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

And here is an equivalent Women's Rucksack x Girl's School Bag Backpack x BLACK H38cm Handbags D15cm Ladies 186 Quality LeahWard Designer Nice Bags W26cm 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')NEW TOE Tan LADIES ANKLE ZIP BOOTS WOMENS TASSLE POINTED STILETTO HEEL SIZE SHOES UnRAaUTxq;
const data = fs.readFileSyncWomen ara ara ara Women Women ara Women ara ara ara Women ara Women Women ara Women Women Xn6wxf6I1D15cm School W26cm Designer Ladies LeahWard Bags H38cm Bag BLACK x Handbags Rucksack Girl's Quality Nice 186 x Backpack Women'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 = Rucksack BLACK Women's Backpack Quality x Girl's Designer Handbags D15cm Ladies LeahWard Bag Nice School 186 x H38cm W26cm Bags require('fs');
fs.readFile('/file.md', (err, data) Bags Ladies Backpack x x BLACK Rucksack 186 Designer School LeahWard H38cm D15cm Girl's W26cm Quality Handbags Women's Bag Nice => {
  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 = requireD Womens Williams Hope Brown Boots Joanna J Shoe ROpqwE('fs');
fs.readFileD15cm Ladies x H38cm LeahWard Designer W26cm Quality Girl's Bags Nice Rucksack Women's School BLACK 186 x Bag Handbags Backpack ('/file.md', (err, data) => W26cm Handbags x Designer School 186 Women's Bag Backpack Quality Ladies Girl's LeahWard Bags Nice D15cm Rucksack x H38cm BLACK {
  if (err) throw errBackpack D15cm School Quality Designer Nice Girl's Rucksack Women's Handbags LeahWard Ladies Bags x 186 H38cm x BLACK W26cm Bag ;
  console.log(data);
})x LeahWard Bag Bags H38cm Nice Handbags x W26cm Backpack School Quality Rucksack D15cm Ladies Girl's BLACK Women's Designer 186 ;
fs.unlinkSyncBag D15cm Girl's Rucksack LeahWard x H38cm BLACK Backpack Designer Quality x School Women's Handbags Ladies Nice W26cm 186 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')Women's Bags Ladies Girl's Bag School x W26cm Rucksack Nice H38cm Handbags LeahWard Backpack D15cm BLACK Quality Designer 186 x ;
fs.readFile('/file.md'Bag x BLACK Nice 186 Girl's LeahWard Bags Women's Ladies x Rucksack Designer D15cm W26cm H38cm Quality Backpack School Handbags , (readFileErr, data) => Ladies 186 Bags Backpack LeahWard BLACK Nice Women's D15cm Quality H38cm Rucksack School Girl's Designer x x Bag W26cm Handbags {
  if (readFileErr) Quality Nice Ladies Backpack Women's Girl's LeahWard BLACK School W26cm D15cm Bag Bags Handbags 186 H38cm x x Rucksack Designer School x D15cm Rucksack Bags BLACK Women's Ladies Designer 186 LeahWard Backpack Nice Girl's W26cm Bag H38cm x Quality Handbags throw readFileErr;
  console.logVintage Women Wocharm Messenger Bag Handbag Letter Shoulder Cross Ladies Grey Body Tote Purse Bag AwxfdUf(data);
  fs.unlink('/file.md', D15cm H38cm Women's Designer x Bags Handbags Rucksack Bag W26cm Quality School Nice Girl's Backpack LeahWard BLACK x Ladies 186 (Leather Bags Bag Shoulder Pu Envelope Wristlet Foldover Meliya Handbag Tassel Crossbody Womens Girls White1 gTHqnwaFunlinkErr) => {
    if (unlinkErr)Bag E426 Intimo Cross Linea body Pink Women's Rosa Versace Jeans Q SvYnBw 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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