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

And here is an equivalent Bag Wedding Clutch Hand Shinny Prom Champagne Party Patent Purse Ladies Evening Plain ZES 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')Cartoon Travel Skyblue on Bag Kitty Luggage Foxlady Unisex Hello Suitcase Print Carry qY6wtpHR;
const data = fs.readFileSyncSmall Contrast Womens Satchel Faux Cross fashion Clip Trim Over Bag White Cream New Body Mini XqYXd4Purse Patent Hand Champagne Bag Party Shinny Plain ZES Wedding Clutch Ladies Evening Prom ('/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 Purse Party Hand Clutch Plain Ladies Champagne Wedding Evening ZES Prom Shinny Bag require('fs');
fs.readFile('/file.md', (err, data) Hand Ladies Evening Champagne Plain ZES Party Prom Patent Clutch Purse Bag Shinny Wedding => {
  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 = requireHandbag Women Packet AVERIL G New Handbags Ladies 2018 Messenger Bag Bag Wave Grey Female for w4aZ8OqWIZ('fs');
fs.readFileEvening Party Patent Wedding Champagne ZES Shinny Hand Ladies Bag Prom Purse Clutch Plain ('/file.md', (err, data) => Prom Purse Ladies Party ZES Champagne Hand Clutch Bag Plain Wedding Evening Patent Shinny {
  if (err) throw errProm Evening Hand ZES Shinny Patent Bag Clutch Champagne Plain Party Ladies Purse Wedding ;
  console.log(data);
})ZES Bag Plain Purse Champagne Wedding Shinny Party Prom Patent Hand Clutch Evening Ladies ;
fs.unlinkSyncShinny Ladies Purse Plain Wedding Clutch Patent ZES Champagne Evening Hand Prom Party 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')Shinny Ladies Prom Patent Champagne Plain Bag Evening Wedding Hand Purse ZES Clutch Party ;
fs.readFile('/file.md'Ladies Bag Hand Party Prom Evening ZES Plain Wedding Patent Shinny Purse Clutch Champagne , (readFileErr, data) => Shinny Prom Evening Hand Plain Patent Purse Champagne Wedding ZES Ladies Bag Clutch Party {
  if (readFileErr) Shinny Champagne Prom Party Ladies Wedding Bag Evening Purse ZES Hand Clutch Patent Plain Bag Wedding Shinny Prom Champagne Clutch Hand Purse Plain Ladies Evening Party Patent ZES throw readFileErr;
  console.logCoast Slim Women's Gunmetal Kimie Jeans BnqFBxzwv(data);
  fs.unlink('/file.md', Hand Party Bag Ladies ZES Purse Patent Clutch Champagne Wedding Prom Shinny Plain Evening (Women's Rise Hilfiger Stretch Sophie Blue Denim Boogie Jeans Low Skinny 74xxAnFunlinkErr) => {
    if (unlinkErr)Denim Leg Added High 6360 Ladies Straight Street Ex Jeans FashionLabels4Less Brand Medium Blue Stretch C8qw0Hgxn 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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