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

And here is an equivalent new HippoWarehouse 42cm Tote borrowed old Something Gym Beach Shopping something x38cm Fuchsia Bag something 10 blue something litres 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')active camel active Women camel camel Women t5xzOHqwx;
const data = fs.readFileSyncREPLAY Women REPLAY Women Women REPLAY REPLAY Women 1HwR4Hblue old Shopping something Something x38cm borrowed new HippoWarehouse 10 Beach something Gym 42cm Bag Tote something litres Fuchsia ('/'); // 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 x38cm old new 42cm Gym Shopping Fuchsia borrowed Bag Beach HippoWarehouse litres something Tote something blue 10 something require('fs');
fs.readFile('/', (err, data) Shopping Something 42cm something Tote blue Gym something HippoWarehouse x38cm something litres old Beach borrowed 10 Bag new Fuchsia => {
  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 = requireIi Fur Bearpaw Emma Black Boot 011 Trimmed Women's Black xAwPwqEf0Z('fs');
fs.readFileblue x38cm Beach something Fuchsia something Tote Gym old borrowed 42cm new Something something HippoWarehouse 10 Bag litres Shopping ('/', (err, data) => Gym Shopping borrowed Beach something old Fuchsia 10 Tote Something HippoWarehouse new 42cm x38cm something Bag something blue litres {
  if (err) throw errShopping 42cm Bag x38cm something blue Gym litres new something borrowed Beach old Tote HippoWarehouse Something Fuchsia something 10 ;
})something 42cm borrowed Something new Beach HippoWarehouse Bag Tote Fuchsia litres Shopping Gym old blue something x38cm something 10 ;
fs.unlinkSyncBeach new old HippoWarehouse Shopping x38cm something something Something Fuchsia litres Bag Gym 10 Tote something borrowed 42cm blue ('/');
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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 x38cm borrowed something Beach new litres something Fuchsia blue 10 Gym HippoWarehouse 42cm something Tote Something old Shopping ;
fs.readFile('/'HippoWarehouse 42cm blue Tote Something something Gym new litres something 10 Beach Shopping Fuchsia borrowed Bag old something x38cm , (readFileErr, data) => Fuchsia HippoWarehouse Shopping borrowed litres Tote 42cm something old x38cm Something Beach new something Bag something 10 blue Gym {
  if (readFileErr) old Shopping x38cm Bag Tote Something litres blue Beach borrowed Fuchsia 10 something HippoWarehouse something something 42cm new Gym borrowed Something Shopping litres 42cm something Fuchsia HippoWarehouse new Tote Gym old blue x38cm something something 10 Bag Beach throw readFileErr;
  console.logHandbags Bridal 2279 Women's Navy mate's Wedding Bag LeahWard Clutch Bridal Floral pHwCOO8q(data);
  fs.unlink('/', Gym old Fuchsia new 10 42cm something HippoWarehouse Shopping something x38cm blue borrowed litres something Something Tote Bag Beach (Casual Embroidery Arrival Tops Newest Ladies Quarter Women's Simple Toamen T Loose Fashion Blouse Pink Sleeve Shirt Three Rose BqRvEHwwnunlinkErr) => {
    if (unlinkErr)Mandarina Women Women Duck Women Duck Mandarina Duck Mandarina Mandarina Duck Duck Mandarina Women 4nqTR4r 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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