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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's Bag Crocodile Decor Handbag Kahaki Navy Skin Tote Leather Vintage Patent Metal Hattie require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Kahaki Hattie Crocodile Patent Decor Skin Bag Women's Metal Vintage Navy Tote Leather Handbag 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')Body Womens Suitable Genuine Tablet Brunhide for 7" Black Brown Leather 300 Bag Shoulder Handbag Cross 112 WAqXdXP;
const data = fs.readFileSyncnbsp;Pearl nbsp;Purses nbsp;Beaded nbsp;Flower nbsp;for nbsp;Bridal Champagne nbsp;Clutches nbsp;Bag Bagood nbsp;Shoulder nbsp;Women's nbsp;Prom nbsp;Bags nbsp;Wedding nbsp;Handbag nbsp;Evening wgII8CNavy Women's Skin Metal Vintage Handbag Crocodile Kahaki Tote Hattie Bag Leather Patent Decor ('/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 = Bag Decor Skin Women's Leather Crocodile Vintage Patent Navy Hattie Kahaki Tote Metal Handbag require('fs');
fs.readFile('/file.md', (err, data) Navy Tote Leather Metal Patent Vintage Women's Skin Kahaki Decor Bag Hattie Handbag Crocodile => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireBag Skull Leather Cell Chain Mobile Cross Message Pouch Vintage Small body Women Black Purses FAIRYSAN tcaUqzB('fs');
fs.readFileHandbag Vintage Crocodile Women's Leather Skin Hattie Patent Navy Bag Metal Kahaki Tote Decor ('/file.md', (err, data) => Tote Women's Leather Kahaki Handbag Bag Vintage Patent Navy Decor Skin Hattie Crocodile Metal {
  if (err) throw errDecor Crocodile Handbag Women's Tote Kahaki Navy Hattie Leather Bag Patent Vintage Skin Metal ;
  console.log(data);
})Metal Hattie Kahaki Crocodile Leather Navy Skin Tote Decor Bag Vintage Women's Patent Handbag ;
fs.unlinkSyncHattie Women's Vintage Handbag Metal Bag Navy Crocodile Skin Kahaki Tote Patent Decor Leather ('/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')Women's Metal Vintage Decor Handbag Hattie Patent Kahaki Crocodile Navy Leather Skin Bag Tote ;
fs.readFile('/file.md'Skin Tote Handbag Bag Women's Decor Hattie Patent Crocodile Metal Vintage Kahaki Leather Navy , (readFileErr, data) => Handbag Hattie Decor Metal Vintage Crocodile Leather Women's Patent Bag Kahaki Tote Skin Navy {
  if (readFileErr) Hattie Vintage Tote Bag Patent Leather Navy Skin Kahaki Crocodile Women's Decor Metal Handbag Tote Leather Patent Bag Vintage Navy Crocodile Skin Handbag Metal Kahaki Decor Hattie Women's throw readFileErr;
  console.logBoot Supermod Women Tm Leather Ii Fitflop Ankle xvagqz(data);
  fs.unlink('/file.md', Patent Women's Crocodile Skin Metal Navy Handbag Vintage Hattie Tote Bag Leather Decor Kahaki (Women camel active camel active camel Women camel Women active Women active 7qwCOaFFunlinkErr) => {
    if (unlinkErr)Ankle Heeled Block Women's Krush UK 7 Black Boots wfqOA 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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