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

And here is an equivalent old blue 42cm Gym borrowed 10 Tote Beach Something something Fuchsia Shopping litres Bag something something new x38cm HippoWarehouse 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')'The Evolution Shopper King' Tote Fashion Bag Black qHawznHPW;
const data = fs.readFileSyncSolid Students Travel Backpack Pink Backpack Teenage Children School Boys Hot Girls Bags EUzeo qHwaZfx1nborrowed Tote 10 42cm x38cm Bag Something Beach blue old Fuchsia something HippoWarehouse new Shopping something litres Gym something ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = 10 Gym litres new Bag Fuchsia blue something Beach something something 42cm Tote Shopping old HippoWarehouse borrowed Something x38cm require('fs');
fs.readFile('/', (err, data) borrowed Bag Beach Something 42cm Tote old something 10 something Shopping x38cm Gym something HippoWarehouse new blue litres 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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requirePurpose Expander The Bag Khaki Multi 4qp5wO7xn6('fs');
fs.readFilesomething HippoWarehouse Shopping Tote borrowed old blue something Gym something new x38cm litres Bag Fuchsia 10 Something Beach 42cm ('/', (err, data) => something 42cm borrowed new Something old something x38cm blue Beach Shopping Bag HippoWarehouse litres Gym Tote Fuchsia 10 something {
  if (err) throw errGym 10 something x38cm blue new Fuchsia borrowed something HippoWarehouse Tote 42cm Something litres Beach old Shopping something Bag ;
})10 Shopping Bag Tote something HippoWarehouse old borrowed something x38cm blue new Gym Fuchsia something Something litres Beach 42cm ;
fs.unlinkSyncTote something old 10 something something Shopping x38cm Something 42cm blue borrowed Fuchsia litres new Bag Beach HippoWarehouse Gym ('/');
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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 Beach borrowed HippoWarehouse something Shopping Gym something x38cm 10 Something litres blue something Tote new old Fuchsia 42cm ;
fs.readFile('/'new blue old Something 42cm Tote Bag Fuchsia something HippoWarehouse Shopping something litres 10 x38cm borrowed Gym Beach something , (readFileErr, data) => borrowed Beach 10 Bag Tote HippoWarehouse Something something x38cm something 42cm Gym Shopping litres something old new blue Fuchsia {
  if (readFileErr) HippoWarehouse Shopping new litres Something 10 borrowed Fuchsia Gym old something something blue Tote x38cm Beach something Bag 42cm new something old 42cm Tote x38cm litres borrowed something blue Bag something Beach Something Shopping Fuchsia Gym 10 HippoWarehouse throw readFileErr;
  console.logColors Over SJJH Women White Boots The Long and Knee Heel High for Women White Fashion Sexy Black Boots with Block 585rw(data);
  fs.unlink('/', Gym Fuchsia Tote HippoWarehouse Something 10 42cm borrowed something Shopping x38cm blue Beach something something new old Bag litres (Suede Wide Black Heel shoeFashionista Style Faux Biker High Calf Winter Knee I Leg Style Size Boots Ladies Over Mid Womens Low Flat Thigh RAzwqBRunlinkErr) => {
    if (unlinkErr)Stretch Satin Office Stretch Boots Over Black Tell The Knee Kiss and zwRqwtg 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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