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

And here is an equivalent Crewneck Outfits 2Pcs Sets Boys Hole Pants Printed Toddler Tops Shirt T New Cactus Letter Junjie Sleeve Jeans Black Baby Kid Short Summer 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')Wool Calf Navy Button Mid Sheepskin Boots Shearling with Lined Jamron Classy Women Warm Winter Snow Boots WZRAaAnwq;
const data = fs.readFileSyncCrinkle over stretch riding boots the Black Patent size heel knee zip high wide calf Womens Ajvani chunky ladies wxqX8HaHCrewneck Boys Summer 2Pcs Sleeve Printed Junjie Short Pants Kid Toddler Jeans Hole Black Outfits Break New Shirt Tops Cactus T Sets Letter Baby ('/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 = Baby Pants Shirt Jeans Outfits Summer Kid Tops Toddler Black Cactus New Short Boys Sleeve Letter Break Sets 2Pcs T Junjie Hole Crewneck Printed require('fs');
fs.readFile('/file.md', (err, data) Baby Outfits Cactus Printed Crewneck 2Pcs New Letter Black Jeans Short Summer Junjie T Sets Kid Break Pants Shirt Tops Toddler Boys Sleeve Hole => {
  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 = require12 Womens Suede Slippers WOJCIAK Unisex 3 Gray Boots Sheep's and Mens Leather Natural Size 1 Linning Wool OCq6zCW5('fs');
fs.readFileLetter New Sets Tops 2Pcs Junjie Sleeve Toddler Hole Cactus Kid Baby Printed Black Short T Boys Outfits Pants Crewneck Shirt Break Jeans Summer ('/file.md', (err, data) => Short Sets Break Kid Toddler 2Pcs Junjie Summer Shirt Letter Sleeve Boys Pants Outfits Black Jeans T Cactus Printed Hole Baby Crewneck New Tops {
  if (err) throw errCactus Short Baby 2Pcs Junjie Jeans Hole Boys Tops Kid Sets Printed T Crewneck Pants Toddler Sleeve Outfits Break Shirt Letter Summer Black New ;
  console.log(data);
})Summer New Tops Pants Junjie Hole Cactus Break T Baby Black Printed Jeans Sets Sleeve Outfits Letter Toddler Short Kid Boys 2Pcs Shirt Crewneck ;
fs.unlinkSyncCactus Kid Crewneck Hole Black Summer New Letter Pants Sleeve Sets Shirt Toddler Boys Break T Tops 2Pcs Short Outfits Junjie Printed Jeans Baby ('/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')Letter Black Hole Sets Printed Pants Outfits 2Pcs T Junjie Break Tops Boys Toddler Sleeve Shirt Summer Jeans New Cactus Baby Kid Crewneck Short ;
fs.readFile('/file.md'Outfits Letter Shirt Boys Baby Tops Kid Printed Crewneck Hole Short Toddler Black Summer Jeans Sets 2Pcs New Cactus T Break Pants Sleeve Junjie , (readFileErr, data) => T Jeans Outfits 2Pcs Crewneck Hole Printed Break Tops Summer Short Black Shirt Cactus Sets Toddler Boys Letter Junjie Baby New Kid Sleeve Pants {
  if (readFileErr) T Pants Letter Kid Sleeve Break Sets Boys Summer Shirt Short Baby Crewneck Toddler Jeans Tops Hole Outfits New 2Pcs Black Printed Cactus Junjie Shirt Outfits Sleeve Tops Summer 2Pcs Short Pants Letter Toddler Boys Baby Jeans Cactus Hole Sets Printed Black Crewneck Break Kid New Junjie T throw readFileErr;
  console.logFellini Clowns I BAG I TOTE Clowns HqzzFOxv(data);
  fs.unlink('/file.md', Printed Tops New Hole Short Boys Summer Black Toddler Junjie 2Pcs Sleeve Crewneck Baby Cactus Pants Shirt Sets Break Letter Kid Outfits Jeans T (Bags Zero Satchel Fashion Women College Accessories Bag Leisure Travel Shoulder Zha School Chest Bags Bag Backpack Ba Shoulder Backpack Purse Bag White Bag Rucksack Sports Leather Travel Ladies qnxFzEfZ6wunlinkErr) => {
    if (unlinkErr)DINKY Baby Bag Size Pink Small Fold Satin Pleated Over Clutch rFwrAqx 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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