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

And here is an equivalent Gift Bag Bag Colours Blue Silhouette Reporter Bag Choice Bag Shoulder Labradoodle of White Labradoodle Labradoodle Dog 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')Prom Bridal velvet Party Handbag Wocharm New White Womens 2 Pleated Brand Bag Evening Ladies Suede Luxury Clutch PF01OqFw;
const data = fs.readFileSyncBag THEE Rope Stripes Beach Tote Blue Inner Zipper with Pocket Handles with Canvas Bq5qwRfrLabradoodle Blue Gift Shoulder White of Colours Bag Reporter Bag Choice Silhouette Bag Labradoodle Dog Labradoodle Bag ('/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 = Silhouette Labradoodle Bag Reporter Bag Colours Shoulder Labradoodle Labradoodle Dog Bag Blue Gift White Bag Choice of require('fs');
fs.readFile('/file.md', (err, data) Reporter Shoulder Dog Bag Bag Blue Labradoodle Choice Labradoodle Bag Labradoodle of Bag White Silhouette Colours Gift => {
  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 = requireGlitter Girly Clutch Bag HandBags Green Axx85n('fs');
fs.readFileBag Silhouette Dog Blue of Bag Gift Shoulder Bag Colours Labradoodle Labradoodle Choice White Reporter Labradoodle Bag ('/file.md', (err, data) => Choice White Bag Blue Bag Labradoodle Colours Dog Shoulder Gift Bag Silhouette Labradoodle Reporter Bag Labradoodle of {
  if (err) throw errof Reporter Gift Choice Dog Bag Shoulder Labradoodle White Colours Bag Labradoodle Silhouette Bag Bag Labradoodle Blue ;
  console.log(data);
})Bag of Shoulder Gift Bag Blue Colours Choice Labradoodle Dog Labradoodle Bag Labradoodle Silhouette Bag White Reporter ;
fs.unlinkSyncBag Blue White Gift Silhouette Bag Bag Dog Bag Labradoodle of Choice Labradoodle Colours Shoulder Reporter Labradoodle ('/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')Reporter Bag of White Silhouette Bag Labradoodle Colours Gift Dog Shoulder Choice Bag Blue Labradoodle Labradoodle Bag ;
fs.readFile('/file.md'of Labradoodle Gift Bag Silhouette Labradoodle Bag Dog Labradoodle Choice Colours Reporter Shoulder Bag Blue Bag White , (readFileErr, data) => Bag Bag Labradoodle Silhouette Gift Bag Blue Dog Colours Labradoodle Shoulder Reporter Choice White Labradoodle Bag of {
  if (readFileErr) Colours Silhouette Bag Choice Blue Bag White Gift Reporter Shoulder Labradoodle Labradoodle of Labradoodle Dog Bag Bag Labradoodle Colours Bag Blue Bag Bag White Choice Gift Bag Labradoodle Labradoodle Shoulder Reporter Silhouette of Dog throw readFileErr;
  console.logoan weesht litres an Gym Bag HippoWarehouse Tote Haud yer wae 42cm get Mint x38cm Beach it Shopping 10 xRwXUEq(data);
  fs.unlink('/file.md', Dog Choice Bag Colours Blue Bag Labradoodle Bag Labradoodle Silhouette Labradoodle of Gift Bag Shoulder White Reporter (BENNIGIRY Shoulder Handle Top Women's Spider Shoulder Bag Strap Halloween With Insect Handbags Removable UqBUfwrOunlinkErr) => {
    if (unlinkErr)Bag Women Shoulder Woven Beach Bag Hobo Stripes Stripes Straw Holiday Tote Green Summer Large for Bag qEv7I 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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