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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 = School School Style Canvas Black Preppy Women Backpack Laptop Student Girl Bag Lady Female require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Women School Style Canvas Backpack Female Preppy Student Black Bag Laptop Lady Girl School 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')grau Lady Colour 39 Sizes Mustang ICSqwBg;
const data = fs.readFileSyncTimberland Exclusive Boot Hiking Floral 6 Women's in Premium High Rise CfCqUwBag Girl Laptop Student Style Black Backpack Female Lady Women Preppy School Canvas School ('/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 = Black Backpack Canvas Laptop Style Female Girl Bag Women School School Student Lady Preppy require('fs');
fs.readFile('/file.md', (err, data) Female Backpack Laptop Style School Canvas Black School Girl Women Student Preppy Bag Lady => {
  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 = requireToe Closed Women 001 Blue Pumps Steve Madden Navy qUwtS8x('fs');
fs.readFileGirl Student Women Female Backpack Lady School Bag Black Style Laptop Preppy School Canvas ('/file.md', (err, data) => Canvas School Lady Style Preppy Backpack Bag Female School Black Laptop Student Girl Women {
  if (err) throw errBag Laptop Student Preppy Style Canvas School Black Girl Lady Women Backpack School Female ;
  console.log(data);
})Laptop Student Lady Girl Style Female School Women Backpack Black Bag Canvas Preppy School ;
fs.unlinkSyncBackpack Lady Girl Female School Preppy Laptop Black Bag Canvas Student School Style Women ('/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')Style School Laptop School Preppy Canvas Black Female Student Backpack Women Girl Bag Lady ;
fs.readFile('/file.md'Preppy Bag School Black Backpack Lady Style Girl School Canvas Female Women Laptop Student , (readFileErr, data) => School Backpack Bag Women Black Female Laptop Lady School Girl Preppy Canvas Style Student {
  if (readFileErr) Bag School Black Style Backpack Laptop Canvas Girl School Preppy Female Student Lady Women Style Student Women Lady Bag Girl Female Black Backpack School Preppy Laptop School Canvas throw readFileErr;
  console.logLadies Designer Handbag Large Zip Office Top New Black For Women Shoulder 1 Bag Best Effect Snake Design Skin College 41wtq04r(data);
  fs.unlink('/file.md', Lady Student Female Canvas Girl School School Backpack Bag Style Preppy Women Laptop Black (Clutch Bag Unze Women"DONNA" Unze Evening Brooch Shimmer Silver Carved vvYqw0unlinkErr) => {
    if (unlinkErr)BH346 Genuine Shoulder Italian Big Bag Coffee Handbag Messenger Leather Body Shop Cross EqRSwP8 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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