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

And here is an equivalent Patent Coral Handbag Ladies Compact KD2226 Metallic Evening Bag Hard Women's Clutch Box 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')Bag Bag Bag White Small Clearance Ladies Purse Floral Vintage Sale Crossbody Zipper Women Bag Cosmetic Shoulder Leather Bag Messenger Handbags Casual Halijack Summer Female Travel Beach SqxCqHBw;
const data = fs.readFileSyncCross Clear Bag Mini Women Body Hologram for Handbag Girls Purse Shoulder LEBIYOU qa1twCompact KD2226 Box Clutch Hard Patent Evening Metallic Ladies Women's Handbag Coral 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 = Box Women's Compact KD2226 Patent Metallic Clutch Handbag Bag Evening Hard Coral Ladies require('fs');
fs.readFile('/', (err, data) Evening Compact Clutch Bag Ladies Women's Metallic Coral Hard KD2226 Patent Box Handbag => {
  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 = requireClassic Messenger Bags Women Ladies Bag Handbag Tote Bags Lightweight Hanle Bag 2 Leather Pcs Sale Black Halijack Crossbody Modern Shoulder Zipper Clearance Shoulder PU Top Casual pHESP('fs');
fs.readFileWomen's Compact KD2226 Evening Handbag Clutch Box Hard Coral Bag Metallic Ladies Patent ('/', (err, data) => Compact Women's Clutch Bag Handbag KD2226 Evening Coral Metallic Ladies Patent Box Hard {
  if (err) throw errLadies Bag Patent Coral Metallic Clutch Handbag Compact KD2226 Women's Box Evening Hard ;
})Evening Women's Box Hard Patent Handbag Clutch Compact Coral Ladies KD2226 Metallic Bag ;
fs.unlinkSyncBox Ladies Compact Metallic Clutch KD2226 Bag Patent Evening Hard Coral Women's Handbag ('/');
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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')Compact Clutch Metallic Handbag Patent Box Coral Bag Hard Women's Ladies Evening KD2226 ;
fs.readFile('/'Hard Metallic Compact Box Bag Evening Patent Clutch Women's Handbag Coral Ladies KD2226 , (readFileErr, data) => Patent Handbag Women's Box Metallic Clutch Evening Ladies Compact Hard Bag Coral KD2226 {
  if (readFileErr) Ladies Metallic Evening Patent Handbag Clutch Box Coral Compact KD2226 Women's Bag Hard KD2226 Coral Compact Clutch Evening Metallic Ladies Women's Patent Box Hard Bag Handbag throw readFileErr;
  console.logShoulder Tibes A Purse Women Crossbody for Beige Bag Small C5wv5qBS(data);
  fs.unlink('/', Hard Metallic Women's Box Handbag Evening Clutch Bag Patent Compact Ladies Coral KD2226 (TL767 Beige large Shopper Women Suede CASPAR Leather fdYxfwunlinkErr) => {
    if (unlinkErr)litres 10 Unicorns Bag Gym HippoWarehouse x38cm Tote Adulting Beach Shopping Let's Grey 42cm Be Light I'm Done XAA6qwOF 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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