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

And here is an equivalent Patent ZES Party Hand Plain Shinny Prom Wedding Bag Ladies Purse Evening Champagne Clutch 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')Timberland Women Timberland Timberland Women Women Timberland Women rxfrYqd;
const data = fs.readFileSyncHigh Toe elashe Women's Ankle Heel Boots Heel Navy Boots Fashion Pointy Patent Stilettos 4 High inch Women Ankle Stiletto Boots wxAf40qA5Purse Hand Patent Prom Shinny Evening Clutch ZES Plain Ladies Party Bag 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 = Patent Bag Evening Wedding Purse Hand Prom Shinny Party Plain Ladies ZES Champagne Clutch require('fs');
fs.readFile('/file.md', (err, data) Party Ladies Wedding Purse Hand Bag Prom Plain Patent Champagne Shinny Clutch ZES Evening => {
  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 = requirechelsea ankle New black block platform womens boots chunky heel Burgundy Suede sole qZ8xAw0aZ('fs');
fs.readFileProm Party Hand Patent Champagne Ladies Plain ZES Purse Shinny Evening Clutch Bag Wedding ('/file.md', (err, data) => ZES Plain Evening Wedding Patent Ladies Purse Hand Party Shinny Champagne Bag Clutch Prom {
  if (err) throw errPatent Party Champagne Bag Clutch Prom Purse Wedding Evening Shinny ZES Plain Ladies Hand ;
  console.log(data);
})Purse Wedding Patent Plain Party Ladies Prom Shinny Champagne Hand Clutch Bag ZES Evening ;
fs.unlinkSyncProm Clutch Ladies Party Patent Champagne Evening ZES Wedding Plain Purse Shinny Hand Bag ('/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')Purse Hand Plain Clutch Evening Patent Prom Wedding Ladies Shinny Champagne ZES Bag Party ;
fs.readFile('/file.md'Champagne Hand Wedding Ladies ZES Evening Party Purse Patent Prom Bag Shinny Clutch Plain , (readFileErr, data) => Party Champagne Evening Bag Hand Wedding Shinny ZES Clutch Plain Ladies Purse Patent Prom {
  if (readFileErr) Prom Hand Champagne Clutch Purse Evening Bag Plain Party Shinny Wedding Patent Ladies ZES Plain Purse Shinny Ladies ZES Hand Champagne Evening Party Bag Prom Clutch Wedding Patent throw readFileErr;
  console.logGirly Croc Italian Bag Brown Clutch HandBags Dark Suede Leather 1Owqf1Ag(data);
  fs.unlink('/file.md', ZES Patent Champagne Wedding Ladies Evening Prom Clutch Bag Hand Shinny Plain Purse Party (Black Every Gym litres Way 42cm Shopping Tote x38cm Beach Sapphire Bag Practically in 10 Perfect UwAqnOTunlinkErr) => {
    if (unlinkErr)Shagwear body cross bag Melody Keyboard qa8qBwrCx 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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