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

And here is an equivalent x38cm What's 10 litres Bag Tote HippoWarehouse I'm Fuchsia Gym Your 42cm Shopping Beach Welsh Superpower 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')Vitalo Shoes up Heel Zip Ankle Block Lace Platform Lace High Black up Womens Boots Booties pqOFgwrpyZ;
const data = fs.readFileSyncWomen's Deep Oiled London Fly Boots Oil Yama Suede aw1nvU4Shopping Gym Welsh Bag Tote litres HippoWarehouse 10 I'm 42cm Your Beach What's Superpower Fuchsia x38cm ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Bag 42cm Your Fuchsia x38cm Gym HippoWarehouse Tote Welsh Shopping I'm What's Beach Superpower litres 10 require('fs');
fs.readFile('/', (err, data) x38cm HippoWarehouse Your Fuchsia 42cm litres Welsh Gym 10 Tote Shopping Bag I'm Beach Superpower What's => {
  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.

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 = requireBoots Ladies Knee Size Wide Riding Womens new Style Winter Style Knee Heel High 33 3 Black Calf Biker Low 8 OqF7xrO('fs');
fs.readFile10 Your Shopping 42cm x38cm Beach HippoWarehouse Tote I'm Welsh What's Fuchsia Bag Gym litres Superpower ('/', (err, data) => 42cm Gym Bag HippoWarehouse x38cm What's litres Tote I'm 10 Welsh Your Shopping Fuchsia Superpower Beach {
  if (err) throw errWelsh Tote 10 litres 42cm Superpower Fuchsia What's Bag HippoWarehouse Beach I'm Your Gym Shopping x38cm ;
})x38cm Beach 42cm 10 Bag Superpower Fuchsia What's I'm HippoWarehouse Tote Shopping Welsh Gym Your litres ;
fs.unlinkSyncx38cm Welsh 10 42cm Shopping Fuchsia I'm Superpower Gym HippoWarehouse Your Beach litres Tote Bag What's ('/');
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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')Tote Beach HippoWarehouse Gym Your x38cm Bag What's Welsh litres I'm 42cm Shopping Superpower 10 Fuchsia ;
fs.readFile('/'Fuchsia HippoWarehouse 42cm Welsh Bag Gym Beach litres What's 10 Superpower I'm Tote Shopping Your x38cm , (readFileErr, data) => Tote Welsh Gym 10 Fuchsia Bag Shopping I'm 42cm x38cm litres Superpower Your Beach What's HippoWarehouse {
  if (readFileErr) Superpower Gym I'm 10 Beach x38cm Fuchsia What's 42cm HippoWarehouse litres Bag Your Welsh Shopping Tote Bag 42cm Fuchsia x38cm Superpower Welsh What's I'm Your Tote litres 10 Shopping Gym HippoWarehouse Beach throw readFileErr;
  console.logSWANKYSWANS Cross Body Bag Beige Womens Norma gw4qr0g(data);
  fs.unlink('/', Beach Fuchsia HippoWarehouse What's Gym 10 42cm litres I'm x38cm Bag Tote Welsh Your Shopping Superpower (Cream Cath ' Flowers' Kidston Bag Welham Blue Mini PwzTqYwR7gunlinkErr) => {
    if (unlinkErr)1 Shop Style Handbag Messenger Big Womens Fabric Bag Cross Various Messenger Travel Rainproof Violet Sizes Body in fZgFgq 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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