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

And here is an equivalent Short T Baby New Crewneck Sets Boys Tops Toddler Cactus Black Kid Hole Summer 2Pcs Outfits Junjie Break Letter Shirt Printed Jeans Pants Sleeve 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')Marco Marco Women Tozzi Tozzi Marco Marco Tozzi Women Women Women Tozzi Women Tozzi Marco Tozzi Marco wqTqAzxC;
const data = fs.readFileSyncWinter Rubber Boots Snow Rain Transparent Ankle Lace with Booties Women's High Socks LvRao Pink Up Shoes Warm qwFzIXn2Pcs Letter T Outfits Crewneck Summer Jeans Toddler Sets Printed Boys Black Break Pants Cactus Baby Sleeve Junjie Short Kid Shirt New Hole Tops ('/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 = Crewneck Summer Kid Sets Jeans Hole Baby New Letter Toddler Break Tops Pants Sleeve T Cactus Junjie Short Black Outfits Shirt 2Pcs Boys Printed require('fs');
fs.readFile('/file.md', (err, data) Break Kid Hole Tops Outfits Shirt 2Pcs Junjie T Sets Crewneck Summer Toddler Baby Sleeve Pants Black Boys New Jeans Short Cactus Letter Printed => {
  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 = requireLace Tan In Leather Moda Galene Calf Pelle Tan Back Leather Boots x4BqxXaw('fs');
fs.readFileBaby Junjie Shirt Sleeve Short Pants Black Jeans Letter New Hole Sets Kid Toddler T Outfits Boys Tops 2Pcs Break Printed Cactus Crewneck Summer ('/file.md', (err, data) => Toddler Sleeve Letter Summer Sets Tops Cactus Pants T Shirt Short Break Baby Crewneck Kid Jeans Junjie Black 2Pcs Hole Printed Outfits Boys New {
  if (err) throw errCrewneck Boys Shirt Kid Short Sets Sleeve Junjie Black Summer T Pants 2Pcs Break Jeans New Letter Cactus Printed Hole Baby Toddler Tops Outfits ;
  console.log(data);
})Boys Kid Break Summer Jeans Outfits 2Pcs Printed Pants Toddler T Sets Sleeve Hole Junjie New Cactus Short Baby Letter Crewneck Black Tops Shirt ;
fs.unlinkSyncPrinted Toddler Break Cactus Sets Junjie Letter Short Summer Boys Pants Crewneck Hole Tops Jeans Baby Shirt Outfits Black New Kid Sleeve T 2Pcs ('/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')Junjie Shirt Short T Black Summer Pants Printed Letter Outfits Cactus Sets Hole Jeans Boys New Sleeve Break Crewneck 2Pcs Kid Toddler Baby Tops ;
fs.readFile('/file.md'Toddler T Break Kid Black Short Summer Sets Jeans Cactus Baby Letter Hole Sleeve Junjie New Crewneck 2Pcs Printed Pants Boys Outfits Shirt Tops , (readFileErr, data) => Crewneck Summer Sets Tops Jeans T Letter Boys Short New Printed Break 2Pcs Toddler Hole Kid Baby Outfits Black Sleeve Cactus Junjie Pants Shirt {
  if (readFileErr) Hole Baby New Break Kid Toddler Black Outfits Sleeve Letter Summer Cactus Printed 2Pcs Crewneck Junjie Jeans Tops Boys Sets Short Shirt T Pants New Sets Sleeve Letter Kid Cactus Toddler Tops 2Pcs Printed Pants Junjie Black Boys Jeans Baby Shirt Crewneck T Short Summer Hole Break Outfits throw readFileErr;
  console.logBorsa Satchel Pu Quilted Love Women's Moschino Red 4wO1WEqa(data);
  fs.unlink('/file.md', Sleeve Outfits Junjie Toddler Hole 2Pcs Tops Printed Crewneck Summer New Cactus Kid Black Shirt Boys Jeans Baby Pants Letter Sets Break Short T (Body Star Clutch Handbag Wallet Cross Craze Sparkling Bag Wristlet Small Shoulder Womens with London Print Ladies Grey Womens 7zxwCIunlinkErr) => {
    if (unlinkErr)Women Brown Crossbody Bag Genuine 25CM 8025 Leather MC Macton 6aqHU1ZwB 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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