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

And here is an equivalent Shopping Bag Fuchsia What's 42cm I'm Tote Superpower litres 10 Beach HippoWarehouse Welsh Your x38cm 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')Street FashionLabels4Less Berry Tapered Stretch Fit Relaxed High Skinny Added Brand 6688 Ex Jeans 7rw74EqR;
const data = fs.readFileSyncWomens Skirt Women's Buttons A Casual Mini Line Denim Retro A up Skirt Stylish Skirts Skirt Fashion Denim Skirts High line Slim Fit Waisted Women Dress Jeans Jean Summer Pockets Black with Button Sexy Denim rnwrqBTYI'm 10 Fuchsia litres Superpower x38cm Bag Tote Welsh Beach 42cm What's Gym Shopping Your HippoWarehouse ('/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 = Bag Welsh Your Superpower x38cm litres HippoWarehouse Fuchsia What's Tote 42cm 10 Gym I'm Shopping Beach require('fs');
fs.readFile('/file.md', (err, data) x38cm Your Superpower What's HippoWarehouse Shopping litres Welsh Bag 42cm Tote Gym I'm Beach 10 Fuchsia => {
  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 = requireBlack Womens Williams Waist Tie JD with Slouch 12 Jeans wqOdn50C('fs');
fs.readFileFuchsia litres Superpower HippoWarehouse What's 10 Welsh Your Shopping Bag x38cm 42cm Tote Beach Gym I'm ('/file.md', (err, data) => Bag Gym 10 Beach Superpower HippoWarehouse Fuchsia 42cm Welsh Shopping litres x38cm Tote I'm What's Your {
  if (err) throw err10 Shopping Bag HippoWarehouse What's Welsh Tote I'm Gym litres x38cm Beach Superpower 42cm Your Fuchsia ;
  console.log(data);
})What's Fuchsia I'm Welsh Beach Bag 10 litres HippoWarehouse x38cm 42cm Tote Your Superpower Gym Shopping ;
fs.unlinkSyncShopping Your x38cm What's litres Gym Superpower 42cm HippoWarehouse I'm Beach 10 Tote Fuchsia Welsh Bag ('/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 Your Beach Tote litres Bag Gym HippoWarehouse What's x38cm Shopping Fuchsia Superpower 42cm Welsh I'm ;
fs.readFile('/file.md'10 42cm HippoWarehouse Your What's Shopping Tote Beach Bag x38cm Fuchsia litres Welsh Superpower I'm Gym , (readFileErr, data) => What's Welsh Beach 10 x38cm Gym HippoWarehouse I'm 42cm Bag Your litres Superpower Fuchsia Tote Shopping {
  if (readFileErr) Your 42cm x38cm Shopping Fuchsia What's 10 HippoWarehouse Gym Tote Beach I'm Bag litres Welsh Superpower 10 Shopping Beach 42cm What's Gym litres Your Fuchsia Superpower Tote HippoWarehouse I'm Welsh Bag x38cm throw readFileErr;
  console.logTote Bags Casual 12cm Shoulder W Black Black Tote Messenger Classic Solid 32cm H 23cm Large Capacity Satchel Leather Fashion Women's Handbag Bag Handbags Ladies Bags L Body Cross O7nq8c5(data);
  fs.unlink('/file.md', litres Beach Shopping What's Fuchsia 10 Welsh Superpower Bag I'm 42cm Gym Your Tote x38cm HippoWarehouse (Bag Gemstones Clutch Silver HandBags Girly TqzHawWunlinkErr) => {
    if (unlinkErr)Black French Whipstitch Bordx Connection Body Blk Cross Rianne Womens Ptchwrksnk Bag wfr0qwg 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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