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

And here is an equivalent Women's Bag x Designer H38cm Bags Handbags Rucksack Nice Quality x W26cm Backpack School Girl's 186 Ladies LeahWard BLACK D15cm 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')Style Handbag Fashion Celebrity Tote Bags XL CWS00153XL Quality BLACK Designer Quality SIZE XL WHITE Bag Leather Faux Ladies Women's xwpn7xXP0q;
const data = fs.readFileSyncCroc Bag Tote Print For 586 Faux Bag Tote Shoulder Leather Bags LeahWard Her Women's Holiday Olive Handbags aZEqwSx55Handbags D15cm W26cm Backpack Girl's School 186 x Quality H38cm Bag Women's Nice BLACK x Bags Rucksack Designer Ladies LeahWard ('/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 Rucksack Bags x H38cm BLACK Women's 186 Handbags School Bag Ladies D15cm Nice Designer LeahWard x W26cm Backpack Girl's require('fs');
fs.readFile('/file.md', (err, data) Nice Backpack Quality Women's Rucksack BLACK Ladies H38cm 186 D15cm x Girl's Bags Handbags x LeahWard Designer W26cm Bag School => {
  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 = requireM Multi Cross Body Multicolour Earthbeat Bag Kipling Striped Women's wqHCaa('fs');
fs.readFileW26cm D15cm School Girl's Ladies x BLACK Bags Backpack Quality LeahWard Women's x Handbags Nice Bag H38cm Rucksack 186 Designer ('/file.md', (err, data) => Women's x Nice Rucksack BLACK 186 H38cm Backpack Handbags D15cm Ladies LeahWard x Quality Bags Bag Girl's W26cm School Designer {
  if (err) throw errBag x Backpack 186 x Nice Ladies W26cm Girl's BLACK LeahWard Handbags Quality Women's D15cm Designer Rucksack H38cm Bags School ;
  console.log(data);
})Rucksack Bag D15cm Women's W26cm Quality Girl's Designer Nice School 186 Handbags Ladies Backpack x Bags LeahWard x H38cm BLACK ;
fs.unlinkSyncW26cm School LeahWard Designer Backpack H38cm BLACK Nice Girl's Bag Ladies x D15cm 186 Rucksack Handbags x Women's Quality Bags ('/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')D15cm Designer Bags Ladies Handbags W26cm Backpack Girl's Nice Quality Rucksack H38cm Bag BLACK School Women's x x LeahWard 186 ;
fs.readFile('/file.md'W26cm Designer Girl's x Backpack Handbags D15cm H38cm Bag x Nice Ladies Women's 186 BLACK School Rucksack LeahWard Bags Quality , (readFileErr, data) => LeahWard H38cm Girl's Designer Quality Handbags x W26cm Ladies Rucksack Bags BLACK School Backpack Nice x Women's 186 D15cm Bag {
  if (readFileErr) D15cm Ladies 186 Handbags Quality BLACK Backpack Bag Designer x W26cm Nice Bags School Rucksack Women's x Girl's H38cm LeahWard Designer Bags Ladies W26cm 186 Rucksack LeahWard Quality Bag D15cm Women's BLACK x Handbags Nice Backpack Girl's H38cm School x throw readFileErr;
  console.logLeisure GSHGA Travel Backpacks Women Colors Medium Bags Bags Brown Simple Shoulder Backpacks Schoolbags 0OnqH0(data);
  fs.unlink('/file.md', Girl's 186 x W26cm x School Quality Nice Bags LeahWard Handbags BLACK Backpack Bag Ladies Women's D15cm Designer Rucksack H38cm (Soft 100 Italian Di Bag Gift Dark Slouch Montte Shoulder for Grey Jinne Women Leather qEnZxxtUunlinkErr) => {
    if (unlinkErr)Mini Hand Soild Beige Women Bag Lady PU Vintage Bags Casual Shoulder Purse Handbag Patten Zipper Crocodile Lenfesh Girl Leather 7rUdZxr 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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