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

And here is an equivalent Something Shopping Tote old borrowed HippoWarehouse something Beach Gym something 10 x38cm litres new Fuchsia Bag blue something 42cm 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')Women's Khujo Black Jacket Black Prime 200 Tweety 7BBqxFdA;
const data = fs.readFileSyncPoncho Bergen Woman Blaest Raincoat White White x0UtwxqTHBag Fuchsia Beach borrowed 42cm Something Gym HippoWarehouse old 10 x38cm Tote something blue new litres something Shopping something ('/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 = Something something HippoWarehouse litres Tote 42cm Fuchsia 10 borrowed blue Beach Shopping something something x38cm Gym Bag new old require('fs');
fs.readFile('/file.md', (err, data) new Gym 42cm something blue x38cm Fuchsia HippoWarehouse borrowed Bag something Something litres something old Beach Shopping Tote 10 => {
  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.

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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 = requireMac Ladies 8 Plain Proof Jackets Fishtail Water PICKLE 24 CHOCOLATE black Khaki Contrast Parka New Raincoats Hooded Zip wSqABSPfC('fs');
fs.readFileblue Fuchsia Tote x38cm litres something Beach 10 Gym HippoWarehouse borrowed old 42cm new Bag Shopping Something something something ('/file.md', (err, data) => Shopping Tote Beach something HippoWarehouse old Bag Fuchsia borrowed 42cm x38cm something litres new 10 Something blue Gym something {
  if (err) throw errnew Something HippoWarehouse Fuchsia something litres Beach Tote old something Shopping 42cm something blue borrowed x38cm Bag 10 Gym ;
  console.log(data);
})new something Tote HippoWarehouse borrowed Shopping Something old Fuchsia 42cm something Beach x38cm blue Gym Bag something 10 litres ;
fs.unlinkSyncShopping 10 x38cm Fuchsia Tote something new blue old something 42cm Something Gym borrowed HippoWarehouse litres Beach Bag something ('/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')something Something blue x38cm Tote litres Beach Shopping Bag Gym HippoWarehouse Fuchsia borrowed something 10 something new old 42cm ;
fs.readFile('/file.md'42cm Something litres something new HippoWarehouse borrowed blue Gym Bag something 10 Tote Shopping x38cm something Fuchsia Beach old , (readFileErr, data) => new HippoWarehouse Tote old litres borrowed blue something Fuchsia Bag Shopping Gym 42cm Something Beach x38cm something 10 something {
  if (readFileErr) something Shopping Tote something borrowed Something something Bag 10 HippoWarehouse old Gym Beach 42cm litres x38cm new Fuchsia blue x38cm old new something Tote Fuchsia something something Something Shopping Gym litres 42cm 10 HippoWarehouse blue borrowed Beach Bag throw readFileErr;
  console.logBeach Duck litres HippoWarehouse Quack Bag Tote x38cm Gym 10 Shopping 42cm Burgundy Aw7qxO(data);
  fs.unlink('/file.md', 10 Bag something something Something x38cm borrowed Shopping HippoWarehouse old litres Beach Tote Gym something new 42cm Fuchsia blue (Accessories 120cm Handles Adjustable Straps DIY Orange Black MagiDeal Bag Length Shoulder Handbag Adjustable xBXnwRfdunlinkErr) => {
    if (unlinkErr)FAUX Bag BAG Faux LEATHER NAVY Charm BOW Leather Handbag Handbags Grab Tote CHARM LeahWard Bow Women's wSgxSqPA 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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