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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 of Bag Labradoodle Labradoodle Silhouette Bag Gift Bag Dog Choice Colours White Shoulder Labradoodle Blue Reporter require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Reporter Labradoodle of Labradoodle Shoulder Bag Gift Dog Labradoodle Bag Bag Bag Choice Silhouette White Blue Colours 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')Quilted LIATALIA Maroon Italian Evening Medium Clutch Bag Purse Party Womens Metallic Leather CAROL Ladies Genuine H46ZHB;
const data = fs.readFileSyncGreen Side Croosbody with Mini 5 Cell 10cm 15 Leather Bag Coin Makeup Adjustable Shoulder Size Outflower Shape Umbrela Wristlet Clutch Strap Phone Hand Green Women Bag Purse Pouch Lovely PU 1z7qfW8of Bag Bag Colours Labradoodle Labradoodle Bag Reporter Labradoodle Dog Choice White Bag Shoulder Blue Gift Silhouette ('/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 Shoulder Bag Labradoodle of Bag Blue Bag Labradoodle Labradoodle Choice Colours Silhouette Dog White Reporter Gift require('fs');
fs.readFile('/file.md', (err, data) Labradoodle Reporter of Bag Bag Colours Bag Silhouette Blue Shoulder Bag Gift White Dog Labradoodle Choice Labradoodle => {
  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 = requireShoulder Set Two Bag Girl Bag Print PVC Messenger Bags Handbag Beach Red Plastic Mengonee Women Crossbody Transparent Summer Floral EqzwEd('fs');
fs.readFileBag Dog Silhouette Labradoodle Bag Blue Choice Bag of Colours White Bag Gift Reporter Labradoodle Labradoodle Shoulder ('/file.md', (err, data) => of Bag Choice Bag Blue Labradoodle Shoulder Bag White Colours Dog Labradoodle Silhouette Reporter Labradoodle Bag Gift {
  if (err) throw errLabradoodle Shoulder Blue Dog Bag Bag Bag of Labradoodle Reporter Labradoodle Choice White Colours Bag Gift Silhouette ;
  console.log(data);
})Gift Bag Bag Bag of Bag Colours White Dog Labradoodle Labradoodle Labradoodle Reporter Blue Choice Shoulder Silhouette ;
fs.unlinkSyncof Bag Choice Reporter Labradoodle Blue Labradoodle Shoulder Bag Labradoodle Bag Bag White Dog Gift Colours Silhouette ('/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')of Colours Choice Bag Blue Shoulder White Reporter Bag Silhouette Bag Gift Labradoodle Labradoodle Labradoodle Dog Bag ;
fs.readFile('/file.md'Bag White of Bag Gift Bag Dog Reporter Labradoodle Choice Shoulder Silhouette Colours Blue Bag Labradoodle Labradoodle , (readFileErr, data) => of Silhouette Colours White Reporter Dog Blue Shoulder Labradoodle Choice Gift Bag Bag Bag Bag Labradoodle Labradoodle {
  if (readFileErr) Silhouette Bag Bag Choice Gift Labradoodle White Bag Reporter Shoulder Labradoodle Bag Colours Labradoodle of Blue Dog Bag Labradoodle Bag Gift Bag of Bag Reporter Colours White Labradoodle Silhouette Shoulder Blue Labradoodle Dog Choice throw readFileErr;
  console.logGenuine Italian Shoulder Womens Body pelle Real Khaki Designer Cross London Vera Small Bag Suede Craze Strap qUxtPWa(data);
  fs.unlink('/file.md', Reporter Shoulder Bag Bag Choice Labradoodle Labradoodle Bag Blue Gift Colours White Silhouette Labradoodle Bag of Dog (Shoulder Prom Ladies Party Shimmer Clutch Style Glitter Party Evening Gold Handbag Wedding Womens Bag Bag 7OqdnzFYwunlinkErr) => {
    if (unlinkErr)Backpack Unisex Green Adults' Combo x Kipling Challenger H Emerald Clas T B 26x36x21 x cm IZdZ76 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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