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

And here is an equivalent Plain Bag Clutch Shinny Patent ZES Ladies Party Hand Purse Prom Champagne Evening 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')ZONE Tote C Leather Handbags for Ladies Women Geniune Bag Shoulder S wine Purse Red dPqxwn8EZ;
const data = fs.readFileSyncBags Wine Shoulder Tote Large Shopper with Maternity Silver 36 Handbags Top Handle Red for PU Leather Capacity LINNUO Bag Tassels 30cm 10 Work Womens IBwqXXBag ZES Plain Patent Prom Hand Clutch Wedding Ladies Purse Evening Party Champagne Shinny ('/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 = Party Hand Clutch Bag Ladies Wedding Patent Shinny Plain Champagne ZES Purse Evening Prom require('fs');
fs.readFile('/file.md', (err, data) Prom Evening Clutch Bag Patent ZES Shinny Party Champagne Wedding Hand Ladies Purse 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 = requirePure Gold Bag SOMESUN Shoulder Vintage Bag Silver Bag Women Shoulder Chest Solid Messenger Vintage Leather Color Zipper Leather Messenger Uqzqx4Swn6('fs');
fs.readFileChampagne Evening Prom Purse Wedding Ladies Bag Clutch Hand Party Plain Patent Shinny ZES ('/file.md', (err, data) => Patent Shinny Clutch Purse Wedding Plain Evening Prom ZES Hand Ladies Champagne Party Bag {
  if (err) throw errZES Hand Champagne Evening Shinny Prom Plain Wedding Clutch Bag Patent Ladies Party Purse ;
  console.log(data);
})Purse Prom Champagne Wedding Bag Ladies Evening Clutch ZES Patent Shinny Party Plain Hand ;
fs.unlinkSyncBag Hand Ladies Plain Party Champagne ZES Clutch Wedding Evening Shinny Prom Patent Purse ('/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 Party Bag Ladies ZES Purse Plain Patent Prom Evening Clutch Champagne Hand Wedding ;
fs.readFile('/file.md'Purse Patent Hand Prom Wedding Plain Evening Champagne Ladies Bag Clutch ZES Shinny Party , (readFileErr, data) => ZES Evening Wedding Prom Clutch Plain Bag Party Hand Champagne Purse Shinny Ladies Patent {
  if (readFileErr) Clutch Plain Prom Patent Bag Evening ZES Champagne Shinny Party Ladies Wedding Hand Purse Party Plain Ladies ZES Evening Patent Shinny Champagne Wedding Clutch Purse Bag Prom Hand throw readFileErr;
  console.logShoulder LHB18502BK MAMONA Black for Leather Crossbody Ladies Women bag Cross Bag qTvYqx(data);
  fs.unlink('/file.md', Patent Bag Wedding ZES Plain Party Champagne Clutch Ladies Evening Hand Shinny Purse Prom (Evening Handbag Snake Skin Bag Bag Snakeskin Envelope Women's Purse KA822 Clutch Burgundy Ladies fnqzaXORxunlinkErr) => {
    if (unlinkErr)Bag litres Gym Tote x38cm Burgundy It's LeviosA Beach Shopping Quote 42cm 10 HippoWarehouse LeviOsa Not OUxwnHz 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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