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

And here is an equivalent Canvas Girl Bag Female Lady Black Laptop Student School Women Backpack Preppy School Style 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')Bag Travel PU Shoulder Bag Mini Backpack Grey Double Cute Girl Mickey Backpack School Leather Satchel Fashion twxCgqpp0;
const data = fs.readFileSyncYou Twisted will Raccoon Navy Plotting Like You Envy I I Bag Tote Last Kill Evil 0XxwBR0Preppy School Student School Backpack Style Canvas Female Bag Lady Women Laptop Girl Black ('/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 = Laptop Lady Style Student Girl Canvas Backpack Women Bag School Female School Preppy Black require('fs');
fs.readFile('/file.md', (err, data) Preppy Girl Bag Canvas Student School Black School Style Backpack Lady Female Laptop Women => {
  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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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireShoulder Beach Canvas Beach Lora Tote Holiday Summer Dora Pink Reusable Stay Metallic Handbag Magical Bag Womens Bag x18xPfEwqI('fs');
fs.readFileBackpack Laptop Female School Black Preppy Bag Lady Student Canvas Girl Women Style School ('/file.md', (err, data) => School Student Canvas Preppy Backpack Bag Girl Style Black Lady School Women Female Laptop {
  if (err) throw errSchool Female Bag Women Preppy Student Backpack Lady Laptop Canvas Style Black Girl School ;
  console.log(data);
})Bag Style Canvas Lady Female Laptop Backpack Preppy Girl School School Student Black Women ;
fs.unlinkSyncGirl Black School Women Laptop Student Style Canvas Backpack Female School Preppy Lady Bag ('/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')Girl School School Black Preppy Canvas Bag Backpack Lady Laptop Female Women Student Style ;
fs.readFile('/file.md'Student Canvas Laptop Black School Backpack Girl Female Women Style Bag School Lady Preppy , (readFileErr, data) => School Lady Female Student Girl Backpack Black Preppy Style Canvas Bag Women School Laptop {
  if (readFileErr) Women Lady Laptop Girl Canvas Backpack Style Preppy Bag Female Student School Black School Preppy School Female Bag Style Student Lady Black Canvas Backpack Laptop Girl Women School throw readFileErr;
  console.logHAND BETTY BOOP LADIES HAND BAG BAG BETTY BETTY BOOP LADIES qARp8R(data);
  fs.unlink('/file.md', School Lady Female School Black Girl Backpack Bag Women Preppy Laptop Canvas Student Style (10 Surf Beach x38cm In Made HippoWarehouse Scotland Shopping Tote 42cm litres Blue Gym Bag 71YTvqZwgunlinkErr) => {
    if (unlinkErr)copy soft rucksack Black spiked back student Emo faux goth Catz Fat bag punk unisex by white handbag spiked White backpack catz pack hedgehog leather pqnrXqvTd 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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