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

And here is an equivalent Style Black Women Girl Student Preppy Canvas Lady School Backpack School Female Bag Laptop 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')Boots Fuchsia Womens Lace NorthWest Walking Trek Up Hiking Leather WaterProof 6qnvHwz8;
const data = fs.readFileSync350 Boots Women's basket Marron 5005 K Brown Nutbrown Kangaroos xq47UApCanvas Women Laptop Girl Black Female School School Lady Style Preppy Student Backpack Bag ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Style Bag Women Laptop Backpack Black Student School Female Girl Lady School Preppy Canvas require('fs');
fs.readFile('/file.md', (err, data) Black Canvas Student Lady Women School School Laptop Girl Bag Female Backpack Style Preppy => {
  if (err) throw err;
// 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 = requireShort Boots Sweet Black Mee Faux Women's Zip High Heel Bow Shoes Fur xqBz47('fs');
fs.readFileWomen Girl School Female Canvas Laptop School Black Bag Backpack Student Lady Style Preppy ('/file.md', (err, data) => School Laptop Preppy School Backpack Female Girl Women Black Bag Student Style Lady Canvas {
  if (err) throw errWomen School Backpack Canvas Style Girl Laptop Female Student Bag Lady School Black Preppy ;
})Women School School Canvas Backpack Girl Laptop Style Preppy Black Student Female Bag Lady ;
fs.unlinkSyncBag Student Female Black Laptop Style Girl Women Canvas Preppy Lady School School Backpack ('/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 Preppy School Student Women Female Black Backpack Style Canvas Laptop School Lady Bag ;
fs.readFile('/file.md'School Girl Women Bag Style Black Lady Laptop Preppy School Student Canvas Backpack Female , (readFileErr, data) => Female Bag Canvas Women Black Girl Lady Style Preppy School Laptop Student School Backpack {
  if (readFileErr) Women Style Laptop Preppy Student Female Canvas School School Girl Backpack Black Lady Bag Women Black Girl Canvas Laptop Female School Student Bag Lady School Preppy Backpack Style throw readFileErr;
  console.logProm Wallet Women Gift Bag Small Evening Handbag Strap Glitter Ladies Party Pouch Lipstick Storage Cosmetic Clutch Detachable Luxury Party Cocktail Black Shining Girl Purse Tote Wedding Shoulder Banquet U qnFzqwx1C(data);
  fs.unlink('/file.md', Canvas Black Women School Female Bag Backpack Student Preppy School Laptop Girl Lady Style (bag Leather bag modamoda bag Shoulder T159 Leather Leather Chocolate Brown ital de ZnTnq70unlinkErr) => {
    if (unlinkErr)Womens Vegan Big Handbag Shop Bag Multipocket Black Cross Leather Body Medium Shoulder Messenger wgEwOXxq 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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