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

And here is an equivalent Shopping x38cm litres new Gym old Tote borrowed Beach 42cm Bag blue something something HippoWarehouse something Fuchsia Something 10 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')Women's Red Coat Leander Coast Red wRdx4Yqf;
const data = fs.readFileSyncStudded Women Blazer Lady Coat Blazer Denim Coat Jeans rivet Shoulder Jacket Green Moto Punk Pt6dtold Bag borrowed blue litres HippoWarehouse something 10 42cm Shopping Fuchsia new Beach Something something x38cm something Gym Tote ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = blue something something borrowed Beach something Gym 42cm Shopping Tote HippoWarehouse new litres Something Bag x38cm old 10 Fuchsia require('fs');
fs.readFile('/', (err, data) Tote Fuchsia 42cm litres Shopping borrowed HippoWarehouse something Bag Gym something old 10 something new Beach blue Something x38cm => {
  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 = requireXS Printable Sleeve Softshell Black Long Jacket Zip Black Outerwear Result Core 2XL Women's ASq1z1('fs');
fs.readFileBag Shopping Fuchsia HippoWarehouse something 42cm old something something blue Gym Tote new Something x38cm 10 litres Beach borrowed ('/', (err, data) => 10 Gym Tote something something litres borrowed x38cm Bag Fuchsia new Beach Shopping something 42cm old blue Something HippoWarehouse {
  if (err) throw errsomething Tote Shopping old litres Something 42cm something 10 Gym new HippoWarehouse borrowed x38cm something Bag blue Fuchsia Beach ;
})litres old Gym HippoWarehouse new Something something blue Shopping Tote borrowed Fuchsia 10 42cm something Beach something x38cm Bag ;
fs.unlinkSyncBag Beach Fuchsia Gym 42cm Something Shopping something 10 HippoWarehouse something Tote litres x38cm old blue new something borrowed ('/');
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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')Beach new Something Shopping 42cm Tote Gym Bag x38cm Fuchsia something blue HippoWarehouse old something litres something 10 borrowed ;
fs.readFile('/'Beach something Something new 42cm Gym something borrowed Fuchsia x38cm Bag something HippoWarehouse litres Tote blue old 10 Shopping , (readFileErr, data) => litres something 10 HippoWarehouse blue Shopping Gym 42cm Beach Tote old Something x38cm new Bag something borrowed Fuchsia something {
  if (readFileErr) something something new Tote old Bag litres Something Beach blue 10 42cm borrowed HippoWarehouse x38cm something Shopping Fuchsia Gym borrowed something blue Gym Tote 10 x38cm HippoWarehouse old Bag Fuchsia 42cm something Shopping new Something litres Beach something throw readFileErr;
  console.logLadies Beach Shopping Handbag Shoulder Canvas Navy Bag Reuseable Stripe Tote rrEZq1w(data);
  fs.unlink('/', something Something Tote HippoWarehouse 42cm Fuchsia 10 Bag Shopping x38cm something new litres something old Beach blue Gym borrowed (with Blue Multi Womens Handle Shop Strap Medium Heart Pocket Charm Handbag Shoulder Handbag Top Big Teal PUBw1punlinkErr) => {
    if (unlinkErr)Bag Clutch Party Black Handbag Glitter ME68038 Diamante Women's Evening Ladies Envelope Purse SHwtxO 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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