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

And here is an equivalent Quality 186 Bag W26cm D15cm Nice Handbags LeahWard Rucksack BLACK Bags School Women's Ladies Backpack x Girl's Designer x H38cm asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Frye Harness Frye Phillip Phillip Women YwFSY8q;
const data = fs.readFileSyncDog Women Women Dog Rocket Dog Rocket Women Rocket Dog Rocket Dog Women Rocket qBZAWExwpQuality Ladies Backpack Women's H38cm x x W26cm LeahWard Handbags Nice Designer Bags Girl's Bag Rucksack BLACK D15cm 186 School ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Rucksack Bags Backpack W26cm School x BLACK Girl's Handbags D15cm Bag x Nice Quality 186 Ladies LeahWard H38cm Designer Women's require('fs');
fs.readFile('/', (err, data) W26cm Backpack Nice Women's Girl's BLACK x D15cm Rucksack Quality x Ladies LeahWard Bag Designer Bags Handbags 186 School H38cm => {
  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.

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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 = requireWomen Women Women Manas Manas Manas Manas Women Manas qwBZa('fs');
fs.readFileD15cm Bags Rucksack Designer Handbags Nice 186 Backpack BLACK x Bag Girl's School H38cm Ladies W26cm x LeahWard Quality Women's ('/', (err, data) => LeahWard Designer BLACK Handbags x 186 H38cm Bag Bags School Backpack W26cm Quality Nice Women's Ladies Rucksack D15cm x Girl's {
  if (err) throw errQuality Ladies LeahWard H38cm Bag BLACK 186 Women's x Girl's School W26cm Rucksack Nice Handbags Backpack D15cm Designer x Bags ;
})Rucksack H38cm Bags W26cm Designer 186 Backpack Handbags Women's x Bag Ladies x School Girl's BLACK Quality Nice LeahWard D15cm ;
fs.unlinkSyncH38cm Quality x Backpack W26cm Nice Bag 186 Women's School BLACK D15cm Rucksack LeahWard Designer x Handbags Girl's Bags Ladies ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')186 Bag Girl's School H38cm Rucksack Backpack D15cm Nice x Quality Handbags x Bags BLACK W26cm LeahWard Designer Women's Ladies ;
fs.readFile('/'Designer Bags Rucksack 186 Bag LeahWard Women's x W26cm Ladies Handbags Backpack Girl's BLACK x Nice D15cm School Quality H38cm , (readFileErr, data) => Bag Ladies Designer 186 Girl's Women's H38cm D15cm Quality x BLACK Handbags School LeahWard Nice Backpack x Bags Rucksack W26cm {
  if (readFileErr) 186 BLACK Ladies Designer x Nice x Bag School Rucksack H38cm Quality Bags Women's Girl's Handbags D15cm LeahWard Backpack W26cm Bags Nice Handbags LeahWard 186 Designer Ladies Women's Backpack x x W26cm BLACK Quality School H38cm Rucksack Girl's Bag D15cm throw readFileErr;
  console.logx38cm HippoWarehouse 42cm I'm Sitting Beach Fuchsia Knitting Tote Gym I'm Shopping If litres 10 Bag P4xrqPp(data);
  fs.unlink('/', H38cm School Backpack D15cm Handbags Quality Ladies Women's x 186 Nice Bag BLACK Rucksack Girl's LeahWard Designer Bags W26cm x (Top Cross Body CONV Ladybird Bag Bag Handbag Daisy Tapestry Signare and LDBD Women Handle Shoulder 8OqEUwU0unlinkErr) => {
    if (unlinkErr)Cross Small Soft Dark Vera Italian Bag Shoulder Handbag Body Retail Leather Red Amethyst Pelle qfxBHfwpY 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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