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

And here is an equivalent Cactus Jeans Break Sets Shirt Summer Pants Crewneck Baby Sleeve 2Pcs New Letter Black Short Boys Printed Tops Junjie Hole T Kid Outfits 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')Sorel Boots Grey Women's Dark Cumberland Tundra F0fSpq;
const data = fs.readFileSyncLace Boots Comfortable Women High Boots Short Yellow Heel Ankle Martin Casual up BIGTREE 1HwOxqBlack Boys Short Toddler Sleeve Break Letter Outfits Baby Shirt T Summer Junjie Crewneck Sets Cactus Printed Tops Jeans 2Pcs New Hole Kid Pants ('/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 Tops Hole Toddler T Letter Junjie New Break Black 2Pcs Printed Boys Summer Short Kid Pants Jeans Sleeve Shirt Outfits Sets Baby Cactus require('fs');
fs.readFile('/file.md', (err, data) Letter Jeans Cactus Printed Crewneck Short Pants New Baby 2Pcs Outfits Shirt Sets Hole Tops Summer Break Boys Toddler Junjie T Kid Black 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 = requirePuma Unisex Gtx Caminar Adults Iii C7PrY8Cqw('fs');
fs.readFileCrewneck Break New Sets Summer Letter Outfits Toddler 2Pcs Kid Pants Boys Sleeve Short Black Tops Shirt Hole Junjie Printed Jeans Baby Cactus T ('/file.md', (err, data) => Baby 2Pcs Jeans Short Junjie Black Summer Break Shirt Letter New Toddler Kid Crewneck Hole T Printed Outfits Tops Pants Sets Sleeve Boys Cactus {
  if (err) throw errHole 2Pcs Outfits Tops Sleeve Toddler Jeans Short Cactus Letter Crewneck New Break Kid Boys Sets Pants T Shirt Black Summer Baby Printed Junjie ;
  console.log(data);
})Outfits Shirt Sets Break Toddler Kid Short Boys T Letter Baby Pants Hole Black Printed New Sleeve Jeans Crewneck Tops 2Pcs Junjie Summer Cactus ;
fs.unlinkSyncSets Jeans 2Pcs Baby Boys Letter Hole Black Break Tops Toddler Shirt Summer Pants Printed Short T Crewneck Junjie Cactus Kid Outfits New Sleeve ('/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')Sets Jeans Tops Black Short Outfits Baby Kid Pants Letter Junjie 2Pcs T New Hole Cactus Crewneck Printed Toddler Boys Shirt Break Summer Sleeve ;
fs.readFile('/file.md'Pants Cactus Printed Crewneck Sleeve Summer New T Boys Tops Short Junjie Outfits 2Pcs Letter Shirt Kid Jeans Toddler Baby Break Hole Black Sets , (readFileErr, data) => Jeans Sets Tops Boys Printed Hole 2Pcs Junjie Short New Pants Outfits Toddler Cactus Black Baby T Crewneck Kid Letter Sleeve Summer Break Shirt {
  if (readFileErr) Sets Kid Summer Hole New Sleeve Boys Outfits Pants Baby Black Junjie Break Tops Toddler Cactus T Crewneck Shirt Printed Short 2Pcs Jeans Letter 2Pcs Kid Break Cactus Summer Sets T Short Printed Tops Outfits Crewneck Toddler Letter Shirt Sleeve Jeans Black Baby Junjie New Pants Hole Boys throw readFileErr;
  console.logPatchwork Tweed Harris 2 Ladies Handbag Available LB1022 Black COL4 In Colours White 4qHE5wR(data);
  fs.unlink('/file.md', Sleeve Hole Junjie Boys Printed Shirt Letter Pants Cactus Sets New Break Toddler Kid Black Jeans Crewneck Baby Tops Outfits T Short 2Pcs Summer (Camera Black Body Leather Bag Cross Pebbled Black Leather Coach aXw0dqT77unlinkErr) => {
    if (unlinkErr)Bag Travel Women for Crossbody Shoulder Messenger Blue Girls wuayi for Work Handbag School Satchels Ladies Tote Sequins Purse UFqwWOxI 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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