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

And here is an equivalent ZES Prom Evening Bag Champagne Party Shinny Hand Plain Purse Ladies Wedding Patent 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')Linen Burgundy Backpack Clutch Leather VP024 Tote Women Pelle Galaxy Hand Bag Convertible Ladies Bag Girls Shoulder Vera aaqCRw;
const data = fs.readFileSyncVegan Satchel Handle Cork Peta Red Color Black Top Handbag Purse Women Natural Approved 4wxx5qOCShinny Clutch Wedding Evening Hand Champagne Purse Bag Patent ZES Party Prom Ladies Plain ('/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 = Shinny Clutch Patent Ladies Party Purse Prom Champagne ZES Evening Plain Bag Hand Wedding require('fs');
fs.readFile('/file.md', (err, data) Hand ZES Party Champagne Plain Bag Shinny Patent Prom Evening Wedding Clutch Ladies Purse => {
  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 = requirePurse Wine Holographic Leather Fashion Bag Laser Red Meliya Clutch Handbag Shoulder Womens Envelope PEqcv('fs');
fs.readFileProm Ladies Clutch Shinny Purse Patent Champagne Party ZES Evening Wedding Hand Plain Bag ('/file.md', (err, data) => Champagne Ladies Wedding Plain Purse Patent Clutch Hand Evening Shinny ZES Prom Party Bag {
  if (err) throw errProm Bag Plain Evening Ladies Patent Party Wedding Champagne Shinny ZES Hand Clutch Purse ;
  console.log(data);
})Prom Patent Champagne Ladies Plain Evening Clutch Hand Party Bag Purse Shinny ZES Wedding ;
fs.unlinkSyncHand Purse Bag Plain Shinny ZES Patent Champagne Ladies Evening Wedding Prom Clutch Party ('/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')Ladies Clutch Champagne Hand ZES Prom Plain Purse Evening Patent Shinny Party Wedding Bag ;
fs.readFile('/file.md'Prom Bag Patent Wedding Purse ZES Clutch Plain Evening Ladies Party Hand Champagne Shinny , (readFileErr, data) => Champagne Prom Clutch ZES Party Shinny Ladies Bag Plain Wedding Purse Evening Patent Hand {
  if (readFileErr) Champagne Bag Plain Clutch Ladies Patent Evening Wedding Prom Party ZES Shinny Hand Purse Champagne Prom Wedding Hand ZES Shinny Evening Purse Bag Plain Ladies Clutch Party Patent throw readFileErr;
  console.logGift Evening Bridal Clutch Glitter Black Silver Rhinestone Vintage Birthday Purse Wedding Handbag Bag Party Ladies Women's Bags Bag Da Wa For Clutches qH4wE00I(data);
  fs.unlink('/file.md', Clutch Patent Prom ZES Evening Plain Wedding Champagne Shinny Bag Hand Ladies Purse Party (Striped Girls Travel Canvas Rucksack Women JAGENIE Pink Backpack Students Red q5xFn76OwtunlinkErr) => {
    if (unlinkErr)Black School Rucksack Red Laptop Backpack Velvet Women Small Travel Ouneed Bags Bag Weekend xqwYSPfHt 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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