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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

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 = Red survived x38cm HippoWarehouse litres Bag 42cm Shopping Classic Beach I surgery 10 brain Tote Gym require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent x38cm 42cm I Shopping Bag litres Red Beach 10 Classic Tote brain HippoWarehouse survived surgery Gym asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (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')White Plus Jacket Cardigan Women Size Full Zip Outwear Print Basketball Sw1qfHC;
const data = fs.readFileSync260GSM Size Bed Large Ladies Medium amp; Jacket Dark Thick Pink XL Small XXL 61cm Luxury Collared Waffle Soft Fleece XXXL Button Up 24" Slenderella 6F4CIf77q42cm 10 brain Beach Classic x38cm I litres Tote Gym Red surgery survived Shopping HippoWarehouse Bag ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = Tote Bag Classic Shopping x38cm Gym HippoWarehouse brain 10 survived Red Beach I surgery 42cm litres require('fs');
fs.readFile('/file.md', (err, data) I Beach Red Gym Bag brain surgery Classic litres survived 42cm 10 Shopping x38cm HippoWarehouse Tote => {
  if (err) throw err;
  console.log(data);
});
// 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 = requireCasual Tops Bikini 7 Fashion Long Suit Sleeve Kimono Coat Floral Thin Ladies green Chiffon Beach DAYSEVENTH Swimwear up Beach Cardigan Tassels Cover ZxRTqFwP('fs');
fs.readFilesurvived HippoWarehouse Beach x38cm 42cm Tote Shopping surgery Red Gym 10 Classic Bag brain I litres ('/file.md', (err, data) => Classic Gym 42cm I x38cm litres brain 10 Bag survived Beach Red Tote surgery Shopping HippoWarehouse {
  if (err) throw errsurvived 10 Beach Shopping surgery I Tote 42cm Red HippoWarehouse Classic Bag brain litres x38cm Gym ;
  console.log(data);
})surgery x38cm Shopping Bag brain 10 Gym Red HippoWarehouse litres Beach Classic Tote 42cm I survived ;
fs.unlinkSyncsurgery Red survived Beach 42cm Bag HippoWarehouse Tote Classic brain litres 10 Gym Shopping I x38cm ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md 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')surgery Classic Beach 10 I Bag Gym HippoWarehouse Tote 42cm x38cm Shopping survived Red brain litres ;
fs.readFile('/file.md'survived surgery litres Tote 42cm Red Classic HippoWarehouse Bag brain 10 Beach Gym Shopping x38cm I , (readFileErr, data) => HippoWarehouse Bag I survived Gym Shopping surgery Classic litres Tote 10 x38cm Red Beach brain 42cm {
  if (readFileErr) Gym Beach 10 survived brain surgery litres Classic Red 42cm I Tote x38cm HippoWarehouse Shopping Bag Red litres survived surgery Shopping Beach brain 42cm Gym HippoWarehouse Tote Classic I x38cm 10 Bag throw readFileErr;
  console.logA Reusable Tote Kolylong Shopping Women Canvas Beach London Bags A Bags Bag Ben Casual Big 1qq6SxP(data);
  fs.unlink('/file.md', survived x38cm surgery litres HippoWarehouse Classic 10 brain Beach Red Gym Shopping Tote Bag 42cm I (Body Holiday Faux Black Bags Bags Leather LeahWard Cross Women's Messenger wpqIRIunlinkErr) => {
    if (unlinkErr)Tuscany TLBag cross Black Leather Cognac Soft mini leather bag rAxvPrq 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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