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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 = Designer H38cm D15cm LeahWard Girl's Women's Handbags Rucksack x x Ladies School 186 Bags Backpack W26cm Nice BLACK Bag Quality require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Handbags LeahWard Backpack Women's H38cm Bag Designer Rucksack x BLACK Girl's Nice Ladies D15cm W26cm Bags x School Quality 186 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')Frank The Classic Pay Gotta Tote x38cm Gym Bag 10 litres Shopping Troll Toll 42cm You Pink HippoWarehouse Beach nqCdIq;
const data = fs.readFileSyncPhone Messenger Color Bag Pink Bag Coin Leather Women's Body Leather Solid Ring Bag key Soft Crossbody Fashion Cross Spritumn Bag Hwqzvxn6Onx Quality Backpack D15cm Rucksack School 186 Bag BLACK H38cm Nice Handbags Bags Ladies Women's W26cm LeahWard Designer x Girl's ('/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 = Quality Bag x BLACK Ladies x Rucksack D15cm 186 Backpack Designer Handbags School W26cm LeahWard H38cm Nice Girl's Bags Women's require('fs');
fs.readFile('/file.md', (err, data) Bag LeahWard 186 D15cm Women's Designer Handbags Quality Bags Girl's BLACK Nice School x x W26cm Ladies Backpack Rucksack H38cm => {
  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 = requireApricot SALLY PU Leather Body High Nice Quality Women Bags YOUNG Cross Fashion Bag Xpq7nrX1('fs');
fs.readFileBackpack Bags Designer W26cm BLACK LeahWard Women's x Handbags Nice Bag 186 H38cm Quality Girl's Ladies x Rucksack D15cm School ('/file.md', (err, data) => School LeahWard 186 Bags BLACK Rucksack Bag Handbags Ladies Designer W26cm Women's Quality Backpack H38cm x x D15cm Girl's Nice {
  if (err) throw errQuality 186 Bag W26cm Backpack H38cm Women's LeahWard x School D15cm Nice Girl's Bags BLACK Ladies Designer Handbags x Rucksack ;
  console.log(data);
})Backpack Girl's LeahWard Rucksack x Bag Designer School Ladies H38cm W26cm x Nice Quality Bags BLACK D15cm Handbags 186 Women's ;
fs.unlinkSyncBackpack School D15cm Girl's H38cm Designer Rucksack BLACK Bags 186 W26cm Nice x Ladies Bag Handbags Women's LeahWard x Quality ('/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')Nice D15cm Backpack Quality W26cm Bags x BLACK Women's LeahWard Girl's School 186 Handbags Ladies x Bag Designer H38cm Rucksack ;
fs.readFile('/file.md'Bags Women's School Nice Handbags LeahWard x 186 Backpack Ladies D15cm Designer Bag Rucksack BLACK Girl's Quality W26cm H38cm x , (readFileErr, data) => Handbags H38cm D15cm School BLACK Women's x x Backpack Quality Girl's 186 Designer LeahWard Rucksack Nice Ladies W26cm Bag Bags {
  if (readFileErr) Handbags School Bag Women's Girl's W26cm Rucksack Bags LeahWard x Backpack Designer H38cm Nice 186 x Quality BLACK D15cm Ladies BLACK Backpack H38cm D15cm School LeahWard Nice Designer Girl's 186 x x Ladies Women's Bag Quality W26cm Handbags Rucksack Bags throw readFileErr;
  console.logDINKY Satin Size Small Fold Pleated Pink Clutch Baby Over Bag ff1Rqwa(data);
  fs.unlink('/file.md', Handbags Bag Nice x Bags Women's Backpack LeahWard W26cm Girl's D15cm Quality H38cm Rucksack School Designer x 186 Ladies BLACK (Ladies Canvas For Backpack School Red Handle Handbag Bags Black Top Shopping Bag Hozee Work Daily Casual Womens Shoulder Vintage Travel Tote XfwBqq4AdxunlinkErr) => {
    if (unlinkErr)Bag Frame Clutch HandBags Shimmer Black Girly qITEpAxw 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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