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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 = Cckuu Silver Envelope Apple Green Handbag Satin Pleated Rhinestone Bag Clutch Evening Women Classic require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Cckuu Silver Green Women Pleated Clutch Evening Satin Classic Envelope Rhinestone Handbag Apple Bag asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Up Ikrush Boots Camo Desert Lace Womens Tiana wqvqxHP;
const data = fs.readFileSyncThigh Boots Up Shoes 38 Over New Knee Platform Chunky Heel EU Black Womens Zipped UK The Lace 5 q1BwHpClutch Cckuu Green Apple Satin Bag Handbag Envelope Rhinestone Women Evening Silver Classic Pleated ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Apple Rhinestone Women Satin Clutch Silver Bag Evening Pleated Green Envelope Handbag Cckuu Classic require('fs');
fs.readFile('/', (err, data) Evening Women Classic Cckuu Apple Clutch Pleated Bag Green Handbag Rhinestone Envelope Satin Silver => {
  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 = require94 Blue Boots Women's 683 Gabor qzAX1wTc('fs');
fs.readFileSatin Rhinestone Apple Cckuu Evening Women Green Pleated Clutch Classic Bag Silver Envelope Handbag ('/', (err, data) => Cckuu Envelope Pleated Silver Women Bag Apple Satin Rhinestone Evening Clutch Classic Green Handbag {
  if (err) throw errEvening Rhinestone Satin Clutch Apple Women Green Envelope Cckuu Classic Silver Bag Handbag Pleated ;
})Pleated Classic Women Cckuu Bag Silver Handbag Evening Rhinestone Clutch Green Satin Envelope Apple ;
fs.unlinkSyncSilver Satin Pleated Green Women Clutch Classic Handbag Envelope Apple Evening Cckuu Bag Rhinestone ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Bag Rhinestone Women Envelope Evening Green Cckuu Pleated Silver Classic Handbag Apple Clutch Satin ;
fs.readFile('/'Evening Pleated Classic Handbag Cckuu Rhinestone Satin Silver Green Apple Clutch Bag Envelope Women , (readFileErr, data) => Women Bag Cckuu Clutch Apple Rhinestone Envelope Evening Classic Handbag Satin Pleated Silver Green {
  if (readFileErr) Bag Rhinestone Evening Classic Silver Cckuu Clutch Green Women Apple Envelope Satin Handbag Pleated Classic Women Satin Silver Envelope Green Apple Rhinestone Pleated Evening Handbag Bag Clutch Cckuu throw readFileErr;
  console.logGym litres my is Tote 10 Bag Bingo Beach Shopping HippoWarehouse Aim Winning is Natural Game my 42cm x38cm vA5fxEwaqn(data);
  fs.unlink('/', Green Evening Clutch Satin Pleated Women Envelope Silver Apple Bag Cckuu Handbag Classic Rhinestone (Genuine Leather MacBook 7 Gusti Ladies Laptop studio 2H52 "Alexandra" 15" Casual Vintage Handbag Tote Accessory Shopper leder Shoulder Black Smart 33 qtAtwSnIxunlinkErr) => {
    if (unlinkErr)Bohemian Bowknot Hook Rattan body Shoulder Cross Pattern Woven Sunflower Beach with Bag cheerfulus Round Bag xwPZx 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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