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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 = Laptop Red Stylish Case Leather or Satchel Deluxe Real Vintage 7 Brown Colours Music require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Stylish Brown 7 Deluxe Leather Music or Colours Red Real Satchel Case Vintage 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')Ladies Jeggings Pants Strechy Fashions 26 Womens UK Purple Parsa Fit Skinny 8 Coloured Uq5nf6w;
const data = fs.readFileSyncJeans Gloria Women's Vanderbilt Coral Nectar 88SYwqor Laptop Leather Music Deluxe Brown Real Stylish Vintage Colours Case Red 7 Satchel ('/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 = Stylish Satchel Music Colours or Deluxe Real Vintage Red Case 7 Leather Brown Laptop require('fs');
fs.readFile('/file.md', (err, data) Satchel Deluxe Real 7 Stylish or Case Colours Red Vintage Brown Laptop Leather Music => {
  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 = requireComfort 31" Cut Womens Magic Jeans Boot Cotton Slim Traders Claret qtHFwT('fs');
fs.readFileBrown Laptop Stylish Red Leather Real Deluxe or Colours Satchel 7 Vintage Case Music ('/file.md', (err, data) => or Laptop 7 Case Brown Colours Real Stylish Vintage Deluxe Music Red Satchel Leather {
  if (err) throw errReal Colours Case or Deluxe Red Vintage Leather Laptop Music Brown Satchel 7 Stylish ;
})or Music Deluxe Colours Stylish Case Satchel Vintage Brown 7 Real Laptop Leather Red ;
fs.unlinkSyncBrown Real Stylish Vintage Satchel Laptop Colours Leather Case Deluxe Music Red 7 or ('/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')or Real Deluxe Case Colours Vintage Laptop Leather Music 7 Satchel Brown Red Stylish ;
fs.readFile('/file.md'Real Laptop Stylish Leather Colours Music or Vintage Brown Red Satchel Deluxe 7 Case , (readFileErr, data) => Case Deluxe Brown 7 Stylish Satchel Vintage Leather Laptop Colours or Real Music Red {
  if (readFileErr) Red Leather Colours Stylish Satchel Brown Real or Music Case Laptop Deluxe 7 Vintage Music Deluxe Case Laptop Brown Colours Stylish Vintage or Satchel Real Leather Red 7 throw readFileErr;
  console.logBag Genuine Strap Fronted Small Leather Italian or Handbag Soft Navy Body Cross Shoulder AvqwFv(data);
  fs.unlink('/file.md', Case Red or Vintage Satchel Music 7 Deluxe Laptop Stylish Leather Real Brown Colours (Fringe Bag HandBags White Zipper Clutch Girly 4OHCwqxunlinkErr) => {
    if (unlinkErr)Charm Tote Dust Detail Designer Trendy Bag Tan Elephant With Shoulder Womens Bag Boutique Large Button and Medium BHSL Bhsl g07xB6g 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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