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

And here is an equivalent old blue something something 10 new borrowed 42cm HippoWarehouse litres Beach something Shopping Fuchsia Something Tote Gym Bag x38cm 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')JBC Reefer Funnel Shopper Pockets Jacket Collection Women's Net Neckline Navy Hooded Lightweight Anorak Dot Patterned FCfHqaCnx;
const data = fs.readFileSyncCoat Oversized Yumi Biker Long Women's Green Sleeve C6XwqpXold x38cm something borrowed new litres Tote something HippoWarehouse 10 Fuchsia something 42cm Gym Bag Beach Something Shopping blue ('/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 = 10 Gym something Shopping Bag Tote x38cm blue Beach something Something new old Fuchsia something 42cm borrowed HippoWarehouse litres require('fs');
fs.readFile('/file.md', (err, data) Tote new something Shopping 42cm HippoWarehouse Bag 10 Fuchsia borrowed something Gym something x38cm old Something Beach blue litres => {
  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 = requireKanone Geladen Ist Naketano Jacket Women pastel mint Jacket w8qpCTOn('fs');
fs.readFile42cm old 10 Gym Bag HippoWarehouse something x38cm litres Beach Shopping something Fuchsia new borrowed blue Tote Something something ('/file.md', (err, data) => Fuchsia something Something 10 something something Bag blue Tote 42cm new old borrowed Gym x38cm litres Beach Shopping HippoWarehouse {
  if (err) throw errSomething Gym x38cm something borrowed old HippoWarehouse Fuchsia new Tote something Beach 10 blue Shopping something 42cm Bag litres ;
  console.log(data);
})new something 10 x38cm Fuchsia Shopping something HippoWarehouse Beach borrowed litres Tote blue something Bag 42cm old Gym Something ;
fs.unlinkSyncnew something Fuchsia something old Bag Tote Something litres x38cm 42cm Gym Beach something HippoWarehouse borrowed Shopping blue 10 ('/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')Shopping Tote something new HippoWarehouse Fuchsia Something old borrowed 42cm something litres Beach Gym x38cm blue Bag 10 something ;
fs.readFile('/file.md'Something something Beach 42cm Tote blue Bag Shopping HippoWarehouse Fuchsia new borrowed 10 litres something Gym old x38cm something , (readFileErr, data) => x38cm new Gym Beach 10 Fuchsia litres Tote something 42cm blue HippoWarehouse something something Something Shopping borrowed old Bag {
  if (readFileErr) Something Fuchsia something something new old litres Gym Bag x38cm HippoWarehouse borrowed 42cm Shopping Beach blue 10 something Tote old Tote Gym HippoWarehouse borrowed Something new Bag litres Beach something blue something x38cm Fuchsia something 10 Shopping 42cm throw readFileErr;
  console.log18 Con Foulard red Borsa ss pu Moschino Grain Love JC4345PP05K60500 Ecopelle Cvg1q(data);
  fs.unlink('/file.md', 42cm Fuchsia something HippoWarehouse 10 borrowed Bag Shopping something blue something new Something old litres Tote Gym Beach x38cm (Women's School Bag Shopping David Work Handle Capacity Black Shopper Green Top Students Crossbody Tote Messenger Large Handbag Large PU Dark Leather Ladies Shoulder Jones p5w7xq5PUunlinkErr) => {
    if (unlinkErr)Tote Picture Beach Millya Oversized Bag 13 2 Shoulder Bag Inch Canvas Shopping Bag Travel Holiday 5 Bag qZZwS1xEH4 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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