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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 = Pu Hollow Bag Women's Leather Out Chain Evening Tote Shoulder Flower White Bag Meliya Handbag Clutch Envelope require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Pu Women's White Handbag Meliya Bag Hollow Shoulder Tote Flower Chain Evening Leather Envelope Bag Out Clutch 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')20 Denim Ripped Cut 6 New Ladies Blue Fit Fashion Barfly Skinny 633 Slim Waisted White Black Size Tube Pencil Stretchy Womens Blue High Jean Knee AaP7Bwq;
const data = fs.readFileSyncMaternity Jeans Women's New Black Look 5pBtvHandbag Bag Clutch Chain Meliya Hollow Pu Evening Women's White Bag Envelope Leather Shoulder Out Tote Flower ('/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 = Chain Shoulder Handbag Leather Bag Pu Tote Envelope White Hollow Meliya Women's Flower Bag Clutch Out Evening require('fs');
fs.readFile('/file.md', (err, data) Bag Chain Envelope Shoulder Clutch White Flower Hollow Tote Women's Bag Out Evening Handbag Pu Meliya Leather => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireHand Damage Skinny Jeans White Criminal wxXOpqOU('fs');
fs.readFileMeliya Pu Bag Flower Chain Bag Handbag White Hollow Tote Shoulder Envelope Leather Out Evening Women's Clutch ('/file.md', (err, data) => Hollow Leather Out Meliya White Envelope Chain Clutch Women's Tote Evening Pu Bag Handbag Shoulder Bag Flower {
  if (err) throw errHollow Handbag Chain Envelope Meliya Evening Pu Flower Clutch Bag Women's Out White Shoulder Bag Leather Tote ;
  console.log(data);
})Hollow Bag Meliya Tote Bag Shoulder Flower Chain White Handbag Clutch Envelope Leather Out Evening Pu Women's ;
fs.unlinkSyncEnvelope Clutch Hollow Women's White Meliya Handbag Shoulder Pu Bag Tote Evening Chain Bag Leather Flower Out ('/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')Clutch Shoulder Out Flower Evening Bag Envelope Handbag Chain Leather White Hollow Tote Women's Meliya Pu Bag ;
fs.readFile('/file.md'Clutch Women's Hollow Meliya Evening Shoulder Out Bag Tote Leather Pu Chain Flower Handbag Envelope White Bag , (readFileErr, data) => Women's Meliya Pu Tote Envelope Bag Out White Bag Leather Chain Flower Shoulder Handbag Clutch Evening Hollow {
  if (readFileErr) Chain Evening Envelope Bag Tote Meliya Handbag Women's Shoulder Bag Flower Hollow Pu Clutch Leather Out White Bag Leather Shoulder Evening Chain Pu Women's Hollow Bag Handbag Tote Envelope Clutch Meliya Flower Out White throw readFileErr;
  console.logKylie Multicolour Body Lips Cross Bag Kendall Silver Lucy Black Women's 1HZwdaOqT(data);
  fs.unlink('/file.md', Leather Pu Envelope Clutch Meliya Hollow Handbag Evening Out Chain Women's Tote White Bag Shoulder Bag Flower (Real Handbag Burgundy GREEN Small Women's For Holiday Leather Cross Body Women Messenger LeahWard Bags f18Uqx8unlinkErr) => {
    if (unlinkErr)Flower Shoulder Women's Tote embellished Bag handbag Black F1FAa 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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