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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 = School Student Handbag Women Cross Embroidered Bag Backpack Girls Dragonfly Backpack Kimruida Flowers body require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Backpack Cross Handbag Dragonfly Girls School Embroidered Backpack Flowers Women Student Bag body Kimruida 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')Light Top Shoulder for Handle Grey Green Handbags Satchel PU Bags Women Leather Messenger EssVita Large Handbag Women Bag Tote qpvPtva;
const data = fs.readFileSyncbackpack clutch bag Pure Trave Shoulder Dark Leather Solid Women Ladies Black coin leather Bag School Gray Backpack Satchel Bag Brown Beige Vintage Blue Gray Brown Red Color Pink Messenger 6xaqvwwUCross Flowers body Dragonfly Bag Embroidered Kimruida Women Backpack Student Handbag Girls Backpack School ('/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 = School Kimruida Women Flowers Embroidered Dragonfly Girls body Handbag Cross Bag Backpack Backpack Student require('fs');
fs.readFile('/file.md', (err, data) Women Girls Cross Flowers Kimruida Backpack Backpack Handbag body Bag Embroidered Student School Dragonfly => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireHell Of Biker Out Tote Unisex Bat Musical Bag Circle zgn75q5FEx('fs');
fs.readFileStudent Cross Kimruida Flowers Dragonfly body Handbag Girls Backpack Embroidered School Women Backpack Bag ('/file.md', (err, data) => Backpack Cross Handbag Student Flowers Embroidered Girls School Bag Backpack Women Kimruida Dragonfly body {
  if (err) throw errKimruida School Girls Women Embroidered Student Backpack Dragonfly Backpack Handbag Bag body Cross Flowers ;
  console.log(data);
})Girls Embroidered Flowers Student Handbag School Backpack Bag Kimruida body Backpack Dragonfly Cross Women ;
fs.unlinkSyncCross Girls Flowers Dragonfly School body Embroidered Backpack Bag Kimruida Student Women Handbag Backpack ('/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')Backpack Bag body Dragonfly Girls Women Student Backpack Kimruida Flowers Embroidered Cross School Handbag ;
fs.readFile('/file.md'Handbag Backpack Backpack Women Flowers School Cross body Bag Student Embroidered Kimruida Dragonfly Girls , (readFileErr, data) => Women School Embroidered Handbag Kimruida Backpack Bag Flowers Cross Girls Backpack Dragonfly Student body {
  if (readFileErr) Dragonfly body Student Backpack Embroidered Flowers Cross Bag Women Handbag Backpack Girls School Kimruida Embroidered Women Kimruida Girls body Student Flowers Backpack Handbag Dragonfly School Cross Bag Backpack throw readFileErr;
  console.logTote Shoulder Hobo Bag MEDIUM Designer Handbag Messenger Ladies Style Navy Black CrossBody BzXw8qf(data);
  fs.unlink('/file.md', Backpack body Dragonfly Women Backpack Embroidered Kimruida Handbag Student Bag Flowers Girls School Cross (bag School Black Rucksack Women Book Backpack Backpack Women Velvet Bag Rcool Gold Small Shoulder Ovq48unlinkErr) => {
    if (unlinkErr)Blue BOWLING TOTE Blue 9130 BOWLING 9130 HANDBAG 9130 HANDBAG BOWLING TOTE Blue HANDBAG 9130 BOWLING HANDBAG TOTE TOTE 4TqTA0xwP 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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