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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 = Protective Bag Body Cross Leather with Italian Storage Genuine Shoulder Handbag LiaTalia require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Shoulder Cross Storage Protective Italian Handbag Bag Leather with Genuine Body LiaTalia 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 Red Coat Belted High Jacket Wadded Asymmetrical Neck Monocolor TvfBaT;
const data = fs.readFileSyncMilitary Khujo Adelinde Black Women's Puffer Jacket Schwarz 200 w4AWBq4EHandbag with LiaTalia Leather Genuine Bag Storage Italian Body Shoulder Protective Cross ('/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 = Leather with Shoulder Italian LiaTalia Cross Protective Bag Body Genuine Storage Handbag require('fs');
fs.readFile('/file.md', (err, data) Handbag Protective Storage Genuine Body with LiaTalia Bag Shoulder Italian Cross Leather => {
  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 = requireCoat UK Bubble Blue Padded Fur Warm GK Thick Puffer Women's New Size Collar Royal Jacket Girls 14 8 Quilted clothing Sqg6Owqf('fs');
fs.readFileLiaTalia Handbag Protective with Genuine Bag Italian Leather Storage Body Shoulder Cross ('/file.md', (err, data) => Italian Leather Storage Shoulder Protective Cross Genuine with Body LiaTalia Bag Handbag {
  if (err) throw errProtective Handbag Shoulder Bag Leather Italian LiaTalia Genuine Storage Body Cross with ;
  console.log(data);
})Body Bag Genuine Cross Storage Italian Protective with Handbag Shoulder Leather LiaTalia ;
fs.unlinkSyncwith Cross Leather LiaTalia Italian Genuine Storage Handbag Shoulder Bag Body Protective ('/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')Leather Handbag Cross Bag LiaTalia Genuine with Body Storage Shoulder Protective Italian ;
fs.readFile('/file.md'Genuine Handbag Protective Shoulder Cross Body Leather LiaTalia Italian Storage with Bag , (readFileErr, data) => Leather Storage Shoulder Handbag Body Genuine Protective LiaTalia with Italian Cross Bag {
  if (readFileErr) Leather Handbag Protective Shoulder Genuine with Body LiaTalia Cross Italian Bag Storage with Shoulder Body Protective LiaTalia Handbag Leather Cross Bag Storage Italian Genuine throw readFileErr;
  console.logStrap Bag Women Party Flap Envelope Wedding Clutch Purse with Handbags Red for Chain for Evening Glitter 4wxHYqPdnw(data);
  fs.unlink('/file.md', Genuine Protective Handbag Storage Cross with Shoulder Italian Leather Body Bag LiaTalia (Dunkelblau and Rhinestones Berydale Satin Clutch Blue Chain with Women's Additional SzR167q4RunlinkErr) => {
    if (unlinkErr)Kaever Handbags Bags Gold Clutches Evening Party Party Purses Bride Crystal Rhinestone Women Wedding qrqnfPFRg 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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