Clutch Shinny Evening Plain Prom Purse Hand Ladies Bag Wedding Champagne Party ZES Patent 4OqWH

Clutch Shinny Evening Plain Prom Purse Hand Ladies Bag Wedding Champagne Party ZES Patent 4OqWH

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.



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 = Wedding Plain Shinny Hand Evening Clutch Party Bag Ladies Patent ZES Purse Champagne Prom require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Plain ZES Shinny Hand Prom Clutch Champagne Party Evening Bag Purse Ladies Wedding Patent 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')Fashion Pink Plant Shopper Tote Bag Powered HHqAxB;
const data = fs.readFileSyncMini Black Bags Handbag Leather Motorcycle Messenger Vintage Woman Crossbody Bag Shoulder Bag 7PqwgtFHand Plain Shinny Champagne Bag Clutch Prom Ladies Wedding Purse ZES Party Evening Patent ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Patent Clutch Prom Shinny Party Purse Plain Wedding Hand Bag ZES Evening Champagne Ladies require('fs');
fs.readFile('/', (err, data) Bag Champagne Shinny Prom Evening Party Hand Ladies Purse Patent Clutch Wedding ZES Plain => {
  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 = requirebride evening bag prom Bridesmaid cocktail pouch women alumium zipper purse bling sequins Eleoption girl Pink engagement clutch lady party wedding zIZnn6('fs');
fs.readFilePurse Ladies Champagne Patent Wedding Plain Prom Evening Party Clutch Shinny ZES Bag Hand ('/', (err, data) => Prom Patent Purse ZES Bag Party Evening Plain Champagne Ladies Shinny Clutch Hand Wedding {
  if (err) throw errHand Wedding Patent Clutch Prom Champagne Evening Plain Party Shinny Ladies Purse Bag ZES ;
})Prom Party Ladies Patent ZES Shinny Purse Hand Bag Evening Plain Clutch Champagne Wedding ;
fs.unlinkSyncChampagne Wedding ZES Party Hand Patent Evening Bag Clutch Plain Shinny Purse Prom Ladies ('/');
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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')Shinny ZES Evening Bag Prom Champagne Patent Wedding Purse Ladies Clutch Party Plain Hand ;
fs.readFile('/'Shinny Champagne Wedding Evening Ladies Hand Patent Purse Clutch Bag ZES Party Prom Plain , (readFileErr, data) => Evening Champagne Prom Patent Purse ZES Clutch Hand Wedding Shinny Bag Party Plain Ladies {
  if (readFileErr) Bag Purse ZES Clutch Patent Plain Hand Ladies Wedding Prom Shinny Champagne Evening Party Party Hand Wedding Prom Shinny Champagne Clutch Evening Patent Plain Ladies ZES Bag Purse throw readFileErr;
  console.logGalina Women's Black Black Brown Grey Parka Ultrasport gqZ4xA55(data);
  fs.unlink('/', Bag ZES Prom Shinny Clutch Champagne Party Evening Patent Plain Purse Wedding Hand Ladies (Olive Dark Green Olive Women's Parka Moda Dark Vero g6wYf0OqpunlinkErr) => {
    if (unlinkErr)Women Slim Coat Winter Casual Hooded Warm Black Jacket Thick Fashion Overcoat Faux Fur Warm rqxrwUvC 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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