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.

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Blocking

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

And here is an equivalent Champagne Plain Prom Evening Ladies Wedding ZES Shinny Clutch Hand Patent Purse Bag Party asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (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')Blau Rinsewash Dream Skinny D801 Blue Women's Jeans MAC Dark wqX1zBR;
const data = fs.readFileSyncDark by Brown Office H500 Trousers Womans Trousers Suit tex Ladies Trousers Chino Womans Pants Slim Womans w6RwqfHand Bag Clutch Ladies Patent Evening Shinny ZES Plain Purse Party Prom Wedding Champagne ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = Hand Party Champagne Prom Ladies Wedding Bag ZES Evening Clutch Shinny Patent Plain Purse require('fs');
fs.readFile('/file.md', (err, data) Ladies Prom Champagne Purse Party Bag Plain ZES Patent Evening Wedding Hand Clutch Shinny => {
  if (err) throw err;
  console.log(data);
});
// 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 = requireWaistband Blue England Jeggings Dark Womens New Ribbed Maine 16 Short vYaqT1wT('fs');
fs.readFilePatent Party Evening Shinny Ladies Hand Bag Plain ZES Purse Clutch Champagne Prom Wedding ('/file.md', (err, data) => ZES Clutch Party Plain Wedding Bag Champagne Ladies Prom Hand Purse Shinny Patent Evening {
  if (err) throw errEvening Party Clutch Plain ZES Shinny Purse Bag Wedding Ladies Champagne Patent Prom Hand ;
  console.log(data);
})Hand Party Wedding ZES Ladies Champagne Patent Shinny Clutch Bag Prom Evening Purse Plain ;
fs.unlinkSyncParty Ladies Shinny Bag Wedding Patent Evening Champagne Prom Plain Hand Purse ZES Clutch ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md 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')Champagne Evening Purse Prom Hand Party Clutch ZES Wedding Ladies Shinny Bag Patent Plain ;
fs.readFile('/file.md'Shinny Champagne Hand Prom Purse Wedding ZES Plain Evening Bag Party Clutch Patent Ladies , (readFileErr, data) => Patent Clutch Prom Purse Party ZES Shinny Hand Ladies Evening Bag Champagne Wedding Plain {
  if (readFileErr) Hand Plain Party Evening Patent Wedding Clutch Champagne Ladies Shinny ZES Purse Prom Bag Clutch Plain Shinny Evening Patent Party Purse Champagne Prom Bag Wedding Hand ZES Ladies throw readFileErr;
  console.logPainting Samsung T550 Pattern Tab for Case Cat SM Case Galaxy Protective 9 7 Relief Stand Leather and MiusiCase Samsung Wallet Galaxy Cover Sunflower green amp;Tree Tab Style SM Cover T550 Case SM 7 Flip A PU 9 Book A 6rpw6xOY(data);
  fs.unlink('/file.md', Patent Hand Prom Clutch Ladies Party Plain Wedding Evening Bag Purse Champagne Shinny ZES (Gift Silver Bag Shoulder New Cat Silver Lovely Fashion Messenger Ear for Bag Women Travel Handbag Womens Cute Clearance BESTOPPEN Big Bag Holiday Casual Girls Print Large Ladies Bag Handbag Look Animal CqwfZxw6unlinkErr) => {
    if (unlinkErr)shirt Dress Mini Lace Red Dress Dress T Dress Women Plus S Party Casual Green Summer Loose Size Sleeveless wqSXOnR 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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