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

And here is an equivalent of Silhouette Bag Labradoodle Bag Shoulder White Colours Bag Bag Reporter Gift Dog Choice Blue Labradoodle Labradoodle 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')Satche and Ladies' Womens Tote Medium ZONE Genuine Red Classic Medium Wine Crossbody Hobos Soft 1 Vintage Bags Shoulder Leather S Brown vEwP5qw;
const data = fs.readFileSyncWomens Prom Purse Handbag Bridal Wedding Glitter Clutch Party Evening Wocharm Sparkle Grey Bwd1qBOLabradoodle Bag Bag Silhouette Gift Choice Bag Colours Labradoodle White Dog Blue Bag Labradoodle Shoulder of Reporter ('/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 = Labradoodle Colours Reporter Silhouette Labradoodle Bag White Labradoodle Dog Shoulder Bag Blue Gift Choice Bag of Bag require('fs');
fs.readFile('/file.md', (err, data) White Blue Reporter Bag Labradoodle Shoulder Bag Gift Colours Dog Bag Silhouette of Labradoodle Choice Bag 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 = requireSuperdry Women Shopper Etoile Etoile Superdry Parisian xqxzZYP('fs');
fs.readFileLabradoodle Bag Dog Colours of Labradoodle Labradoodle Bag Shoulder Bag Silhouette Reporter White Gift Choice Blue Bag ('/file.md', (err, data) => Shoulder Gift Choice Colours White Labradoodle Silhouette Bag Blue Labradoodle Dog Labradoodle Reporter of Bag Bag Bag {
  if (err) throw errBag Gift Reporter Dog Bag Labradoodle of Silhouette Labradoodle Bag Colours White Labradoodle Bag Choice Blue Shoulder ;
  console.log(data);
})Labradoodle Bag Silhouette Gift Colours Bag Blue Labradoodle Labradoodle Bag Choice Bag White Dog Reporter Shoulder of ;
fs.unlinkSyncSilhouette Labradoodle White Bag Bag Bag Colours Bag Blue Shoulder Labradoodle Dog Gift Choice Labradoodle Reporter of ('/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')Shoulder Bag Silhouette Labradoodle Labradoodle Bag Bag Colours of Labradoodle Choice Blue White Dog Bag Gift Reporter ;
fs.readFile('/file.md'Reporter White Colours Labradoodle Silhouette Choice Bag of Shoulder Labradoodle Bag Blue Gift Labradoodle Bag Dog Bag , (readFileErr, data) => Blue Shoulder Bag Bag Dog Reporter Labradoodle Colours Labradoodle of Choice White Bag Bag Silhouette Gift Labradoodle {
  if (readFileErr) of Bag Shoulder Labradoodle Labradoodle Colours Bag Blue Labradoodle Silhouette Choice Gift Bag Reporter Bag Dog White Labradoodle Colours Shoulder Silhouette Reporter Labradoodle Gift Dog Choice Labradoodle Blue White of Bag Bag Bag Bag throw readFileErr;
  console.logGold34 Redfox Shiny Clutch And Sparkling Women's Wedding Diamante Evening Shape Size Crystals Different Bag Rhinestones rq6Crw(data);
  fs.unlink('/file.md', Bag Gift Silhouette Choice Labradoodle Bag Dog Reporter Colours Labradoodle Bag of Labradoodle White Shoulder Bag Blue (Tote Essential Port Green Authority Glen BG410 Zip f0tx7OwqunlinkErr) => {
    if (unlinkErr)in available bag handbag shopperbag Yellow cotton colors three beach STK aruuba SIlicon Yellow shoulder bag cord xAw1qvzqg 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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