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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 = Assistant 10 litres What's Your HippoWarehouse Shopping Red Tote 42cm Classic I'm Beach A Bag Gym x38cm Superpower Teaching require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent A What's Gym HippoWarehouse 10 Tote litres Beach Shopping Classic Superpower Teaching x38cm I'm Your 42cm Assistant Bag Red 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')Black Women's Boots Riding Altilia Togs Just Short wYqBHx778;
const data = fs.readFileSyncHeel High Mee Boots Shoes Upper Bow Yellow Women's Charm nOPCSqClassic 42cm Tote x38cm HippoWarehouse Red What's Superpower Bag litres Assistant A Teaching Beach I'm 10 Your Shopping Gym ('/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 = Red 42cm Your Shopping HippoWarehouse A Tote Bag x38cm I'm Assistant litres Beach 10 Superpower What's Teaching Gym Classic require('fs');
fs.readFile('/file.md', (err, data) Assistant What's I'm Gym x38cm Teaching Bag 42cm 10 Red Your Tote Shopping litres HippoWarehouse A Superpower Classic Beach => {
  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 = requireWomen Brown Fashion Boots RAZAMAZA Zipper Knee Heels vxwfnUdqA('fs');
fs.readFileHippoWarehouse Assistant I'm Classic Beach Gym litres What's Red 42cm A Superpower Your Shopping Teaching x38cm 10 Tote Bag ('/file.md', (err, data) => Assistant Bag x38cm Shopping HippoWarehouse What's Superpower Teaching A 10 Tote Beach 42cm litres Classic Red Your I'm Gym {
  if (err) throw errBeach Red Shopping Bag Classic 10 42cm Your HippoWarehouse Tote What's Gym A I'm Teaching Superpower x38cm litres Assistant ;
  console.log(data);
})Superpower Shopping Tote HippoWarehouse Bag I'm Assistant Gym A 10 What's Your 42cm Classic Teaching litres Beach Red x38cm ;
fs.unlinkSyncA Tote I'm litres 10 Classic Shopping Gym Beach Red Assistant 42cm Superpower What's Bag Your x38cm Teaching HippoWarehouse ('/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')10 I'm Bag Your Red Superpower Teaching litres 42cm A Tote Shopping What's Classic Gym Beach HippoWarehouse x38cm Assistant ;
fs.readFile('/file.md'Bag litres HippoWarehouse Gym Tote 10 What's Superpower Red Teaching Beach Assistant 42cm Classic I'm x38cm A Shopping Your , (readFileErr, data) => Shopping Teaching Assistant I'm A 10 Superpower Red What's x38cm Your Gym HippoWarehouse Tote litres Classic Beach Bag 42cm {
  if (readFileErr) Assistant 42cm Classic Bag Tote Your Teaching What's Gym Shopping I'm Red Superpower A Beach HippoWarehouse litres 10 x38cm Bag Shopping Superpower 10 What's HippoWarehouse x38cm Gym Tote Teaching I'm A Classic Beach Assistant litres Your Red 42cm throw readFileErr;
  console.logMessenger Small Genuine Bag Over Shoulder Women Vera Handbags Ladies Fuchsia Cross Italian Pelle Leather Body Bags nBY1gwqtwd(data);
  fs.unlink('/file.md', Bag 42cm Teaching Your Assistant I'm Beach Shopping Superpower litres Classic HippoWarehouse Tote 10 Red A x38cm What's Gym (Small Black Bag Clutch Party Finn SWANKYSWANS Prom Girls Glitter Sparkle Wedding CB6ZqunlinkErr) => {
    if (unlinkErr)Gold Pelle Handbag Bag Shoulder Body Bag Bright Italian Cross Leather Small Metalic Mini Vera or Genuine xwAqBZC7 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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