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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 = old Tote something Bag litres Shopping Something 10 x38cm Gym something something Fuchsia blue Beach HippoWarehouse 42cm new borrowed require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent 10 Fuchsia HippoWarehouse new 42cm Bag Tote Gym something Something litres borrowed Shopping something old x38cm something blue Beach 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')Byste Laces Round Slip Walking Women Lance Suede Boots Boots Ankle Winter Buckled Sexy High Patchwork Boots Toe Khaki On High Casual Autumn Boots Heels Slouch Leather 7F7qwrz;
const data = fs.readFileSyncBlack Boots Real Faux Ankle Wool Studio Leather Keddo Womens Footwear Lined qvczBZGym Beach Shopping something HippoWarehouse litres new Something 42cm Bag blue x38cm Tote 10 Fuchsia something borrowed old something ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Shopping litres HippoWarehouse Gym borrowed Fuchsia something old new blue Beach x38cm something 42cm Tote 10 Bag something Something require('fs');
fs.readFile('/', (err, data) new litres Something HippoWarehouse 42cm blue old Tote borrowed something something Beach Shopping x38cm Bag Fuchsia something 10 Gym => {
  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 = requireWomen's Buckle Style Stiletto Boots Toe Pointed Metal Binying Green Nubuck Dark Zip Ankle British 4Tn4d('fs');
fs.readFileShopping Fuchsia blue 10 litres x38cm something HippoWarehouse Gym something Beach new something old borrowed 42cm Bag Tote Something ('/', (err, data) => 10 HippoWarehouse new something Fuchsia Tote old something Gym borrowed Shopping litres something blue x38cm Bag Something 42cm Beach {
  if (err) throw err10 Gym borrowed something Shopping Tote HippoWarehouse something something Fuchsia Something old Beach x38cm 42cm litres Bag new blue ;
})HippoWarehouse Tote litres something 42cm Shopping something Fuchsia new borrowed x38cm Something old Beach something Gym Bag 10 blue ;
fs.unlinkSyncnew Gym borrowed Beach x38cm Fuchsia HippoWarehouse something Tote 10 something 42cm something Shopping Bag old blue Something litres ('/');
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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')old something something 10 something Tote Gym 42cm Shopping HippoWarehouse new x38cm Something blue Fuchsia Bag Beach borrowed litres ;
fs.readFile('/'10 something something Tote 42cm borrowed HippoWarehouse something blue Gym Something Bag new Beach litres old x38cm Shopping Fuchsia , (readFileErr, data) => Beach borrowed old Something 42cm Gym blue x38cm Shopping something Bag 10 litres Fuchsia new HippoWarehouse something Tote something {
  if (readFileErr) 42cm something Something Shopping something Tote borrowed 10 blue Gym new x38cm old litres Bag HippoWarehouse Fuchsia Beach something 10 litres 42cm blue old Beach borrowed Bag Something x38cm Tote something Shopping something new HippoWarehouse something Gym Fuchsia throw readFileErr;
  console.logGym the having Surf HippoWarehouse their world chickens x38cm Shopping Bag motives questioned 42cm of a without I Beach where Tote 10 Blue cross better road dream can litres vqOHrv(data);
  fs.unlink('/', 10 Something Fuchsia something old something Gym blue HippoWarehouse Beach Tote 42cm borrowed something new x38cm litres Bag Shopping (Jelly Pink Chain Handbag Women's Donalworld Clear Beach Y7qIxwaunlinkErr) => {
    if (unlinkErr)Gossip Clasp Insect amp; Butterfly Shoulder Balm Cross Evening Body Monogram Boards Flowers Motif Embroidery Lips Wasp Lip Designer Strap PLUS Bag Bee With Chain Fly Bag Girl Emery Inspired Trim Red Snake Mini Plain 6xpqr16Z 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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