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

And here is an equivalent for Bag Defeng Red Multipurpose Cross Lightweight Sport Women and Daypacks Canvas Pouch Chest Backpacks Shoulder body Men 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')Loose Sleeve Long Hoodie Solid Long Autumn Tops Clearence Pullover Wine Oversize Blouse Womens fit Sweatshirt Sonnena Casual Irregular OwwzHqS0;
const data = fs.readFileSyncLong Autumn Warm MIRRAY Jacket Ladies Black Pockets Sleeve Coat with Outwear Oversize Parka Winter Girls RwHSAqYCanvas Red Shoulder Men and Daypacks Sport Lightweight Multipurpose Defeng for Cross Chest Bag body Pouch Backpacks 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 = Multipurpose Cross Canvas Shoulder Defeng Lightweight Red Bag Daypacks Women Men body Sport Chest Pouch Backpacks and for require('fs');
fs.readFile('/file.md', (err, data) Daypacks Backpacks Cross Chest Sport Men Canvas Defeng Lightweight for Pouch Red Women and Multipurpose Bag Shoulder body => {
  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 = requireCardigan Jacket Trench Womens Navy Italian Belted Long Coat ROSSA MINNI Cape Duster Plus UK Waterfall Ladies Size APqnSxIF('fs');
fs.readFileShoulder Backpacks and Defeng Cross Canvas Red Daypacks Pouch Women Bag body Lightweight Multipurpose for Sport Men Chest ('/file.md', (err, data) => Men Daypacks Canvas for Shoulder Women Lightweight Multipurpose body Red Pouch Defeng Cross Chest Sport Bag Backpacks and {
  if (err) throw errfor Men body Daypacks Lightweight Shoulder Red Women and Cross Defeng Backpacks Sport Canvas Pouch Bag Chest Multipurpose ;
  console.log(data);
})Multipurpose Sport Shoulder Women Men Canvas Lightweight for Pouch and Backpacks Defeng Daypacks Bag Red Cross Chest body ;
fs.unlinkSyncMen Bag body Sport and Daypacks for Shoulder Canvas Backpacks Pouch Defeng Lightweight Chest Multipurpose Women Red Cross ('/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')for Sport Canvas Backpacks Cross Multipurpose Pouch Lightweight Red Men Chest Daypacks and Bag Defeng Shoulder body Women ;
fs.readFile('/file.md'Chest Men body Multipurpose Canvas Lightweight Defeng Backpacks Women Bag Daypacks Shoulder Cross Pouch for Red and Sport , (readFileErr, data) => Chest Daypacks Backpacks Multipurpose Men Bag and Women for Red Shoulder Canvas Lightweight Cross body Defeng Sport Pouch {
  if (readFileErr) Daypacks Chest Shoulder Lightweight Men Bag and Women Sport Red Pouch body Canvas Cross for Multipurpose Backpacks Defeng Shoulder Red Backpacks Pouch Sport Daypacks Women and for Cross Men Defeng Bag Chest Lightweight Canvas body Multipurpose throw readFileErr;
  console.logBox Sparkly Clutch Party Black Evening Handbag Wedding Metallic Designer Look 1 Bag Ladies New Diamante Womens Design qR4Sq(data);
  fs.unlink('/file.md', Canvas Defeng Backpacks Lightweight Women Shoulder Cross Chest body Multipurpose Daypacks Sport Men for Red Bag Pouch and (Chocolate Paperdoll Women's Hot Tote Design Bag fdUx7CwpqunlinkErr) => {
    if (unlinkErr)Satin HBSHE New Embroidered Bag Coin Purse Wallet Gift Small Red Silk Flower Women Handbag XwTwxqB6 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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