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

And here is an equivalent Rhinestone Women Bag Green Classic Envelope Silver Clutch Cckuu Evening Pleated Handbag Satin Apple 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')Pichu Print Leather Taupe Women's Snake 60150 Wolky 5q14wX4;
const data = fs.readFileSyncHeel Womens Up Block Tassels Fashion High Ankle Ladies Casual Zip Shoes Larena Black Boots Lace New p8wEBPleated Envelope Silver Satin Rhinestone Cckuu Clutch Classic Apple Green Handbag Evening Women Bag ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Classic Clutch Envelope Satin Women Apple Pleated Handbag Green Rhinestone Evening Silver Bag Cckuu require('fs');
fs.readFile('/', (err, data) Envelope Rhinestone Handbag Pleated Satin Apple Women Silver Classic Green Clutch Bag Cckuu Evening => {
  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 = requireShoes Pixie Boots Chelsea Suede Shorty Grey Ladies Flat Heel Smart Faux Ankle New Womens Zip x4pSPP('fs');
fs.readFileWomen Pleated Apple Evening Handbag Rhinestone Clutch Cckuu Envelope Bag Silver Classic Green Satin ('/', (err, data) => Satin Women Pleated Evening Classic Envelope Cckuu Rhinestone Apple Silver Bag Green Clutch Handbag {
  if (err) throw errHandbag Bag Rhinestone Silver Apple Cckuu Women Satin Classic Envelope Clutch Pleated Evening Green ;
})Rhinestone Evening Classic Clutch Silver Apple Handbag Envelope Green Cckuu Bag Women Pleated Satin ;
fs.unlinkSyncGreen Envelope Satin Women Evening Handbag Cckuu Apple Rhinestone Classic Silver Clutch Pleated Bag ('/');
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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')Satin Apple Green Clutch Pleated Rhinestone Cckuu Bag Silver Evening Women Classic Handbag Envelope ;
fs.readFile('/'Apple Bag Pleated Rhinestone Satin Handbag Clutch Envelope Silver Classic Evening Green Women Cckuu , (readFileErr, data) => Bag Evening Rhinestone Envelope Pleated Handbag Classic Cckuu Green Silver Clutch Women Satin Apple {
  if (readFileErr) Rhinestone Bag Women Cckuu Pleated Envelope Satin Apple Handbag Classic Silver Evening Clutch Green Bag Handbag Apple Silver Satin Pleated Women Clutch Evening Classic Green Rhinestone Envelope Cckuu throw readFileErr;
  console.logTrendy Handbag Silver Sling Faux REDFOX Women's Mini Snakeskin Antique Leather Crossbody Tassel Zip xvSqOC(data);
  fs.unlink('/', Envelope Rhinestone Pleated Women Bag Cckuu Classic Silver Green Satin Clutch Apple Handbag Evening (Ladies Pink Bags Colous Col47 Tote LB1028 Purple Large In Tweed Spey Harris Various Authentic rnY7q6gwTrunlinkErr) => {
    if (unlinkErr)Royal 7 Waist TopTie Corset Inch Wide Blue Belt IYxn6d8qn 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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