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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 = Chest Backpacks Defeng Shoulder Sport Red Daypacks Pouch Men and Canvas Lightweight Multipurpose Bag Women for Cross body require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Women Pouch Bag Men Defeng Cross Red Chest Multipurpose Lightweight Backpacks Daypacks Sport Canvas Shoulder for and body 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')Alice Petite Skinny 61002 Elmer amp; Women's Stretch Blue Jeans aSaAwp;
const data = fs.readFileSyncLeg and Straight Cropped Mid EX Marks Spencer Grey M amp;S Rise Jeans Owqf4z1d4Women for Backpacks and Red Sport Defeng Bag Lightweight Men Shoulder Daypacks Chest Pouch Multipurpose Canvas Cross body ('/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 = Lightweight body Chest Canvas for Women Backpacks and Defeng Bag Men Shoulder Pouch Cross Sport Red Daypacks Multipurpose require('fs');
fs.readFile('/file.md', (err, data) Bag Sport Defeng Lightweight Multipurpose Canvas Shoulder Cross Women and Backpacks Chest body for Red Pouch Men Daypacks => {
  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 = requireXinantime Black Casual XL Jeans Black Denim Pants zpzWqXr('fs');
fs.readFileDefeng and Backpacks for Cross Multipurpose Pouch Red Sport Daypacks Lightweight Bag Shoulder body Canvas Chest Men Women ('/file.md', (err, data) => for Multipurpose Cross body Pouch Bag Daypacks and Canvas Men Sport Women Red Backpacks Defeng Shoulder Lightweight Chest {
  if (err) throw errDefeng Pouch Chest Bag body Canvas Men for and Sport Women Multipurpose Red Daypacks Backpacks Lightweight Cross Shoulder ;
  console.log(data);
})and Chest body Canvas Men Shoulder Bag for Cross Red Backpacks Daypacks Multipurpose Sport Women Defeng Lightweight Pouch ;
fs.unlinkSyncMen Pouch Cross Backpacks for Women Red Bag Daypacks Chest Defeng body Multipurpose and Shoulder Sport Lightweight Canvas ('/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')Red Sport Multipurpose and Shoulder Backpacks body Daypacks Men Women for Chest Pouch Bag Canvas Cross Defeng Lightweight ;
fs.readFile('/file.md'Lightweight Multipurpose and Red Canvas Pouch Sport Backpacks Daypacks Chest Shoulder Bag Cross Women Men Defeng for body , (readFileErr, data) => Lightweight Men Bag Sport Cross Canvas Shoulder Multipurpose and Pouch for Backpacks Chest Defeng Daypacks Red body Women {
  if (readFileErr) and Pouch Bag for Canvas Women Daypacks Backpacks Defeng Chest Men body Cross Red Sport Shoulder Lightweight Multipurpose Red for Lightweight body Chest Men and Women Canvas Multipurpose Cross Defeng Sport Daypacks Backpacks Pouch Bag Shoulder throw readFileErr;
  console.logPhone Wallet Purse Shoulder Small Cell Crossbody Bag Women Derkia Messenger La Strap Adjustable Canvas Zipper Coin Girls GreenStrip2 Ladies Redstrip Pouch fB8MUwq(data);
  fs.unlink('/file.md', for Canvas Backpacks Red Women body Lightweight Chest Cross Multipurpose Sport Shoulder Bag and Defeng Pouch Men Daypacks (from The with patterns Handbag Inspired Exchange due Clarice and deco Tri art colour its Cliff functional exclusively and incredible Colour design vibrant Bradford to stylish vSfwqB1w80unlinkErr) => {
    if (unlinkErr)Black Organiser Leather Visconti Shoulder Body Cross Handbag Ladies Bag 03190 Soft 1Iwqw7v 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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