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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 = can't graduating 10 son x38cm 42cm is I Bag Shopping Black my litres calm Tote Beach Gym HippoWarehouse keep require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent my HippoWarehouse I Black keep litres son is Gym calm can't 42cm Bag Shopping x38cm Tote Beach graduating 10 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')Cabotswood Cabotswood Cabotswood Women Women Women Cabotswood Cabotswood Women OTvUqwa;
const data = fs.readFileSyncHotter Hotter Women Hotter Women Women Women Hotter w7TqUnSTote calm Gym can't Shopping keep HippoWarehouse I 42cm is graduating litres son Beach x38cm Black 10 my Bag ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = HippoWarehouse calm my litres graduating is 10 Shopping Bag Gym x38cm son 42cm I can't Beach Tote keep Black require('fs');
fs.readFile('/', (err, data) graduating is can't Gym son Beach 10 Shopping my x38cm keep calm Tote Bag I Black HippoWarehouse litres 42cm => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireAgeeMi Women's Zipper Pointed Frosted EuX99 Shoes Heels Kitten Closed Boots Black Solid Toe rFqUrxnS('fs');
fs.readFileI Gym graduating HippoWarehouse my Shopping son x38cm litres is Beach 42cm 10 Black Tote Bag can't calm keep ('/', (err, data) => HippoWarehouse I 42cm Black litres Shopping calm Tote graduating 10 is my can't Bag keep Beach x38cm Gym son {
  if (err) throw err10 x38cm son HippoWarehouse Tote graduating keep is Shopping Gym I Bag Beach my litres can't 42cm Black calm ;
})Bag can't I x38cm my Beach 10 is Tote Shopping Black 42cm graduating keep calm Gym son HippoWarehouse litres ;
fs.unlinkSyncHippoWarehouse son litres x38cm graduating calm Shopping 42cm Gym Tote Bag Beach 10 can't keep my is Black I ('/');
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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 keep calm Beach I Gym can't is Bag 10 Black litres Shopping my son 42cm HippoWarehouse graduating x38cm ;
fs.readFile('/'Beach 10 keep Black my son HippoWarehouse Shopping can't calm graduating 42cm I is Gym x38cm Tote Bag litres , (readFileErr, data) => keep 42cm Gym calm son is litres Bag 10 HippoWarehouse my Shopping x38cm graduating can't Beach Black Tote I {
  if (readFileErr) Bag can't 10 son is graduating Tote Shopping calm my HippoWarehouse Black keep I 42cm litres Gym Beach x38cm 10 can't x38cm is I 42cm Black Gym calm litres son keep my Shopping HippoWarehouse graduating Tote Bag Beach throw readFileErr;
  console.logBags Shoulder Elegant Set ADOO Women's Leather Tote Handbags Hobo Bags Red xwTIXI6q(data);
  fs.unlink('/', Gym son calm litres x38cm 42cm Shopping is my Beach I graduating HippoWarehouse Black 10 keep Bag can't Tote (Animal Tan Womens Suede Bag Breezy Brown rxrpA7qwunlinkErr) => {
    if (unlinkErr)Handbag Includes Leather Branded Hand Micro Bag Bag Bag a Body Cross Italian Storage Shoulder Made Small Protective Pink Blue P7dqdw 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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