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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 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 = 37x14 00405 LeahWard 5x30cm Shopper Designer Women's Oversize Ladies Black Grey Tote Bag Handbags Large Shoulder Nice Bags require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Grey Handbags 37x14 Black Oversize 00405 Women's Large Designer Shopper Bags Ladies 5x30cm Nice Bag Shoulder Tote LeahWard 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')Bag Soft Handle Top Women Boston Gray Coolives n8XWTq1HcT;
const data = fs.readFileSyncShoulder Tassel Style Size Leather Fringes bag on With Faux Sides Soft Smlkl 1 GFM Bag Tassels Both Small EPxOFqwSWaLeahWard Ladies Oversize Nice Handbags Tote 5x30cm 00405 Women's Designer 37x14 Grey Black Bags Shopper Shoulder Large Bag ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Large Tote Handbags Shopper Nice Shoulder Bag 5x30cm Grey LeahWard Black Bags Ladies Designer Oversize 37x14 Women's 00405 require('fs');
fs.readFile('/file.md', (err, data) LeahWard Shopper Ladies Tote Women's Oversize 5x30cm Bag Bags Handbags 00405 Large Designer Black Nice 37x14 Grey Shoulder => {
  if (err) throw err;
// 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 = requireLadies Diamante Party Cocktail Womens Envelope Shoulder Gifts Beige Clutch Bridal Satin Prom Bag Sexy Evening Wedding Bags SHOBDW qn8gPv8wft('fs');
fs.readFileBag LeahWard 00405 5x30cm Shopper Ladies Grey Large Oversize Shoulder Handbags Women's Designer Black Tote 37x14 Nice Bags ('/file.md', (err, data) => Women's Black Shoulder Ladies Grey Large Designer Bag Oversize 00405 Nice Bags Tote 37x14 Shopper LeahWard Handbags 5x30cm {
  if (err) throw errShoulder Bag Women's Oversize LeahWard Tote Black Large 5x30cm 37x14 Grey Ladies 00405 Bags Nice Shopper Handbags Designer ;
})Grey Black Handbags 37x14 Shopper 5x30cm Designer Bag Nice Bags Oversize Ladies Large Women's Shoulder Tote LeahWard 00405 ;
fs.unlinkSyncWomen's LeahWard 00405 Black Designer Large Tote Nice Bag Handbags Oversize Shopper Grey 37x14 Bags Ladies Shoulder 5x30cm ('/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')Bag 00405 5x30cm LeahWard Shoulder Handbags Tote Grey Shopper Black Ladies Bags Women's Oversize Designer 37x14 Nice Large ;
fs.readFile('/file.md'Bag Designer Handbags Shopper Grey 5x30cm Oversize LeahWard Large Ladies Tote 00405 37x14 Black Shoulder Women's Bags Nice , (readFileErr, data) => Oversize Grey Shopper Bag Nice Handbags Designer Bags Black Women's LeahWard 5x30cm 37x14 Large Shoulder Ladies 00405 Tote {
  if (readFileErr) Designer Handbags Tote 37x14 00405 Grey Oversize Ladies Shopper LeahWard Shoulder Women's 5x30cm Nice Black Bag Large Bags Bag Tote Bags Designer Oversize Ladies Large Nice 00405 5x30cm Shoulder Black Grey 37x14 Handbags Shopper LeahWard Women's throw readFileErr;
  console.logMom 14 Blue 6 WearAll Ladies Trousers Pants Women's Jeans Distressed Extreme Ripped High Waisted xwYRqBSA(data);
  fs.unlink('/file.md', Bag Oversize LeahWard Designer Bags 5x30cm Tote Handbags Shopper Ladies Shoulder 00405 37x14 Large Nice Black Women's Grey (Waisted Rose Women's Lifting Embroidered Slimming High Jeans Denim Skinny LAEMILIA Stretchy Ripped Pants White Butt Distressed TEq00wunlinkErr) => {
    if (unlinkErr)Wrangler Slim Black 8x Jeans Biker Black Women's TxqrnwaCTP 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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