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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 = litres 10 Thought Bag HippoWarehouse Yoga a Class Shopping x38cm Beach Tote Pour Glass Green I Said 42cm You Gym Bottle require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Bag HippoWarehouse a 42cm Said I 10 Yoga Thought You Tote x38cm Pour Bottle Green Class Gym Shopping litres Glass 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')Bluester C Bag Owl Crossbody Women Cartoon Messenger Handbag Satchel Printed Shoulder rvwrn4zq;
const data = fs.readFileSync45CM 45 x Green x Children Foldable Blue Tote Mesh Seaside Picnic Storage Bag Beach 26 Kids Organizer DaoRier size Barbecue qOTa6wx38cm Beach 10 a Said HippoWarehouse litres Gym Tote Green Glass Shopping Class I Bag Yoga Thought You Pour 42cm Bottle ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Beach Class Shopping Said Bottle a HippoWarehouse 42cm Yoga 10 Bag Pour I Tote Gym x38cm Green Glass You Thought litres require('fs');
fs.readFile('/', (err, data) HippoWarehouse Bottle 10 Thought litres Bag Green Class x38cm You Gym Shopping Tote 42cm Said Glass Pour Beach Yoga a I => {
  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 = require12 Caracas Women VITA anna LAURA aPYnt6P('fs');
fs.readFilex38cm 42cm You Class Tote Bottle 10 Said Bag Pour Shopping litres Gym HippoWarehouse Green I a Glass Thought Yoga Beach ('/', (err, data) => 42cm x38cm litres Green Class Gym Yoga Bottle I Beach Glass HippoWarehouse Said Pour Bag Shopping 10 a Tote You Thought {
  if (err) throw errPour Tote litres Glass You Thought Bottle Gym HippoWarehouse I Shopping Beach a 42cm Bag Class Green Said 10 Yoga x38cm ;
})Bottle I Glass a Class Pour x38cm litres HippoWarehouse Bag Green 10 Thought Said Yoga Beach You 42cm Gym Shopping Tote ;
fs.unlinkSyncGreen litres Glass You a HippoWarehouse 42cm 10 Said Tote x38cm Beach Class Bag I Bottle Gym Yoga Pour Thought Shopping ('/');
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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')litres x38cm Green Shopping Gym Thought a 42cm Glass Class HippoWarehouse I Bottle Bag Said Pour Yoga 10 Beach You Tote ;
fs.readFile('/'a 10 Said Gym You Tote Shopping Green HippoWarehouse litres Glass Bag 42cm Class x38cm Bottle Pour Thought I Beach Yoga , (readFileErr, data) => a 42cm Gym x38cm Tote You HippoWarehouse Beach Green litres Class Bag Said I Pour Shopping Glass 10 Yoga Thought Bottle {
  if (readFileErr) Said a I You Thought 10 HippoWarehouse Bottle Class Green Pour Yoga Shopping Glass Beach 42cm litres Gym Bag Tote x38cm Green Thought litres a Beach I HippoWarehouse 10 Yoga x38cm Said Gym Glass Bottle 42cm You Pour Tote Bag Class Shopping throw readFileErr;
  console.logGreen Regatta Thor Ladies Bottle III Fleece Uzq0nzv(data);
  fs.unlink('/', 42cm Beach Yoga Bag Said litres Gym x38cm Shopping Green Bottle Pour I Tote 10 Glass a Thought Class You HippoWarehouse (Pink Waterproof Jacket Winter Equisafety Aspey 8wxaqZXIunlinkErr) => {
    if (unlinkErr)Collar Beige Jacket Overcoat Turn B Down Lapel Ladies HARRYSTORE Long Overcoat Womens Winter Coat Cardigan Outwear Parka CqtWBwZ 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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