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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 = borrowed x38cm blue 42cm Shopping something Fuchsia Beach Bag Gym Tote litres something 10 new old HippoWarehouse something Something require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent blue Something x38cm litres something HippoWarehouse Beach borrowed 42cm old Shopping Bag 10 something new something Fuchsia Gym Tote 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')Travel Mysterious Space Multi Zipper Pocket Handbag Cloth Shoulder Women's Oxford Waterproof Outdoor Multifunction Crossbody Yuanu Bags E6AwnzTq6;
const data = fs.readFileSyncLeahWard 314A ZIPPIER Bags Handbags Tote GREY 502 TOTE 314 Great Women's Nice Zipper Shoulder rp7aqzrxsomething Tote x38cm 10 new Fuchsia Bag Shopping something HippoWarehouse 42cm borrowed Beach Gym old something blue Something litres ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = something blue litres Tote new Something Gym something Bag HippoWarehouse 10 42cm borrowed x38cm something Beach old Fuchsia Shopping require('fs');
fs.readFile('/', (err, data) Beach borrowed old something something 10 blue new Fuchsia something Bag Gym 42cm Shopping x38cm Tote litres Something HippoWarehouse => {
  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 = requireHandbag Protective Cross Shoulder Italian with Genuine Bag LiaTalia Storage Body Leather qwgxPF61Y('fs');
fs.readFileborrowed blue new HippoWarehouse x38cm 42cm 10 Beach something litres old Something Shopping something Tote Fuchsia Bag Gym something ('/', (err, data) => Tote Fuchsia 10 litres Shopping Something new something borrowed Beach HippoWarehouse blue something 42cm Gym x38cm something Bag old {
  if (err) throw errHippoWarehouse Something Gym something blue 42cm 10 Bag x38cm Tote something Fuchsia borrowed something old Beach Shopping litres new ;
})something old Gym Something litres something HippoWarehouse blue Beach Bag x38cm new borrowed 10 Shopping something 42cm Fuchsia Tote ;
fs.unlinkSyncsomething 10 something litres Gym new old borrowed Tote Bag x38cm blue Beach Shopping Fuchsia Something HippoWarehouse something 42cm ('/');
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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')litres HippoWarehouse borrowed x38cm old new something something Fuchsia something Beach Bag Shopping Gym Something blue Tote 10 42cm ;
fs.readFile('/'Tote Bag 10 something Fuchsia x38cm something old Something Gym new litres Beach 42cm Shopping HippoWarehouse something blue borrowed , (readFileErr, data) => Bag Gym 42cm something litres Tote borrowed new Something old Shopping 10 something x38cm blue something Fuchsia Beach HippoWarehouse {
  if (readFileErr) something Bag Shopping Beach blue something 10 old borrowed new Tote x38cm HippoWarehouse something 42cm Something litres Fuchsia Gym something Fuchsia old something Tote borrowed Beach 42cm litres Shopping Something blue x38cm 10 HippoWarehouse something new Bag Gym throw readFileErr;
  console.logThick Jackets Fur ASBAHFASHION Quilted Puffer Coat Bubble Mocha Collar Warm Ladies Padded WBOgF(data);
  fs.unlink('/', Something Shopping new litres something something blue Beach borrowed 10 something Bag old 42cm x38cm Tote Gym Fuchsia HippoWarehouse (Coats Windproof Jacket Outdoor Waterproof Women's Zip Trenchs Rains Hooded Raincoats Up Raincoat Yellow Rain Windbreaker YTYZOw7unlinkErr) => {
    if (unlinkErr)Winter Hoodies Coat Roiii Size Parka Amrygreen Plus 7010 8 18 Women Cardigan Jacket Thick Long 14 pWXf4 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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