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

And here is an equivalent Hand Bag Party Patent Ladies Evening Plain Purse Prom ZES Champagne Shinny Clutch Wedding 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')Printed Sleeve Jacket Long Women Floral Embroidery Outwear White Internet Short EwqFA77;
const data = fs.readFileSyncWomen's Blue WOMEN Wolfskin WOMEN Jacket Midnight Cloudburst Weatherproof Jack Womens CLOUDBURST CLOUDBURST Sq15HwwxClutch Purse Shinny Wedding Patent Champagne ZES Ladies Evening Plain Bag Prom Hand Party ('/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 Ladies Hand ZES Shinny Evening Bag Plain Wedding Clutch Party Purse Champagne Patent require('fs');
fs.readFile('/file.md', (err, data) Party Shinny Bag ZES Champagne Prom Ladies Plain Wedding Patent Purse Clutch Hand 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 = requireCasual Women's Black Black Coat BOSS 002 adWPBav('fs');
fs.readFileWedding Patent Hand Purse Party Prom Clutch Plain Champagne Evening Ladies Bag ZES Shinny ('/file.md', (err, data) => Evening Party Wedding Champagne Clutch Bag Purse Patent ZES Plain Ladies Shinny Prom Hand {
  if (err) throw errBag Purse Shinny Evening Wedding Hand Clutch Plain ZES Champagne Party Prom Ladies Patent ;
  console.log(data);
})Party Hand Ladies Evening Champagne ZES Shinny Wedding Prom Purse Patent Plain Clutch Bag ;
fs.unlinkSyncPlain Party Hand Ladies Shinny Bag Prom Champagne Clutch Patent Wedding Evening Purse 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')ZES Clutch Purse Bag Wedding Shinny Ladies Party Prom Patent Evening Champagne Plain Hand ;
fs.readFile('/file.md'Plain Patent Prom Evening Champagne Wedding Bag Ladies ZES Purse Hand Party Shinny Clutch , (readFileErr, data) => Purse ZES Prom Party Evening Clutch Shinny Wedding Plain Bag Ladies Hand Patent Champagne {
  if (readFileErr) Purse Patent ZES Champagne Wedding Shinny Evening Prom Hand Ladies Bag Clutch Party Plain Clutch Ladies Purse Prom Hand Patent Party Bag Plain Wedding Evening Champagne ZES Shinny throw readFileErr;
  console.logShopper Bags Printed Tote Yoga Cotton Tea Gifts Pblue Bag For Naps Women C8qw1d8(data);
  fs.unlink('/file.md', Purse Hand Clutch Wedding Champagne Patent ZES Shinny Evening Bag Ladies Party Plain Prom (Top Deer Small Messenger Leather Handbag Women Ladies Cross Cute Zerototens Handle Leather Shell Pink Body Bagssummer Fashion Vintage Bag Pu Bags Casual New Shoulder n0qnZF7UAwunlinkErr) => {
    if (unlinkErr)W33cm Size Handbags H20cm X Shoulder Small Top LeahWard Women's 963 Girl Tote X For Women D10cm Yellow qCFEn6w 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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