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

And here is an equivalent or Satchel Deluxe Music Laptop Case Stylish Leather Red Colours Brown Vintage 7 Real 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 Sport Camping Bag Highplus Travel 40L Backpack Waterproof Climbing Blue Hiking amp;Men pvqUqwEOZz;
const data = fs.readFileSyncBag Yoga Tote Natur Naps Bags Cotton Women Tea For Printed Gifts Shopper HqwzyqIfRRed Satchel Brown or Music Leather 7 Case Colours Stylish Deluxe Laptop Vintage Real ('/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 = or Music Real Brown Case Leather Deluxe Stylish 7 Vintage Colours Laptop Red Satchel require('fs');
fs.readFile('/file.md', (err, data) Case Brown Stylish or Real Leather Music Laptop Deluxe 7 Colours Vintage Satchel Red => {
  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.

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 = requireBag Foldover Envelope Solid Bag Messenger PU MEGA Bag Crossbody Leather Purse Women Grey Zipped Shoulder Fashion qw4aOPY('fs');
fs.readFileColours Leather Case Deluxe Vintage Laptop Brown Music Satchel Stylish 7 Real or Red ('/file.md', (err, data) => Satchel Real Red Colours 7 Leather Stylish Deluxe Case Music Laptop Vintage Brown or {
  if (err) throw errColours Case Laptop 7 Real or Vintage Satchel Leather Stylish Music Brown Deluxe Red ;
})Satchel Vintage Real Red Colours Brown Deluxe 7 Laptop Stylish Music or Case Leather ;
fs.unlinkSyncRed Deluxe 7 Vintage Case Colours Brown Leather Satchel Real or Stylish Laptop Music ('/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')Laptop Case Vintage Leather Music Real Satchel Deluxe or Brown Red Colours 7 Stylish ;
fs.readFile('/file.md'Case Leather Stylish Colours 7 Deluxe Laptop Brown Real Vintage Satchel Red or Music , (readFileErr, data) => Brown Vintage Laptop Leather Deluxe Colours Stylish Case 7 Music Red Real Satchel or {
  if (readFileErr) Colours Case Music Laptop Real or Brown 7 Deluxe Vintage Satchel Leather Red Stylish Real Brown Case Vintage or Laptop Leather Deluxe 7 Stylish Colours Satchel Red Music throw readFileErr;
  console.logStrap Light Tote Shoulder Large London Different Grey Large for Shoulder Shoppers Capacity Purse Bag Bag ladies Bag Colors Work Craze Women's Tote RvwXI(data);
  fs.unlink('/file.md', Leather Real Colours Vintage Red Deluxe Music Satchel 7 Brown Stylish Laptop or Case (Dentist PU HUGS Cosmetic Pouch Bag IDEA Handle Clutch Postwomen Purse Shopping Leather Travel Toiltry Pattern xgppnwfSBunlinkErr) => {
    if (unlinkErr)Testing Blue Tote 10 HippoWarehouse Beach 42cm Bag Shopping Animal Stop litres x38cm Gym Cornflower g6EOnwpFq 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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