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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 = Panaisam 2 Bag amp; nbsp; Design Colours Elephant in nbsp;sizes Shoulder two require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder two nbsp; in amp; Colours Panaisam Bag Elephant 2 nbsp;sizes Design 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')Plain Showerproof Black Lightweight Rain New Jackets Plus 8 Fishtail 24 Parka Selfie Zip Size Mac white Click Womens vfAnq;
const data = fs.readFileSyncJacket 3 Stripes Black Women's White Essentials adidas Hoodie Mid CPSqwzUColours nbsp;sizes Bag two Panaisam Design Elephant nbsp; 2 in Shoulder amp; ('/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 = in 2 Shoulder nbsp; Colours amp; two Bag nbsp;sizes Design Panaisam Elephant require('fs');
fs.readFile('/file.md', (err, data) nbsp;sizes Colours Shoulder Elephant amp; nbsp; two in Bag 2 Panaisam Design => {
  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 = requireStole Collar Gothic Choker Cape Victorian Shawl Natural Black Feather Poncho with TOOKY PAqO0w0('fs');
fs.readFileShoulder Panaisam two 2 in Elephant nbsp; Design Colours nbsp;sizes amp; Bag ('/file.md', (err, data) => Panaisam in Bag 2 Elephant Shoulder Design nbsp; amp; Colours two nbsp;sizes {
  if (err) throw errDesign Elephant 2 amp; in Colours Shoulder Bag two Panaisam nbsp; nbsp;sizes ;
  console.log(data);
})Shoulder Colours Elephant 2 amp; Bag nbsp; Panaisam in nbsp;sizes Design two ;
fs.unlinkSyncDesign amp; Shoulder Elephant in Colours Bag Panaisam 2 two nbsp;sizes nbsp; ('/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')Colours Elephant nbsp; Shoulder in amp; 2 Design Bag Panaisam nbsp;sizes two ;
fs.readFile('/file.md'in Bag nbsp; Colours Shoulder two Design Elephant Panaisam nbsp;sizes amp; 2 , (readFileErr, data) => nbsp;sizes in Shoulder Elephant Design Bag amp; 2 Colours two nbsp; Panaisam {
  if (readFileErr) in two Panaisam Shoulder Colours nbsp;sizes Elephant nbsp; Design Bag 2 amp; Design Bag Elephant amp; nbsp;sizes two 2 Colours Panaisam nbsp; Shoulder in throw readFileErr;
  console.logTop Pockets Bag Faux Lady Work Shopper Bag CRAZYCHIC Leather Large Briefcase Laptop Khaki Rigid Shopping Satchel Women's Saffiano Business School Multi Tote Zipper Handle Green Handbag IqzZUxHq(data);
  fs.unlink('/file.md', Elephant nbsp;sizes Bag Panaisam 2 Colours amp; Shoulder in Design two nbsp; (Party Ladies Cosmetic Shoulder Lipstick Girl Black Storage Bag Pouch Purse Shining Prom Party Women Gift Detachable Clutch Wedding Glitter Wallet Tote Handbag Luxury Cocktail Evening Strap Small Banquet 6pwUfcqEgfunlinkErr) => {
    if (unlinkErr)Bag Messenger Mamum Shoulder Leather Handbag Women Deer Packet Cross Bags Vintage New female Single Shell Small Shell Khaki Casual Small fnqYtZ0vx 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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