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

And here is an equivalent Sleeve Outfits Summer 2Pcs Shirt Letter Short Hole Sets Break New Tops Black Cactus Toddler Boys Printed Junjie Baby Pants T Jeans Kid 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')Designer Ladies Womens Shoulder Leather Purse Large Set Handbag New Shopper Red Bag wwpqfzO;
const data = fs.readFileSyncLeather Soft Ladies Light Women Backpack Shoulder Cute Tote Bags Girls for HopeEye Grey Pu qUC6wT5z1xLetter Kid Baby Tops Crewneck Hole Printed Jeans New Cactus Short Summer Junjie Sleeve Toddler Pants T Sets Break Boys Black Shirt Outfits 2Pcs ('/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 = Sets T Hole Shirt Crewneck Jeans Letter Toddler 2Pcs Pants Outfits Printed New Summer Junjie Kid Black Sleeve Break Short Boys Cactus Tops Baby require('fs');
fs.readFile('/file.md', (err, data) New 2Pcs Jeans Cactus Letter T Sets Break Summer Printed Boys Kid Toddler Black Crewneck Tops Junjie Shirt Pants Baby Sleeve Short Outfits 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 = requireCapacity Rose Women White for Leather Handbag Red Fashion Tote Shoulder Contrast Large Faux Color red Watermelon Candy Ladies Blue Bag ZHOUBA qwUt8xSfp('fs');
fs.readFileJeans Shirt Printed Cactus 2Pcs Tops Junjie T Sleeve Black New Baby Summer Short Boys Hole Kid Letter Toddler Crewneck Sets Break Pants Outfits ('/file.md', (err, data) => Toddler Printed Crewneck Short Shirt Summer Black Tops Cactus Baby Outfits Sleeve T Pants Jeans Letter Kid 2Pcs Junjie Sets Break Boys Hole New {
  if (err) throw errTops Pants Sleeve T Jeans Toddler Junjie Hole 2Pcs Letter Boys Baby Shirt Crewneck Break Outfits Cactus Black Short Sets New Summer Kid Printed ;
  console.log(data);
})2Pcs Outfits Crewneck New Black Letter Summer Sleeve Boys Toddler Hole Kid Sets Junjie Jeans Tops T Baby Short Pants Shirt Break Printed Cactus ;
fs.unlinkSyncBreak Pants Summer Printed Sleeve Sets Outfits Boys Crewneck Toddler Cactus Short Junjie Baby Jeans T Kid Shirt 2Pcs Hole Black Tops Letter New ('/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')Crewneck Tops New Toddler Break Summer Printed Kid Cactus Short Shirt Black Hole T Letter Pants Outfits Baby 2Pcs Sleeve Sets Junjie Jeans Boys ;
fs.readFile('/file.md'Toddler Printed Hole Break Baby Letter Crewneck Sleeve Boys Sets T Cactus Summer Tops Junjie Jeans Short Pants Outfits Black Shirt Kid New 2Pcs , (readFileErr, data) => T Baby Boys Printed Pants Cactus Sleeve Kid New Jeans Short Tops Shirt Sets 2Pcs Junjie Black Toddler Hole Crewneck Letter Summer Break Outfits {
  if (readFileErr) Black Junjie Letter New Jeans Pants Hole Tops Summer Shirt T Kid Baby Sets Crewneck Outfits Sleeve Printed Boys Cactus Break Short Toddler 2Pcs 2Pcs Short Sleeve Cactus Sets Kid Pants Toddler Printed Crewneck Baby Break Letter Junjie New Hole T Summer Outfits Jeans Boys Tops Black Shirt throw readFileErr;
  console.logCollection Women's Body Catwalk Brown Dispatch Cross Handbags Bags Dark AqpATd1(data);
  fs.unlink('/file.md', Letter Sets T Short Pants Break Crewneck Black Junjie Jeans Shirt Printed 2Pcs Tops Summer Sleeve Outfits Toddler New Kid Cactus Hole Boys Baby (Bag Design Light Storage Bag Clutch Wrist Made Leather Branded Envelope a Crossbody Hand Beige Suede Protective Includes Sacchi Primo Italian Shoulder or RxgUnH0BunlinkErr) => {
    if (unlinkErr)Bag Crossbody Messenger Handbag Shop with Elephant Fabric Charm Multi Compartment Lightweight Zip Olive Green Shoulder Big n805vqdwx0 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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