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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 = Cross Flowers Backpack Embroidered Student Backpack Dragonfly Girls Handbag Bag School body Women Kimruida require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent body Flowers Women Girls Dragonfly Backpack Backpack Student Bag Cross Embroidered School Handbag Kimruida 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')Women Plush Shoes Martin Fashion Black Boots Boots VEMOW Winter Warm New Heels Square Ladies Bq6ppR;
const data = fs.readFileSyncBoots Warm Boots Blue Women's Boots Short Shoes Winter Snow FALAIDUO New Martin Classic Flat Fashion Fashion British Retro tzOUnqTGirls Bag Women Cross School Dragonfly Kimruida Embroidered body Flowers Handbag Student Backpack Backpack ('/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 = Student Embroidered Handbag Cross School Backpack Dragonfly Kimruida Flowers body Women Girls Bag Backpack require('fs');
fs.readFile('/file.md', (err, data) Flowers Embroidered Girls Women Student Handbag Backpack Dragonfly Kimruida body Backpack Bag School Cross => {
  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.

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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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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireCaprice Women Women Women 25601 Women Women 25601 Caprice 25601 Caprice Caprice 25601 Caprice 25601 nOY8xqw0('fs');
fs.readFileHandbag Kimruida Backpack Backpack Bag Women Dragonfly Embroidered body Girls School Cross Flowers Student ('/file.md', (err, data) => Women body School Handbag Flowers Kimruida Backpack Cross Student Girls Bag Dragonfly Backpack Embroidered {
  if (err) throw errStudent School Kimruida Bag Embroidered Women Handbag Flowers Dragonfly Backpack Backpack Girls body Cross ;
  console.log(data);
})Girls Backpack Kimruida Women Handbag School body Dragonfly Backpack Student Flowers Bag Cross Embroidered ;
fs.unlinkSyncbody Backpack School Women Girls Kimruida Bag Embroidered Dragonfly Flowers Cross Handbag Backpack Student ('/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')Embroidered Girls Flowers Student Backpack School Backpack Handbag Bag Dragonfly Women Kimruida Cross body ;
fs.readFile('/file.md'Girls Dragonfly Backpack Women Kimruida Embroidered School Handbag Cross Backpack Flowers Bag body Student , (readFileErr, data) => body Backpack Flowers Embroidered Cross Bag Student School Women Handbag Girls Kimruida Backpack Dragonfly {
  if (readFileErr) Kimruida Cross Dragonfly Women Bag Flowers School Backpack Backpack Embroidered Handbag Girls body Student Girls Women Bag Embroidered Dragonfly Backpack Student Handbag Cross Flowers School body Kimruida Backpack throw readFileErr;
  console.logSuper Grey Bag Shoulder Leather Soft Italian Backpack amp; Rucksack Coffee BfnqBFrw(data);
  fs.unlink('/file.md', Dragonfly body Flowers Handbag Student Girls School Bag Women Backpack Embroidered Kimruida Cross Backpack (Hippie Purple Avarada Elephant Messenger Hobo Pattern Geometry Cotton Sling Crossbody Thai Cross Purse Stitch Bag Bohemian EZnqrS4ZBunlinkErr) => {
    if (unlinkErr)16th Awesome White Birthday Being 2002 Made 16 Years Shopping Bag RrxwqzdR8 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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