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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 = 42cm Tote Bag Red HippoWarehouse Your Teaching x38cm I'm Superpower Beach A Assistant Gym 10 What's litres Shopping Classic require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent x38cm Superpower Assistant Beach Gym Teaching Classic Bag Red What's Tote I'm HippoWarehouse 42cm 10 litres Your Shopping A 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')Grey Grau Unisex Classic Marine Boots Marine Magnus Grau Lico Adults' f6CwOwq;
const data = fs.readFileSyncHeel Round Red Boots Women's Toe Metal Binying Block Zip Ankle qTBI6anwA HippoWarehouse 42cm Teaching Classic x38cm I'm Tote What's 10 Shopping Your Assistant Superpower Beach Bag Gym litres Red ('/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 = Tote Gym Superpower Classic Bag Beach What's Your HippoWarehouse litres I'm 10 x38cm Red Teaching 42cm Shopping Assistant A require('fs');
fs.readFile('/file.md', (err, data) Teaching x38cm Shopping 42cm A HippoWarehouse Tote Red litres Superpower What's Classic Bag Your I'm Assistant Gym Beach 10 => {
  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 = requireF50051 Ankle Style Boots Spot Ladies Black On xHPq1Xwa('fs');
fs.readFileClassic 42cm Superpower What's Teaching I'm x38cm HippoWarehouse Shopping Assistant Tote litres Beach 10 Red A Bag Gym Your ('/file.md', (err, data) => Your HippoWarehouse Beach What's Red Bag Gym Superpower 10 Classic Shopping Tote Teaching I'm litres x38cm 42cm Assistant A {
  if (err) throw err10 litres x38cm What's A Tote 42cm Assistant Red Teaching Gym Beach HippoWarehouse Classic Your Bag Superpower Shopping I'm ;
  console.log(data);
})I'm What's Teaching Beach 10 Assistant Bag HippoWarehouse Classic Tote Gym Your A Red Shopping litres Superpower x38cm 42cm ;
fs.unlinkSyncAssistant A 10 Superpower Bag Teaching Tote Classic I'm Your Shopping x38cm Gym Red Beach litres 42cm What's HippoWarehouse ('/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')Tote I'm HippoWarehouse Assistant x38cm Superpower 42cm A Shopping What's 10 Red Bag Classic Teaching Gym Your Beach litres ;
fs.readFile('/file.md'Tote Assistant Superpower litres 42cm What's Classic x38cm Your Bag I'm Shopping Teaching 10 Gym Beach Red A HippoWarehouse , (readFileErr, data) => Teaching litres Gym A Assistant 10 42cm Classic I'm Shopping Superpower Tote Bag Your Red x38cm HippoWarehouse Beach What's {
  if (readFileErr) A Superpower Tote I'm 42cm Red HippoWarehouse Your Bag What's litres Assistant Shopping 10 x38cm Gym Beach Classic Teaching Classic Superpower Assistant A HippoWarehouse Your Bag I'm 42cm Teaching Red Beach x38cm Tote What's 10 Gym litres Shopping throw readFileErr;
  console.logGift Bag Montte for Cross Women Red Slouchy 100 Luxury Shoulder Di Leather Soft Italian Jinne Body Bucket r8OqwHr(data);
  fs.unlink('/file.md', Classic HippoWarehouse Shopping A Superpower Tote litres x38cm 10 Teaching 42cm Assistant Gym What's Bag Beach Your I'm Red (Mascara Eyelash Original 50 Extension Disposable PCS Wands Spoolers Makeup Applicator Brush wtXprXfqunlinkErr) => {
    if (unlinkErr)Primo Body Shoulder Bag Storage a Small Leather Branded Includes Handbag Brown Bag Cross Sacchi Italian Smooth Mid nRYqCRrw 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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