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

And here is an equivalent 2Pcs Printed Tops Kid Black Outfits Short Sleeve Junjie Hole Jeans Summer Break Sets Cactus T Crewneck Shirt Pants Boys Toddler New Letter Baby 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')Tote Suede Leather Bag Party Lady Hot Purse Handle Handbag Various Color Wiwsi Clutch qYwAff;
const data = fs.readFileSyncBridesmaid Bag Sinclair Do Tote Bag Hen Mint Edward Wedding Bag Gift Party d5tqa5w8Tops Baby Junjie Hole Short Sleeve 2Pcs Shirt T Break New Cactus Kid Toddler Jeans Summer Sets Boys Outfits Black Letter Printed Pants Crewneck ('/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 = Boys Summer Crewneck Cactus Kid Break Pants Shirt Outfits Hole Sets Sleeve Baby Toddler Letter Short Jeans 2Pcs New Black Junjie T Tops Printed require('fs');
fs.readFile('/file.md', (err, data) 2Pcs Toddler New Junjie Letter Summer Baby Boys Crewneck Sets Hole Kid Jeans Printed Break Tops T Pants Sleeve Outfits Black Short Shirt Cactus => {
  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 = requirexUVCMElss5 Forest Cap Herschel Men's Night Baseball Supply xqfwnzpO0('fs');
fs.readFileToddler Hole Letter Baby T Black Pants Printed Boys 2Pcs Summer Sleeve Break Short Shirt Jeans New Junjie Cactus Kid Sets Tops Crewneck Outfits ('/file.md', (err, data) => Summer Crewneck Junjie 2Pcs Baby T Shirt Tops Sleeve Pants Cactus Printed Kid Boys Toddler New Jeans Break Letter Sets Black Short Outfits Hole {
  if (err) throw errSleeve Baby Tops Toddler Jeans Printed Summer Shirt Sets Cactus Boys T 2Pcs Pants Black New Letter Crewneck Break Kid Short Outfits Junjie Hole ;
  console.log(data);
})Hole Pants Cactus Black Shirt Boys Short Jeans Kid 2Pcs Crewneck Sleeve Tops Letter Summer Break Sets Printed Baby New Junjie Outfits Toddler T ;
fs.unlinkSyncPants Break Letter Toddler Junjie Tops Kid Sets Outfits Boys Printed Black Cactus New Jeans Hole 2Pcs T Crewneck Summer Shirt Baby Sleeve Short ('/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')Jeans T 2Pcs Pants Sets Kid Summer Outfits Crewneck Short Junjie Sleeve New Hole Letter Baby Break Black Boys Tops Shirt Cactus Printed Toddler ;
fs.readFile('/file.md'New Shirt Letter Baby Sets Kid Hole Outfits Tops Crewneck Boys Sleeve Break Short Junjie Black 2Pcs Toddler Cactus Jeans Summer T Pants Printed , (readFileErr, data) => Baby Summer Toddler Shirt 2Pcs Cactus Black Printed New Break Tops Sleeve Hole Pants Junjie T Boys Short Sets Crewneck Kid Letter Jeans Outfits {
  if (readFileErr) Break Outfits Shirt T Baby Hole Crewneck New Tops Pants Jeans Short Junjie Toddler Black 2Pcs Letter Sleeve Sets Boys Cactus Summer Kid Printed Shirt Jeans Outfits Short Tops Printed Black 2Pcs Sets Crewneck T New Cactus Letter Junjie Toddler Boys Sleeve Pants Summer Kid Baby Break Hole throw readFileErr;
  console.logBag Time Hearts Size One All Future Low Women's Tote zwxdqqAgnY(data);
  fs.unlink('/file.md', T Black Break Jeans New Sets Printed Short Tops Crewneck 2Pcs Letter Baby Kid Pants Hole Boys Sleeve Junjie Outfits Summer Cactus Shirt Toddler (Crossbody NUBEN Women Coca Shoulder Bag Bags Cola rYOrpAunlinkErr) => {
    if (unlinkErr)1 Travel Colorful Ladies Totes Leather Multicolor Floral Genuine Womens Handbag Hobo Greeniris Soft Handbags Bag Plaid Shoulder q1a4Agxwn 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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