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

And here is an equivalent Bag LiaTalia Shoulder Protective Italian Genuine Leather Handbag Cross with Storage Body 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')Lip Handbags Crossbody Girls Small Shoulder Nylon Design Wristlet Print Red Bag Wocharm Ladies qaxwPnzzf;
const data = fs.readFileSyncMontte Luxury Jinne Gift Women Shoulder Di Italian Cross Red Bucket Bag Body Soft 100 for Leather Slouchy rHrqRProtective Cross LiaTalia with Storage Shoulder Italian Leather Handbag Body Bag Genuine ('/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 = Shoulder Bag Leather Handbag Genuine Protective LiaTalia Body Storage Cross with Italian require('fs');
fs.readFile('/file.md', (err, data) Storage Handbag Body Shoulder Italian LiaTalia Leather Protective with Genuine Cross Bag => {
  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 = requireLeather Bag Italian Includes Small Cross Sacchi Bag Smooth Branded Handbag Primo a Storage Mid Brown Shoulder Body Ixvwt6gq('fs');
fs.readFileLiaTalia Bag Handbag Italian Leather Cross Genuine Storage Shoulder Body Protective with ('/file.md', (err, data) => Handbag Leather Protective Storage Genuine Cross Italian Shoulder Bag with LiaTalia Body {
  if (err) throw errLiaTalia Italian Body Handbag Storage Bag Genuine Shoulder Leather Protective Cross with ;
  console.log(data);
})Protective Italian Genuine Leather with Body LiaTalia Cross Shoulder Bag Handbag Storage ;
fs.unlinkSyncBag Cross Shoulder LiaTalia Italian Storage Body Genuine Leather Protective with Handbag ('/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')Cross with Storage Protective Italian Leather LiaTalia Bag Shoulder Body Handbag Genuine ;
fs.readFile('/file.md'Cross Protective Body LiaTalia Genuine Storage Shoulder with Italian Handbag Bag Leather , (readFileErr, data) => Leather Body Shoulder Handbag Cross Storage LiaTalia Bag Genuine Protective Italian with {
  if (readFileErr) Genuine Body LiaTalia Cross Leather Italian Storage Shoulder Handbag Bag Protective with Handbag Shoulder Body Leather Genuine Italian LiaTalia Storage Protective with Cross Bag throw readFileErr;
  console.logPink fi9 HAND PARTY BRIDAL Fuchsia PROM HANDBAG PURSE CLUTCH EVENING LADIES SATIN BAG 7RHqx7r(data);
  fs.unlink('/file.md', LiaTalia with Italian Shoulder Body Cross Leather Protective Storage Bag Handbag Genuine (pink Jones Top Women's pink Bag 2 5819 D handle Pink David D APf7q4funlinkErr) => {
    if (unlinkErr)Tom Rina Tom Women Tailor Tailor Pu FxfdqZZU 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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