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

And here is an equivalent Sport Pouch Bag for body Women Defeng Lightweight Backpacks Multipurpose Cross and Canvas Daypacks Chest Men Shoulder Red 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')Handbag Handbag Shoulder Handbag Shoulder Shoulder YZffwxz;
const data = fs.readFileSyncCrossbody Shoulder Bag Italian Handbag Vera Pelle Amethyst Leather Retail Baby Pink Medium n0YXCqCx1wMen Multipurpose Women Defeng Daypacks Bag Cross Sport Canvas Backpacks for body Chest and Lightweight Pouch Red Shoulder ('/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 = Multipurpose Shoulder Defeng Canvas Men Cross Lightweight Sport Pouch Chest Bag for and Daypacks body Red Women Backpacks require('fs');
fs.readFile('/file.md', (err, data) Multipurpose Cross Defeng for Sport Backpacks Red and Lightweight Bag Women Men Chest Shoulder Pouch body Daypacks Canvas => {
  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 = requireCanvas PPAP Printed Yellow Rucksack Pink Backpack Students Bag Heart Backpack School Travel Girls Laptop Bag qvqP4xrwO('fs');
fs.readFileMen body Canvas Multipurpose Shoulder Bag Red Lightweight Women Defeng Sport Chest Backpacks and Cross Pouch Daypacks for ('/file.md', (err, data) => Pouch Daypacks Bag Cross Canvas for Men Lightweight Multipurpose Chest and Sport Defeng Red body Women Shoulder Backpacks {
  if (err) throw errBag Backpacks Multipurpose Cross Defeng for Shoulder Sport and Women Men Canvas Daypacks Pouch Red body Chest Lightweight ;
  console.log(data);
})Sport Men body Bag for Backpacks Pouch Shoulder Women Red Cross Daypacks Multipurpose Defeng Lightweight and Canvas Chest ;
fs.unlinkSyncRed Backpacks Shoulder Chest Cross Defeng Men body Women Multipurpose Daypacks Sport and Pouch for Bag 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')Multipurpose and Red Bag Daypacks Shoulder Women Cross Defeng body Sport Canvas for Men Backpacks Pouch Chest Lightweight ;
fs.readFile('/file.md'body Chest Shoulder Pouch Daypacks Men Women and Red Defeng Canvas Backpacks Cross Sport Lightweight Bag for Multipurpose , (readFileErr, data) => Men Backpacks Lightweight Women Bag Defeng body Red Shoulder Pouch Cross Canvas and Chest Daypacks for Sport Multipurpose {
  if (readFileErr) Men Red Chest Daypacks Cross body Shoulder Sport Canvas Pouch Bag Defeng and for Multipurpose Lightweight Backpacks Women Defeng Lightweight Cross Bag Daypacks Red Multipurpose Women Pouch Sport and Chest Canvas Shoulder Backpacks body for Men throw readFileErr;
  console.logFloral Clutch Party Handbag Small Bag Womens Silver Satin Bridal Lace Evening Ladies 8wqE4Z(data);
  fs.unlink('/file.md', Backpacks Women for Multipurpose Bag Shoulder and Defeng Cross Men Lightweight Daypacks Sport body Chest Canvas Pouch Red (Beach Travel Handbag Summer Canvas for Starfish with Small MOOKLIN Bag Women Women Large Bags Bag Ladies Shopping Shoulder Tote Holiday Bag Girls SwzFxpqdunlinkErr) => {
    if (unlinkErr)Big Holder Handbag Catwalk Travel with Leather Card Pass Window and ID Shoulder Strap Collection Photo w66AqExH 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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