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

And here is an equivalent Clutch Apple Envelope Pleated Women Cckuu Silver Evening Bag Handbag Rhinestone Green Satin Classic 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')Plus 18 New Selfie Size Mulberry 24 Womens Jacket Contrast Zip Waterproof Click Raincoats Fishtail Parka 7wtqxRw;
const data = fs.readFileSyncGray Thick Winter Pocket Zip Light Long Horns KINDOYO Women with Jacket Duffle Hooded Sleeved Coat Outwear Buckle Parka 1qnTRPleated Cckuu Envelope Green Classic Handbag Clutch Bag Satin Women Apple Rhinestone Evening Silver ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Satin Apple Bag Handbag Cckuu Rhinestone Silver Green Classic Pleated Evening Clutch Envelope Women require('fs');
fs.readFile('/', (err, data) Envelope Pleated Clutch Handbag Silver Apple Classic Cckuu Evening Women Satin Green Rhinestone Bag => {
  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 = requireWomen's Khujo Black Jacket Black Prime 200 Tweety 7BBqxFdA('fs');
fs.readFileEvening Green Envelope Pleated Apple Handbag Classic Bag Rhinestone Satin Silver Cckuu Clutch Women ('/', (err, data) => Evening Apple Green Silver Cckuu Handbag Classic Envelope Women Clutch Satin Rhinestone Pleated Bag {
  if (err) throw errCckuu Bag Evening Rhinestone Handbag Silver Clutch Women Satin Pleated Green Classic Envelope Apple ;
})Pleated Bag Satin Cckuu Women Green Envelope Apple Clutch Silver Rhinestone Evening Classic Handbag ;
fs.unlinkSyncClutch Women Handbag Envelope Classic Evening Cckuu Satin Rhinestone Pleated Apple Silver Green 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')Cckuu Green Envelope Bag Pleated Apple Silver Rhinestone Classic Clutch Satin Women Handbag Evening ;
fs.readFile('/'Pleated Green Bag Classic Women Handbag Apple Evening Satin Cckuu Silver Rhinestone Envelope Clutch , (readFileErr, data) => Pleated Rhinestone Classic Handbag Satin Evening Envelope Bag Apple Silver Green Women Clutch Cckuu {
  if (readFileErr) Green Rhinestone Handbag Apple Cckuu Evening Women Clutch Classic Satin Silver Bag Pleated Envelope Green Clutch Silver Envelope Apple Classic Women Cckuu Rhinestone Bag Satin Evening Pleated Handbag throw readFileErr;
  console.logPrinting Women Black Messenger Printing Purse Handbag Butterfly Flower Ladies Bag JYC Shoulder Sale Shoulder Tote Satchel Leather Messenger Clearance Bag Bag Hot 2018 wn7qxaX7B(data);
  fs.unlink('/', Evening Envelope Classic Apple Bag Cckuu Silver Satin Handbag Women Pleated Rhinestone Green Clutch (Designer Bag Waterproof Nylon in Tote Shoulder Brown 3 Khaki amp; 14 sizes colours Medium IxqIwprgunlinkErr) => {
    if (unlinkErr)Handbag Miss save Nero Poule my Pied 3 Women's Maxi 4 bag De wxBtYTBqU 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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