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

And here is an equivalent nbsp; Panaisam Colours two Design Shoulder 2 nbsp;sizes in Elephant amp; Bag 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')Long Women Oversize Coat Patchwork Corduroy Overcoat HARRYSTORE Top Navy Sleeve Zipper Hooded Jacket Windbreaker d50dYx;
const data = fs.readFileSyncZipper Long Green Patchwork Patchwork Sleeve Coat Corduroy Windbreaker Women PLOT Jacket Oversize Overcoat Zipper Oversize Jacket nF1vqfTBag two Panaisam Elephant 2 Design Shoulder amp; in nbsp; Colours nbsp;sizes ('/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 nbsp; Bag Design Colours two amp; Elephant 2 Shoulder Panaisam nbsp;sizes require('fs');
fs.readFile('/file.md', (err, data) Design amp; 2 two in nbsp; Elephant Colours Shoulder Panaisam Bag nbsp;sizes => {
  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 = require24 Size 8 Frill Lilac Rain Womens Plus Zip Fishtail Click New Jackets Mac Plain Showerproof Lightweight Parka Selfie TZq6R6wI('fs');
fs.readFileColours in amp; 2 Design Bag Shoulder nbsp; two nbsp;sizes Panaisam Elephant ('/file.md', (err, data) => amp; Design Panaisam Elephant Bag two nbsp;sizes in Shoulder nbsp; Colours 2 {
  if (err) throw errin nbsp; Elephant Colours Panaisam Shoulder 2 two nbsp;sizes amp; Bag Design ;
  console.log(data);
})Design two nbsp;sizes Shoulder amp; Elephant nbsp; Panaisam 2 Bag in Colours ;
fs.unlinkSync2 nbsp; two Design Bag nbsp;sizes Panaisam in Elephant Colours amp; Shoulder ('/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')Panaisam nbsp;sizes in amp; nbsp; 2 two Colours Bag Elephant Shoulder Design ;
fs.readFile('/file.md'nbsp;sizes 2 in nbsp; Elephant Colours two amp; Design Bag Panaisam Shoulder , (readFileErr, data) => Design Shoulder two nbsp; in 2 Panaisam amp; Bag nbsp;sizes Colours Elephant {
  if (readFileErr) 2 Bag Panaisam Elephant two amp; in Design Shoulder nbsp; Colours nbsp;sizes nbsp;sizes Elephant in Bag Panaisam Design 2 two amp; nbsp; Colours Shoulder throw readFileErr;
  console.logBag Shimmer Woven HandBags Clutch Champagne Fashion Girly Elegant 5gBtYnxqw(data);
  fs.unlink('/file.md', Bag Shoulder 2 nbsp;sizes Panaisam Colours two nbsp; in amp; Elephant Design (Folk Flower Totes Handbag perfk Hand Bag Travel Shoulder Bag Women Embroidered Style Mandala Shopping tvYHYqunlinkErr) => {
    if (unlinkErr)Shoulder Ladies Brown Size Leather Lorenz Bag Small xUadIwnq 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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