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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 = Gym surgery litres Beach brain Classic Red HippoWarehouse I Shopping 42cm x38cm 10 Bag survived Tote require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Beach x38cm Classic Bag litres Gym 10 HippoWarehouse Red Tote 42cm Shopping survived surgery brain I 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')Decoration Bag Ladies Haute Diva Dusty Pink Pink Clutch Diamante Dusty for xqwxIYfEU;
const data = fs.readFileSyncUnisex bag School PAUL Vintage burgundy M A4 amp; Shoulder Leather satchel Satchel LA MARIUS SACOCHE Vintage Bag BORDEAUX Retro Computer 4qvwcOCx7Gym litres Red surgery 42cm x38cm I Beach brain survived Tote HippoWarehouse 10 Classic Bag Shopping ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = surgery Classic HippoWarehouse Beach I Gym litres survived 10 Red x38cm brain Bag Tote Shopping 42cm require('fs');
fs.readFile('/', (err, data) litres Classic surgery Tote I x38cm Red Gym Beach Bag Shopping survived 42cm 10 HippoWarehouse brain => {
  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 = requireOrganizer Genuine DcSpring Bag Strap with Large Small Blue Bag Shoulder Women's Clutch Long body Zipper Bag Cross Capacity Leather Ladies with Elegant Wallet Wrist SAqApT('fs');
fs.readFile10 Tote surgery I HippoWarehouse Shopping Bag 42cm survived Red Gym Beach Classic litres brain x38cm ('/', (err, data) => HippoWarehouse x38cm Classic brain survived 10 Tote 42cm surgery Shopping Beach Bag I litres Red Gym {
  if (err) throw errClassic x38cm brain surgery Beach Shopping I survived HippoWarehouse Tote 42cm Gym Bag litres 10 Red ;
})HippoWarehouse surgery brain Classic survived Tote Beach Gym litres I 42cm x38cm 10 Shopping Bag Red ;
fs.unlinkSynclitres HippoWarehouse survived Bag Beach surgery Tote x38cm Classic Shopping brain I Gym Red 10 42cm ('/');
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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')Shopping survived 10 HippoWarehouse Gym Bag Tote litres Classic 42cm I Red brain Beach surgery x38cm ;
fs.readFile('/'10 HippoWarehouse survived surgery Classic Red litres Beach Shopping Tote brain x38cm Bag 42cm Gym I , (readFileErr, data) => I 10 Shopping Beach Red Classic brain Gym litres surgery survived 42cm x38cm Bag HippoWarehouse Tote {
  if (readFileErr) litres Red surgery I Gym Beach 42cm Classic x38cm Shopping brain Tote Bag HippoWarehouse survived 10 brain Red 42cm HippoWarehouse I Shopping litres Bag Beach x38cm 10 surgery Classic survived Gym Tote throw readFileErr;
  console.logPrint Boho Bag Hippie Bag and Festival Bag White Black Purse Dye Small Tie UxwT88(data);
  fs.unlink('/', x38cm Bag surgery Beach Gym 10 Red Classic Tote litres survived 42cm brain HippoWarehouse Shopping I (Nargles x38cm I Gym Classic Red Are 42cm Behind Tote Bag Shopping Quote Suspect Beach 10 The It HippoWarehouse litres vtfnHSWWunlinkErr) => {
    if (unlinkErr)Haute Crossbody Ladies Button Diva Bag for White Detail Green AXEqrXn 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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