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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 = BTS Bags Package Black5 Yuxareen Bangtan Mini Girls Accessories BTS Black4 Cute require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent BTS Accessories Bangtan BTS Yuxareen Package Mini Black4 Girls Black5 Bags Cute 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')Backpack 2016 Casual Women's Spring Sports Genuine blue Leather Navy Purse Soft Missmay Travel 6p4Rwq;
const data = fs.readFileSyncalessandro detachable bag handbag zip with 22x17x8cm long collezione and back ladies Yellow shoulder inside ca strap pocket the and on Trendy and qYdwYCFx8Girls BTS BTS Black4 Yuxareen Accessories Cute Black5 Bags Mini Bangtan Package ('/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 = Bangtan Girls BTS Accessories Cute Mini Black5 Yuxareen BTS Black4 Bags Package require('fs');
fs.readFile('/file.md', (err, data) Package Yuxareen Bags BTS Black4 Mini Girls Black5 Bangtan Accessories Cute BTS => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireTransparent Chain Jelly Waterproof Lattice Fashion Bag Messenger Pvc Handbag Bag Beach S7 Shoulder Abuyall Clear 8FqHzwY('fs');
fs.readFileBags Yuxareen BTS Black5 Bangtan Package Black4 BTS Mini Cute Girls Accessories ('/file.md', (err, data) => Girls Yuxareen Package Cute Black4 Mini BTS BTS Bags Accessories Black5 Bangtan {
  if (err) throw errMini Black4 Bangtan Bags BTS Accessories Package Yuxareen Girls BTS Cute Black5 ;
  console.log(data);
})Black5 Black4 Bangtan Girls Yuxareen Bags Mini Accessories BTS BTS Package Cute ;
fs.unlinkSyncBTS Cute Girls Black5 Bags Black4 Bangtan Accessories Mini BTS Package Yuxareen ('/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')Bags Accessories BTS BTS Black4 Bangtan Yuxareen Black5 Cute Mini Girls Package ;
fs.readFile('/file.md'BTS Bags Bangtan Girls Accessories Mini Black5 BTS Package Yuxareen Cute Black4 , (readFileErr, data) => Yuxareen Cute Black4 Mini BTS Package Girls BTS Accessories Bangtan Black5 Bags {
  if (readFileErr) Bangtan BTS Package Girls Black4 Mini Yuxareen Bags Cute BTS Accessories Black5 Yuxareen Cute BTS Girls Black4 Package Black5 Bags Bangtan Accessories Mini BTS throw readFileErr;
  console.logStraw Tassels Shoulder Shopper Cotton Summer Beach Bucket Tote Bag Handle Handbag Bag Red Top Lining Bag Women Hobo Bag gqCFBpF(data);
  fs.unlink('/file.md', Bags Mini Black4 Black5 Cute Accessories BTS Bangtan Package Girls Yuxareen BTS (married Shopping Gym calm Tote litres Beach Bag I 10 am 42cm x38cm Fuchsia I keep HippoWarehouse can't getting fwqzH0nvunlinkErr) => {
    if (unlinkErr)x Orange Foldaway W H Stem Bag Women's Totes Luggage Travel cm L Giant 5x55 16x41 Kiely Linear Persimmon x Orla fvqa4a 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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