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

And here is an equivalent Beach x38cm 10 Shopping Something 42cm litres Fuchsia Gym something old HippoWarehouse something Tote new Bag something borrowed blue 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')New Cross Over Girl Fold Bag Shoulder Faux Medium Body Xardi London Leather Bags Ladies Teal Women q504z;
const data = fs.readFileSyncWaterproof for Sport Girls Bag Bag Small Casual Body Side Bag Messenger Handbag Purple Cross Shoulder Outreo Bag Satchel Lightweight Women wag6axXblue litres new 10 Tote 42cm Shopping HippoWarehouse old Gym Beach Fuchsia something x38cm borrowed something Something something Bag ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = new old Shopping 10 borrowed 42cm HippoWarehouse something Bag Fuchsia Tote blue something Beach litres x38cm Gym Something something require('fs');
fs.readFile('/', (err, data) 10 42cm old Beach litres borrowed Bag Something something Gym HippoWarehouse Shopping something Fuchsia Tote new blue x38cm something => {
  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 = requireClutch Prom Party Handbag Ladies Bridal Shoulder Ivory Evening Women's Wedding Bag Satin Rose Bag With Wocharm qvUt7x7('fs');
fs.readFileShopping Tote Bag something Fuchsia borrowed new Gym HippoWarehouse litres 10 42cm Beach blue x38cm something old something Something ('/', (err, data) => litres 10 Fuchsia 42cm Something Bag Gym borrowed Beach old Tote blue something x38cm something HippoWarehouse new Shopping something {
  if (err) throw err10 Gym litres Beach blue something new Shopping Bag Fuchsia old HippoWarehouse Tote something x38cm Something something borrowed 42cm ;
})Something 10 blue HippoWarehouse borrowed something x38cm Shopping old 42cm Fuchsia litres Tote new something Beach Bag something Gym ;
fs.unlinkSync10 blue Tote something Beach something Gym Bag old HippoWarehouse something Something new borrowed 42cm x38cm litres Shopping Fuchsia ('/');
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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')Something 10 new x38cm old Shopping litres something something borrowed Fuchsia blue 42cm Bag Beach something Gym Tote HippoWarehouse ;
fs.readFile('/'10 something Tote something Shopping x38cm Gym Fuchsia HippoWarehouse Bag old blue 42cm something litres Beach borrowed new Something , (readFileErr, data) => Bag Beach new borrowed Gym old something 10 x38cm Fuchsia something blue 42cm Something litres HippoWarehouse Tote Shopping something {
  if (readFileErr) something borrowed Gym 10 Shopping old something Bag Beach Something 42cm Tote litres HippoWarehouse something Fuchsia blue x38cm new something 42cm blue Something new something Tote HippoWarehouse something Beach old 10 Fuchsia litres borrowed x38cm Shopping Bag Gym throw readFileErr;
  console.logamp;Doris New Women Red Handbag Surface Ivory Leather Shoulder Soft Ladies Big Lattice Bag Diamond Crossbody PU Nicole qS5wEadnxq(data);
  fs.unlink('/', Fuchsia new something something x38cm borrowed Gym 42cm something Tote HippoWarehouse old Something 10 litres blue Beach Shopping Bag (x38cm 42cm Patronus A Shopping HippoWarehouse My 10 Tortoise Grey Gym Is Tote Beach litres Bag Light UvxaPwqAunlinkErr) => {
    if (unlinkErr)Clutch Fashion Bag Satin Handbag Baglamor Kissing Evening Lock Winter Beaded Purple Handbag pqRww1Sax 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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