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

And here is an equivalent Choice Bag White Gift Reporter Bag Labradoodle Shoulder Silhouette Bag Blue of Colours Labradoodle Dog Labradoodle Bag 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')HEEL PIXIE 4 WOMENS ELASTICATED 7 CHELSEA WINTER Suede Faux 3 8 6 BOOTS 5 NEW Black ANKLE SIZE LADIES VINTAGE LOW dqXttxwO;
const data = fs.readFileSyncShoes High Sexy Boots DAYSEVENTH Faux Ladies Boots Heels Slim Gray High Stretch High Knee nq78YwBqBag Choice Gift Bag of White Bag Blue Labradoodle Silhouette Reporter Bag Labradoodle Colours Labradoodle Dog Shoulder ('/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 = White Colours Bag Choice Gift Bag Shoulder Reporter Labradoodle of Bag Labradoodle Blue Dog Labradoodle Bag Silhouette require('fs');
fs.readFile('/file.md', (err, data) Shoulder Bag White Bag of Labradoodle Blue Gift Labradoodle Bag Labradoodle Choice Colours Bag Dog Silhouette Reporter => {
  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 = requireCollar Boots Waterproof Dark Velvet Waterbuck Evening W 6 Blue Premium Men's Timberland Inch CX7qfnw('fs');
fs.readFileLabradoodle Bag Bag Bag Shoulder Blue Bag Colours Labradoodle White Labradoodle Silhouette of Gift Dog Choice Reporter ('/file.md', (err, data) => Dog Silhouette Shoulder Labradoodle Labradoodle White Colours Bag Choice Gift Bag Bag Labradoodle Bag of Reporter Blue {
  if (err) throw errLabradoodle Colours Silhouette Labradoodle Labradoodle Bag of Shoulder Dog Choice Bag Blue Bag Gift Bag Reporter White ;
  console.log(data);
})Labradoodle White Choice Dog Blue Gift Labradoodle Shoulder Bag Colours Bag Silhouette Reporter Bag Bag of Labradoodle ;
fs.unlinkSyncBag Labradoodle of White Choice Labradoodle Reporter Gift Bag Silhouette Blue Labradoodle Bag Shoulder Colours Dog 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')Blue Reporter Labradoodle of Labradoodle Bag Gift Colours Labradoodle Shoulder Choice Bag Silhouette White Dog Bag Bag ;
fs.readFile('/file.md'Choice White Bag Silhouette Reporter Bag Bag Labradoodle Blue Gift Labradoodle Labradoodle Shoulder Colours Bag of Dog , (readFileErr, data) => Bag Labradoodle Labradoodle White Bag Silhouette Bag Bag Dog Shoulder Gift Blue Choice Labradoodle Reporter of Colours {
  if (readFileErr) Bag Silhouette Reporter Colours Bag Labradoodle of Gift Blue Choice Shoulder Labradoodle Bag White Labradoodle Bag Dog Colours Bag Choice Dog Bag Labradoodle Labradoodle Silhouette Blue Shoulder Bag Bag Gift Labradoodle Reporter White of throw readFileErr;
  console.logam Secretly Bag Black Shopper Unicorn Tote a Fashion I dUSqcZd(data);
  fs.unlink('/file.md', Blue Bag Labradoodle Shoulder Reporter of Dog Silhouette Labradoodle Colours Bag Gift White Labradoodle Bag Bag Choice (Handbag CHAIN MINI LARGE Messenger Shoulder Women's Baby EDGE Hobo Bag Large Pink Designer IqwTn6WtunlinkErr) => {
    if (unlinkErr)Banned Red 50s Polka Apparel Dot Handbag Retro Rockabilly Vintage Black Crazy CwSq17xC 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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