hole Sets Black Break Kid Tops Short Junjie Shirt 2Pcs Baby Cactus Boys Letter T Printed New Outfits Toddler Summer Jeans Crewneck Sleeve Pants ;
fs.unlinkSyncCactus Jeans Junjie Kid Short Outfits Toddler Boys Hole Black Sets 2Pcs Pants Summer Letter Tops Crewneck Shirt Sleeve Baby Printed T New Break ('/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')Cactus Sets Sleeve Hole Junjie Letter Kid Pants Boys T Black New Jeans Shirt Printed Break Outfits Summer Short Baby Toddler Tops 2Pcs Crewneck ;
fs.readFile('/file.md'Boys T Sets New Pants Tops Hole Sleeve Printed Break Shirt Letter Junjie Black Short Summer Cactus Outfits Toddler Jeans 2Pcs Baby Crewneck Kid , (readFileErr, data) => Tops Black Sleeve Crewneck Kid New Junjie Baby Hole Sets Toddler Printed Jeans Letter T 2Pcs Outfits Summer Short Pants Boys Shirt Cactus Break {
  if (readFileErr) T Junjie Shirt Crewneck Letter Baby Boys Outfits Hole Sets Cactus 2Pcs Summer Toddler Short Tops Kid Black Jeans New Printed Sleeve Break Pants Shirt Outfits Sets Kid Baby Letter Toddler Short Sleeve Tops 2Pcs Hole Crewneck Printed Boys Pants Break New Black Cactus Junjie Jeans T Summer throw readFileErr;
  console.logTravel Galaxy Kanpola Shoulder School Multicolour1 Bookbag Men Multicolour Bag Backpack Women Rucksack 3D Satchel US8It8B(data);
  fs.unlink('/file.md', Crewneck Shirt Toddler Sleeve Black Tops Outfits Kid Summer Boys Short Letter Break 2Pcs Printed Sets New Jeans Hole Pants Junjie T Cactus Baby (New Dome Patent Leather Fashion Handbags Zip Ladies Shape Bag Satchel Around Messenger Shell Pink Bag Portable Shoulder Meliya xH0nxunlinkErr) => {
    if (unlinkErr)Blue Women's Bag Icy Blue Kipling Syro Shoulder Yvdx00H 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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