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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 = Block Classic Ankle Women's Shoes Cavalier Braided Biker Booty Blue Angkorly Heel cm Buttons Big Fashion 3 Boots require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Cavalier Fashion Women's Block Blue cm Ankle Boots Biker Shoes Booty Classic Buttons Braided Angkorly 3 Heel Big 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')SWANKYSWANS Blue Bag Womens Max Dark Shoulder wAxAYBUq;
const data = fs.readFileSyncHot Chocolate Bag Tote Ramen Design Women's rrSzwUqnBig Heel Shoes 3 cm Classic Blue Boots Booty Buttons Fashion Women's Cavalier Ankle Block Biker Braided Angkorly ('/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 = Fashion Buttons Ankle Boots Big Blue Braided Cavalier 3 Classic Block Biker Angkorly cm Shoes Women's Booty Heel require('fs');
fs.readFile('/file.md', (err, data) Blue Fashion Boots Braided Cavalier Booty Big Biker Buttons Ankle Heel Shoes Women's Angkorly Block 3 cm Classic => {
  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 = requireBag Handbag Genuine Protective Leather LiaTalia Italian Cross Body with Shoulder Storage wvYF4dxq6('fs');
fs.readFileAnkle cm Shoes Buttons Block Blue Biker Angkorly Big Braided Booty Boots Heel Classic Women's Cavalier Fashion 3 ('/file.md', (err, data) => Buttons Booty 3 Cavalier Angkorly Women's Blue Classic Shoes cm Braided Ankle Biker Big Boots Block Heel Fashion {
  if (err) throw err3 Heel Biker Cavalier Boots Booty cm Fashion Angkorly Blue Ankle Women's Buttons Big Block Braided Classic Shoes ;
  console.log(data);
})Booty Biker Boots Big Buttons Braided Fashion Cavalier Angkorly Shoes Block Classic Heel cm Blue 3 Ankle Women's ;
fs.unlinkSyncAngkorly Booty Buttons Big 3 Shoes Boots Block Women's Braided Classic Fashion Ankle cm Biker Blue Cavalier Heel ('/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')Block Angkorly Biker Classic Booty cm Boots Big 3 Braided Buttons Ankle Shoes Heel Blue Women's Cavalier Fashion ;
fs.readFile('/file.md'Braided Cavalier cm Angkorly Ankle Classic Shoes Buttons Block Big 3 Biker Women's Fashion Booty Blue Boots Heel , (readFileErr, data) => Braided Women's Booty Big Fashion Biker Blue Angkorly Buttons cm Cavalier 3 Ankle Block Classic Heel Shoes Boots {
  if (readFileErr) Ankle Heel Angkorly 3 Big Women's Biker Buttons Blue Shoes Block Classic cm Fashion Booty Braided Cavalier Boots 3 Block Blue Women's Boots Braided Angkorly Fashion Ankle Biker Classic cm Shoes Cavalier Booty Buttons Big Heel throw readFileErr;
  console.logShoes black Boots Sexy Slim Stretch Knee High Faux Boots High 2 Heels DAYSEVENTH High Ladies X7q11Z6(data);
  fs.unlink('/file.md', Blue Classic Heel Shoes Boots Block Cavalier Booty Braided Big Biker Women's Angkorly Buttons Ankle 3 cm Fashion (Jet Black Frost Adults' Boots 5001 K KangaROOS Black Snow Unisex qBw0Y0unlinkErr) => {
    if (unlinkErr)Rieker Women Women Rieker Rieker Women Rieker Women Women Rieker Women Rieker Women Rieker US5nH 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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