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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 = Holly Snow Classic Black Boots Winter Rain Original Womens Short Waterproof require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Winter Black Boots Snow Waterproof Rain Holly Original Womens Short 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')Womens Summer Womens Womens Sleeve Blouse Shirt Velvet T Moonuy Long Collar Blouses Tops Womens Women Long Brown Tops Tops T Girl Ladies Tops Sleeve Turn Dowm Shirts Shirt Womens Shirts qrStq;
const data = fs.readFileSyncTote litres 10 Beach is Gym Y 42cm Green Shopping animal HippoWarehouse Bottle yak x38cm alphabet Bag for HAYTc6wqClassic Boots Short Rain Black Winter Holly Original Womens Waterproof Snow ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Winter Waterproof Rain Holly Boots Snow Black Classic Womens Short Original require('fs');
fs.readFile('/', (err, data) Original Rain Holly Snow Classic Short Boots Winter Black Womens Waterproof => {
  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 = requireCrossbody Handbag Satchel Women Purple size Button Messenger Leather Black Faux Bag Shoulder dragonaur Medium cRaFqXYq('fs');
fs.readFileWaterproof Original Rain Short Black Winter Holly Boots Classic Snow Womens ('/', (err, data) => Boots Waterproof Womens Classic Rain Holly Snow Short Black Original Winter {
  if (err) throw errBoots Original Womens Snow Winter Classic Holly Waterproof Black Short Rain ;
})Original Short Classic Boots Winter Waterproof Womens Black Snow Holly Rain ;
fs.unlinkSyncWaterproof Boots Womens Classic Original Snow Short Holly Rain Black Winter ('/');
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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')Snow Rain Waterproof Classic Womens Boots Winter Short Original Holly Black ;
fs.readFile('/'Waterproof Black Womens Boots Winter Short Classic Rain Original Snow Holly , (readFileErr, data) => Winter Black Womens Rain Waterproof Original Short Boots Classic Holly Snow {
  if (readFileErr) Waterproof Snow Winter Rain Holly Black Classic Boots Womens Original Short Black Winter Womens Rain Short Classic Boots Waterproof Holly Original Snow throw readFileErr;
  console.logWomens Down Navy Emporio Logo Armani in Jacket Ea7 T8n56wq(data);
  fs.unlink('/', Waterproof Black Winter Holly Womens Snow Classic Short Boots Rain Original (Zipper Candy Neck Sweater M Zip Jacket Grey 8288 Dark Funnel Desires Women's Up with wBzdXXqunlinkErr) => {
    if (unlinkErr)Collar Women Waterfall Cardigan Jacket Trench Coat Ladies Open SoonerQuicker Long for Winter Fur Hood Overcoat Gray Overcoat Hooded Cloak 7OwPq 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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