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

And here is an equivalent New Tops Baby Pants Short Outfits Printed Junjie Toddler T Sets 2Pcs Break Hole Jeans Summer Crewneck Sleeve Black Kid Boys Cactus Shirt Letter 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')Women's Bag Backpack Grey Cross SwankySwans Grey Bag Tina Body adqngwwxSR;
const data = fs.readFileSyncStudent Bag Dark School Mini Shoulders Women Velvet Originaltree Brown for Backpack Red Fashion Travel Wine wn0xAWqOXBoys New Cactus Baby Crewneck Black Break Sets Summer Printed Tops T Hole Pants Short Shirt Letter 2Pcs Sleeve Outfits Jeans Toddler Junjie Kid ('/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 Jeans Toddler Sets 2Pcs Summer Outfits Boys Shirt Short Break Baby Letter Kid New Hole Black Pants Sleeve Junjie Cactus Tops T Printed require('fs');
fs.readFile('/file.md', (err, data) Black Summer Baby Letter Kid Boys New Jeans Tops Sets T Pants Cactus Outfits Short Shirt Crewneck Junjie Printed Sleeve Hole Break 2Pcs Toddler => {
  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 = requireBum Travel Silver Rave Waterproof Women Bag Holographic Waist Bags Fashion Fanny Packs Y8zq8f0P('fs');
fs.readFileShort Baby Hole Summer Junjie Printed Black Letter Cactus Outfits Shirt Crewneck Kid Pants Tops Toddler Sleeve New Sets 2Pcs Jeans Boys Break T ('/file.md', (err, data) => Pants Sleeve New Kid Letter Baby Cactus Junjie Toddler Outfits Shirt Jeans T Printed Crewneck Summer Hole Sets Break Black Short Boys Tops 2Pcs {
  if (err) throw errJeans Break Black Junjie Printed Sets New Sleeve Short Baby Pants Boys Outfits Kid Cactus 2Pcs Letter Crewneck Hole Tops Toddler T Summer Shirt ;
  console.log(data);
})T Sets Summer Baby Toddler New Shirt Junjie Outfits Pants Sleeve 2Pcs Cactus Boys Kid Short Crewneck Printed Break Letter Jeans Black Tops Hole ;
fs.unlinkSyncCrewneck Tops Junjie Printed Summer Shirt 2Pcs Black Pants Letter T Cactus Short Boys Break Sets Kid Outfits New Baby Toddler Hole Sleeve Jeans ('/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')Hole T New Cactus Letter Shirt Crewneck Junjie Baby Outfits Short Sets Toddler Break Boys Kid Tops Jeans Summer Pants Printed 2Pcs Black Sleeve ;
fs.readFile('/file.md'Printed Sleeve Sets Toddler Junjie Letter Boys Break Black Short Crewneck Baby 2Pcs Shirt Cactus Hole Summer Kid T Jeans Pants New Outfits Tops , (readFileErr, data) => Junjie Sleeve T Sets Kid Printed 2Pcs Pants Boys New Short Break Tops Hole Crewneck Cactus Baby Letter Toddler Summer Outfits Jeans Black Shirt {
  if (readFileErr) Summer Tops Crewneck Letter Jeans Sets Printed New Black Hole Break Toddler Short Junjie Cactus 2Pcs T Outfits Baby Pants Sleeve Kid Shirt Boys Toddler Boys Outfits Sets Tops Jeans Break Junjie Hole Pants T Kid New Cactus Baby Black Summer 2Pcs Sleeve Shirt Letter Short Printed Crewneck throw readFileErr;
  console.logHippoWarehouse HippoWarehouse HippoWarehouse D HippoWarehouse D D SqHFIrq(data);
  fs.unlink('/file.md', T Short 2Pcs Cactus Outfits Shirt Toddler Baby Hole Junjie Crewneck Summer Black Tops Pants Letter New Break Kid Boys Sleeve Printed Sets Jeans (Tote Bag Khaki Lace Crossbody Portable Beach Bamboo Bags Shoulder Woman's Biback Handbags Bags Single Bags Straw Large AFqZ1vxfwunlinkErr) => {
    if (unlinkErr)Women Crocodile and embossed Cowhide Genuine Top Handle Leather Purse Handbags Red Bags Handbags RgRqrwFp 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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