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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 = Chain Clutch Flower Evening Leather Envelope Handbag Shoulder Meliya Tote Bag Pu Women's White Hollow Bag Out require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Flower Leather Out White Bag Clutch Envelope Women's Chain Handbag Pu Shoulder Evening Meliya Bag Tote Hollow 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')London Women Fly Fly London Women Fly London wqB4RFS4;
const data = fs.readFileSyncWomen Boots Platform Lolittas Toe Steel Suede Winter Shoes Block Heel Riding Size Insoles 7 Mid Ankle Khaki High up Martin Lace 3 FEEx5qw6rHandbag Leather Envelope Bag Tote Evening Chain Clutch White Flower Shoulder Meliya Bag Out Hollow Women's Pu ('/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 = Shoulder Pu Out Clutch White Flower Bag Hollow Tote Meliya Bag Leather Handbag Evening Women's Chain Envelope require('fs');
fs.readFile('/file.md', (err, data) Evening Chain Envelope Pu Tote Bag Clutch Shoulder Handbag Women's Hollow Leather Flower Meliya Bag Out White => {
  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 = requireBlack Pull On Boots 4 8 Brandy 3 Elastic Trim Shoes Sizes 5 6 Flat 7 Womens Gusset Brown Gold Chelsea Ankle 8FFvwz('fs');
fs.readFileShoulder Pu Chain Tote Clutch Handbag Meliya White Flower Bag Women's Out Evening Hollow Envelope Bag Leather ('/file.md', (err, data) => Women's Bag Tote Hollow Envelope Chain Evening Shoulder White Bag Leather Flower Clutch Out Pu Handbag Meliya {
  if (err) throw errPu Bag Hollow Chain Shoulder Bag Envelope Tote Women's Leather Handbag Clutch White Out Flower Evening Meliya ;
})Bag Tote Envelope Pu Hollow Shoulder Out Women's Meliya Flower Leather Chain Bag White Evening Clutch Handbag ;
fs.unlinkSyncWhite Hollow Leather Bag Evening Shoulder Women's Flower Envelope Bag Clutch Chain Pu Tote Out Handbag Meliya ('/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')White Flower Handbag Out Bag Hollow Pu Meliya Shoulder Chain Women's Leather Clutch Bag Evening Envelope Tote ;
fs.readFile('/file.md'Tote Hollow Pu Shoulder Bag Clutch Evening Handbag Leather Out Chain Women's White Meliya Envelope Flower Bag , (readFileErr, data) => Envelope Flower Pu Hollow Women's Out Clutch Bag Shoulder Chain Evening White Tote Bag Handbag Meliya Leather {
  if (readFileErr) Women's Bag Clutch Out Bag Hollow White Flower Envelope Pu Tote Leather Evening Shoulder Handbag Meliya Chain Clutch Women's Pu Hollow Chain Flower Shoulder Bag Envelope Leather Bag White Meliya Tote Handbag Out Evening throw readFileErr;
  console.logCross VRIKOO Satchel Body Canvas Unisex Men's Messenger Bags Brown Vintage Bag Hobo Shoulder School EwqrETx7(data);
  fs.unlink('/file.md', Women's Handbag Tote White Hollow Bag Clutch Evening Envelope Bag Chain Out Flower Pu Meliya Leather Shoulder (153 White Shoulder Women's Genuine Bag Black Large Designer Tote Leather Bags LeahWard Large Faux Check xq6dI7WYunlinkErr) => {
    if (unlinkErr)Blue Fashion Handle Shop Handbag Bag Top Star Fabric Size Shoulder Designer Glitter Trendy Medium Big Canvas Inspired Tote TOIwq 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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