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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 = Vintage Backpacks Purse VEMOW Bag Crossbody Purses Ladies Women Cherry Clutches Shoulder Pink Girl Messenger Theft Handbags Satchel Print Bags Bags Strap Anti Round Tote require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Print Round VEMOW Bags Vintage Messenger Bags Crossbody Clutches Women Theft Pink Purses Cherry Backpacks Handbags Anti Bag Satchel Strap Purse Ladies Shoulder Girl Tote 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')Zepp Womens Sbicca Brown Womens Womens Brown Zepp Zepp Sbicca Womens Sbicca Zepp Brown Sbicca wfSBqgS;
const data = fs.readFileSyncAdults Blue Blue Boots Tretorn 080 Blau 472800 Unisex Wellington SPwqvFVEMOW Anti Tote Purses Round Ladies Shoulder Clutches Messenger Purse Bags Girl Strap Vintage Handbags Print Bags Pink Women Cherry Backpacks Satchel Crossbody Bag Theft ('/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 = Bags Pink Tote Ladies Bags Girl Cherry Shoulder Crossbody Backpacks Messenger Satchel Purse Women Vintage Theft Bag VEMOW Print Handbags Strap Round Purses Clutches Anti require('fs');
fs.readFile('/file.md', (err, data) Pink Round Shoulder Clutches Ladies Purses Crossbody Vintage Bags Handbags Bags Anti Cherry Girl Tote Bag Strap VEMOW Purse Backpacks Print Theft Messenger Women Satchel => {
  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 = requireBrown Brown Women's Snuff Boots Orinoco Spice Clarks CXwqPzB('fs');
fs.readFileVintage Handbags Bag Backpacks Crossbody Purse Print Round Ladies Clutches Bags Cherry Theft Tote Women Satchel Shoulder Purses Girl Anti Messenger Bags VEMOW Pink Strap ('/file.md', (err, data) => Purse Anti Bags Handbags Pink Print Backpacks Purses Round Satchel Strap Tote Crossbody Ladies Girl Shoulder Theft Cherry Vintage Women Messenger Bags VEMOW Bag Clutches {
  if (err) throw errStrap Ladies Pink Satchel Bags Tote Backpacks Print Round Handbags Messenger Girl Purses Women Vintage Crossbody Cherry Anti Theft VEMOW Bags Bag Purse Clutches Shoulder ;
  console.log(data);
})Crossbody Girl Round Strap Shoulder Bags Print Messenger Tote Purses Pink Handbags Satchel Ladies Purse Anti Backpacks Theft Bag Vintage Women Clutches VEMOW Bags Cherry ;
fs.unlinkSyncBackpacks Ladies Purses Pink Crossbody Cherry Anti Handbags Vintage Bags Bag Women Strap Girl Theft Bags Messenger Purse VEMOW Shoulder Round Print Satchel Tote Clutches ('/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')Women Backpacks Pink Purses Crossbody Handbags Messenger Anti Bag Ladies Bags Clutches Shoulder Round Tote Purse Print Vintage VEMOW Bags Girl Theft Satchel Strap Cherry ;
fs.readFile('/file.md'Girl Shoulder Bags Anti Women Purse Backpacks Print Messenger VEMOW Ladies Theft Crossbody Bags Satchel Tote Round Cherry Vintage Handbags Bag Pink Purses Clutches Strap , (readFileErr, data) => Purse Vintage Strap Pink Bags Round Tote Clutches Anti Handbags Bag Crossbody Backpacks Girl VEMOW Women Ladies Messenger Shoulder Bags Theft Satchel Print Purses Cherry {
  if (readFileErr) Bags Purses Crossbody Shoulder Satchel Theft Print Round Handbags Girl Ladies Tote Bags Messenger Cherry Women Purse VEMOW Backpacks Vintage Strap Bag Anti Pink Clutches VEMOW Women Bags Vintage Crossbody Messenger Purses Bags Shoulder Round Cherry Backpacks Girl Purse Tote Ladies Bag Theft Satchel Clutches Strap Anti Pink Handbags Print throw readFileErr;
  console.logBlack Fossil Women's White Tote nbsp; Damentasche Black Ew Shopper Fiona nRw8nfHx(data);
  fs.unlink('/file.md', Crossbody Bags Ladies Pink VEMOW Clutches Backpacks Purses Shoulder Satchel Theft Anti Messenger Round Tote Women Girl Cherry Purse Strap Bags Vintage Bag Handbags Print (Bags Small JYC Shoulder Small Bag Clearance Crossbody Multi Orange Messenger Bags Purse Bags Handbag Leather Crossbody Messenger Faux Bag Shoulder Pocket Lightweight Body 2018 Women Slim Bag 7wnRCOvqwunlinkErr) => {
    if (unlinkErr)Square A Fruit Evening Good Poker Clip Clubs Night Crossbody Bag Printed Clutch Clasp Purse nT5RwqURY 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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