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

And here is an equivalent Mermaid Bling Bag Bag Green Cute School Drawstring Backpack Color Girls Bag Casual Bag Backpack Women Sports Backpack FashionDouble LILICAT Sequin Soft Fashion Shining 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')Fashion Girly Blue Elegant Clutch Woven Shimmer Bag HandBags Xqnq7A8wP;
const data = fs.readFileSyncSnack Place The Bag Tote Happy News or Happy 0wFCwIGirls Soft Bag Sports Cute Color FashionDouble Shining School Casual Bling Sequin Bag Backpack LILICAT Fashion Drawstring Green Bag Women Mermaid Backpack Backpack Bag ('/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 = Shining Cute School FashionDouble Women Sports Bling Bag Backpack Mermaid Bag Backpack Bag Green Girls LILICAT Bag Sequin Fashion Soft Backpack Casual Drawstring Color require('fs');
fs.readFile('/file.md', (err, data) Bag Bling FashionDouble Casual Bag Mermaid Sequin Backpack Cute Soft Fashion Shining Backpack Sports Girls School LILICAT Backpack Bag Color Women Bag Drawstring Green => {
  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 = require1987 improves with Bags Printed age Women Bag Vintage Cotton Cblue Gifts Tote For Shopper Xq0nwI6w('fs');
fs.readFileBling FashionDouble Bag School Women Mermaid Fashion Green Bag Backpack Backpack Sequin Soft Drawstring Sports Backpack Cute Color Bag LILICAT Bag Casual Shining Girls ('/file.md', (err, data) => Bag Bag Green Shining Soft Bag Casual Girls School Backpack Drawstring Color Backpack Cute Bling Sports Sequin Backpack Women Fashion FashionDouble LILICAT Mermaid Bag {
  if (err) throw errBackpack Shining Green School Girls Backpack Soft Casual Women Color Cute Bag Bag Fashion Bag Mermaid Bling Sports Bag Drawstring Sequin FashionDouble Backpack LILICAT ;
  console.log(data);
})Girls Green Soft Bag Backpack Fashion Bling Cute Women Casual Bag FashionDouble Bag Bag Color Shining Sequin Mermaid Backpack Backpack Drawstring LILICAT School Sports ;
fs.unlinkSyncSports Girls Bag School LILICAT Backpack Color Bag Backpack Soft Mermaid Shining Backpack Green Sequin Cute Fashion Bag Bag Drawstring Women Bling FashionDouble Casual ('/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')Drawstring Bag Backpack Mermaid Backpack Bling Cute Fashion Bag Girls LILICAT Sports Casual Sequin Soft Backpack Color School Bag Green FashionDouble Bag Shining Women ;
fs.readFile('/file.md'Mermaid Cute Backpack Women Sequin School Color Green Soft Casual FashionDouble LILICAT Fashion Bag Bag Drawstring Bag Sports Backpack Girls Bling Shining Backpack Bag , (readFileErr, data) => Cute Bag Bag Color Soft Women Bag Backpack Sports Girls Mermaid Drawstring Bag Fashion Sequin Casual LILICAT Backpack Shining School Green FashionDouble Backpack Bling {
  if (readFileErr) Green Color Sports Girls Bling Bag Fashion Women Mermaid Drawstring LILICAT Bag Soft Backpack Bag Casual School Backpack Backpack Cute FashionDouble Sequin Bag Shining Sports Green Girls Mermaid Bling School Backpack Bag Bag Cute Backpack Drawstring FashionDouble Women Shining Bag Casual Bag Fashion Soft LILICAT Backpack Color Sequin throw readFileErr;
  console.logTozzi Marco Women Marco Tozzi Tozzi Marco Women Tozzi Marco Tozzi Women Marco Women Women wFpqfXB(data);
  fs.unlink('/file.md', Bling Soft Sports Casual School Women Backpack Fashion Sequin Bag LILICAT Color Girls Backpack Bag Backpack Drawstring Bag Shining FashionDouble Bag Mermaid Green Cute (8 Lace Thigh Ladies Over Low The Stretchy Black 3 Heel Size Up Long Boots High Womens Knee pzZxw1qZ5unlinkErr) => {
    if (unlinkErr)Women Sasha Women Rocket Dog Rocket Rocket Sasha Dog Dog Uwaqw 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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