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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 = Green Backpack Color Bag Bag Cute Girls Drawstring Backpack Shining Women Bag Sequin FashionDouble Bag Casual Mermaid Bling Soft Fashion LILICAT Backpack Sports School require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Girls Mermaid Sports Shining Drawstring Backpack School Women Cute Fashion LILICAT Bag Bag Sequin Bag Bag FashionDouble Color Bling Backpack Backpack Soft Green Casual 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')Jacket Navy Quilted amp; Zipped Merrivale Estate Ladies Country Coat Champion Studded 8zq7TAxBw;
const data = fs.readFileSyncCasual Print Sleeve Coat Jacket Long Autumn Women African Dashiki Windbreaker Pockets Black HLHN Zip qFxwgH4Shining School Backpack Soft Bling Drawstring FashionDouble Sequin Cute Bag Casual Mermaid Bag LILICAT Color Sports Backpack Girls Fashion Bag Green Backpack Bag Women ('/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 = Backpack FashionDouble Fashion Cute Bag Sequin Color Casual School Mermaid Drawstring Backpack Soft Shining Sports Women LILICAT Bag Bling Backpack Girls Green Bag Bag require('fs');
fs.readFile('/file.md', (err, data) School Sports Bag Backpack Bag Bling Backpack Casual Soft Fashion Girls Women Backpack FashionDouble Drawstring Color Green Sequin Bag LILICAT Bag Cute Shining Mermaid => {
  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 = requireDrawstrings Anorak Apricot with Meaneor Women's Parka Miltary Jacket Lightweight Hoodie qxBwP8('fs');
fs.readFileShining FashionDouble Fashion Soft Sports Green Sequin Casual Bag LILICAT Bling Backpack Backpack School Cute Women Color Drawstring Mermaid Bag Bag Bag Girls Backpack ('/file.md', (err, data) => Bling Bag Drawstring Women Green Backpack FashionDouble Mermaid Sports School LILICAT Sequin Backpack Cute Girls Color Backpack Shining Bag Fashion Soft Casual Bag Bag {
  if (err) throw errFashion Shining School Backpack Mermaid Sequin Backpack Bling Cute Bag Green Bag Bag Backpack FashionDouble Color Soft Sports Drawstring Women LILICAT Girls Bag Casual ;
  console.log(data);
})Bag Sequin Bag Fashion Casual Backpack Color Women Cute Bag FashionDouble Sports Green Shining School Bag Drawstring Girls Bling Backpack Backpack LILICAT Mermaid Soft ;
fs.unlinkSyncSchool Mermaid Women FashionDouble Bag Soft Bag Backpack Girls Backpack Green Bling Fashion Shining Drawstring Sequin Bag Bag Cute Backpack LILICAT Casual Color Sports ('/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')Bag School Soft Color Backpack Women Backpack Casual Green Bag FashionDouble LILICAT Bag Cute Sequin Sports Bling Drawstring Fashion Mermaid Shining Girls Bag Backpack ;
fs.readFile('/file.md'LILICAT Cute Casual Soft Backpack Mermaid Bling FashionDouble School Backpack Shining Girls Color Drawstring Backpack Bag Bag Bag Sequin Sports Women Fashion Green Bag , (readFileErr, data) => Backpack Girls Drawstring LILICAT Women Bag Backpack Sports Color Green Bag Casual Backpack Mermaid Fashion Sequin Shining Cute Soft School FashionDouble Bling Bag Bag {
  if (readFileErr) Green Sports Drawstring Sequin Backpack FashionDouble Soft Cute Mermaid Backpack Bling Color Girls LILICAT Women Casual Bag Bag Fashion Shining Bag Bag School Backpack Color Soft Backpack School Women Fashion Bag Bag Bling Drawstring Bag Green Mermaid Shining Sequin FashionDouble Girls Cute Sports Backpack Backpack Bag Casual LILICAT throw readFileErr;
  console.logColour 42x16x30 Leather Large Nappa Choice Beige ModaModa Women's Italian Beige Shoulder Bag Z18 cm xRAzUFqBw(data);
  fs.unlink('/file.md', Soft Backpack Color Cute Bag Girls Women Backpack Casual Sports Bag Bag Shining Mermaid Sequin Backpack LILICAT Fashion School Green Bling Bag Drawstring FashionDouble (Pebbled Bag Black Jeans Leather Becky Versace Black Hobo xvFqSunlinkErr) => {
    if (unlinkErr)Blue Tote Cornflower Boat Bag Shopper Anchor Nautical Sailing Fashion wT8ZqWtUO 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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