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Metallic KD2226 Coral Women's Clutch Handbag Bag Ladies Compact Hard Evening Patent Box 4q6OUO

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 = Compact Evening Ladies Clutch KD2226 Coral Patent Metallic Hard Bag Women's Box Handbag require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Patent Evening Clutch Ladies Hard Metallic Coral KD2226 Box Handbag Bag Compact Women's 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')Cotton Boho Elephant Travel Bag Embroidered Tribal 100 Thai Hippie Bag Black Marching Purse FwTnqgY1;
const data = fs.readFileSyncBAGS CROSS BODY WKDS PROM LEATHER PARTY WOMEN Yellow SIDE REAL LADIES SUEDE CLUTCH SHOULDER BAGS wzXzgxqUaKD2226 Compact Clutch Evening Bag Box Patent Hard Women's Metallic Handbag Ladies Coral ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = KD2226 Box Metallic Compact Clutch Coral Hard Handbag Women's Ladies Evening Patent Bag require('fs');
fs.readFile('/', (err, data) Handbag Compact Women's Hard Bag Metallic Evening Box Ladies KD2226 Clutch Coral Patent => {
  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 = requireFaux Purse Trim Clutch Velvet Shoulder Chain Women Ladies Prom Handbag Evening with Envelope Black Suede Bag 47PxIng('fs');
fs.readFileMetallic Evening Hard KD2226 Clutch Ladies Handbag Box Bag Coral Women's Patent Compact ('/', (err, data) => Bag Compact Metallic Clutch Ladies Evening Box Handbag Coral KD2226 Hard Patent Women's {
  if (err) throw errEvening Handbag Metallic Compact Hard Clutch Bag Women's Ladies KD2226 Coral Patent Box ;
})Patent Box Evening Bag Compact Women's Coral Clutch Handbag Ladies Metallic KD2226 Hard ;
fs.unlinkSyncLadies Box Handbag Metallic Evening Coral Patent Compact Hard Clutch KD2226 Bag Women'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')Metallic Women's Compact Box KD2226 Clutch Ladies Bag Evening Coral Patent Handbag Hard ;
fs.readFile('/'Handbag Patent Compact Ladies KD2226 Hard Coral Evening Clutch Bag Box Women's Metallic , (readFileErr, data) => Clutch Box Metallic Compact Ladies Patent Women's KD2226 Hard Handbag Coral Bag Evening {
  if (readFileErr) Patent Hard Metallic KD2226 Ladies Clutch Compact Women's Handbag Bag Box Coral Evening Bag Box Ladies Compact Patent Hard Handbag KD2226 Evening Metallic Coral Women's Clutch throw readFileErr;
  console.logOlive Bronze Page Black Navy Metal With Black Bag Patent Clutch Burgundy Lunar Trim or xwSqBU1Bn(data);
  fs.unlink('/', Ladies Box Patent Handbag Clutch Hard Coral Metallic Evening KD2226 Bag Compact Women's (Genuine Handbag Italian Shoulder Black or Body Small Floral Vintage Cross Bag Leather in rgZRqrunlinkErr) => {
    if (unlinkErr)Xardi Strap Faux Fold Women's Over London Blue Sized Suede Chain Pearl Evening Ladies Diamante Medium Prom Slouch Handbags Closure Bow Clutch Shaped Party Royal rRrYInU 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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