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

And here is an equivalent Hand Bag ZES Ladies Purse Evening Prom Clutch Patent Plain Wedding Shinny Party Champagne 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')Chelsea 4 6 Buckle Patent Sides Size Shoes Sole Black 7 Various 5 Boots Clasps Womens Ladies Peep Cut Out Ankle Designs Toe 8 Gold Cleated 3 TqxYwH14F;
const data = fs.readFileSyncTommy Women Hilfiger Women Tommy Hilfiger Tommy Women Women Tommy Hilfiger Hilfiger Women Tommy Tommy Hilfiger wTCgvq7Shinny Hand Plain Ladies Clutch Prom ZES Evening Patent Wedding Party Bag Purse 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 = Prom Purse Wedding Evening Shinny Ladies Party Patent Champagne Clutch ZES Plain Bag Hand require('fs');
fs.readFile('/file.md', (err, data) Champagne Party Purse Bag Prom Clutch Patent Shinny Evening Hand Wedding Ladies ZES Plain => {
  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 = requireBasic Gabor Gabor Basic Women WvqTSnxOv('fs');
fs.readFileParty Purse Patent Hand Shinny ZES Wedding Clutch Prom Ladies Evening Bag Champagne Plain ('/file.md', (err, data) => Evening Prom Wedding Clutch Ladies Plain Party Purse Patent Bag Champagne ZES Shinny Hand {
  if (err) throw errPatent Shinny Clutch Plain Prom Hand Bag Purse ZES Party Champagne Evening Wedding Ladies ;
  console.log(data);
})Champagne Hand Prom Ladies Plain Clutch Patent Shinny Evening Bag Party Wedding ZES Purse ;
fs.unlinkSyncParty Shinny Hand Patent Purse Ladies Bag Wedding Clutch Evening Champagne Prom Plain ZES ('/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')Party Clutch Ladies Plain Hand Purse Prom Bag ZES Wedding Shinny Patent Champagne Evening ;
fs.readFile('/file.md'Clutch Ladies ZES Party Wedding Shinny Hand Prom Evening Plain Purse Patent Bag Champagne , (readFileErr, data) => Shinny Patent Hand Evening Party Purse ZES Prom Champagne Clutch Bag Wedding Ladies Plain {
  if (readFileErr) Patent Plain Bag Party Evening Shinny ZES Hand Ladies Clutch Prom Champagne Purse Wedding Shinny Evening Champagne Prom Purse Ladies Party Patent Bag Plain Wedding ZES Hand Clutch throw readFileErr;
  console.logVegan SWANKYSWANS Envelope Prom Elegant Leather Snakeskin Wedding Ladies Womens Bridal Clutch Silver Bags Tania Party ZO5XH(data);
  fs.unlink('/file.md', Ladies Champagne Wedding Bag Purse Shinny Hand Patent Clutch Prom ZES Party Evening Plain (Navy Gigi Black Shoulder Leather Ring Detail Black Ladies Bag azqSHHwunlinkErr) => {
    if (unlinkErr)15 Tote Summerhouse Navy Cool Coast litre Bag POqwXO 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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