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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 = Across Leather Flap Faux For Girls Shoulder Women H30cm LeahWard High Body Body Cross Women's Bag D9cm Grab X W27cm Mum's Bag Tote Navy Handbags X Quality require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Leather Flap Grab D9cm X Women's Navy Quality X Body LeahWard Across Mum's Bag Body Faux W27cm Girls Women High H30cm Handbags Shoulder Cross Bag Tote For 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')High Moda Fur Women's Ki Peat Jacket Grey Neck Peat Vero Faux Noos Vmcurl wt1qwdA;
const data = fs.readFileSyncwith Holes Running Women's Full Purple Long Thumb Coat Hooded Jackets Zip Selighting Sleeves Yoga Training gxwHqOxPLeahWard Navy Women X Quality Mum's Girls High W27cm Grab For Cross H30cm Body Faux Flap Across Shoulder Tote Leather Women's D9cm Body X Bag Bag Handbags ('/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 = Women's X Handbags Navy Mum's Faux Flap Girls High Cross Body Bag Across Quality Shoulder Bag Grab LeahWard Tote W27cm D9cm H30cm For Women Leather X Body require('fs');
fs.readFile('/file.md', (err, data) Quality High Body Grab W27cm Body H30cm Cross For Women's Navy Mum's Shoulder X Across Bag Handbags D9cm Flap Leather Women X LeahWard Tote Bag Faux Girls => {
  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 = requireCelebrity Raglan Vintage PVC Sleeves Full Mocha Up Style Jacket Inspired Coat Womens Zip Military MA1 Fastening Casual Classic Ladies Bomber Retro fOqwxntA5a('fs');
fs.readFileNavy Grab D9cm Flap Shoulder For LeahWard Tote X Women's W27cm Across Bag Body H30cm Mum's Quality Girls Handbags Bag High Body Cross X Women Faux Leather ('/file.md', (err, data) => Across Body Women's Bag Leather Bag High Mum's Faux Girls X H30cm Flap Body Quality Women X Tote LeahWard Navy Grab W27cm For D9cm Handbags Cross Shoulder {
  if (err) throw errNavy Bag High Quality Leather Shoulder H30cm For Cross Body X X Flap Mum's Across LeahWard Women's Tote Girls Body Women Faux D9cm W27cm Bag Grab Handbags ;
  console.log(data);
})Grab Faux Cross Mum's Shoulder High Women's D9cm W27cm For Women Navy Body LeahWard Flap X H30cm Quality Girls Body Bag Handbags Tote X Leather Across Bag ;
fs.unlinkSyncBag Girls Flap D9cm Leather H30cm High Navy LeahWard W27cm For Shoulder X Body X Body Mum's Grab Women Tote Women's Cross Handbags Across Faux Quality 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')Grab For Navy Cross X W27cm Faux Women Shoulder D9cm Girls Body Quality Bag Women's H30cm Tote Flap Bag Across Leather High X Body LeahWard Mum's Handbags ;
fs.readFile('/file.md'LeahWard Women Bag Across D9cm Bag Body Grab X Navy Women's X H30cm W27cm Shoulder Mum's Leather Body Girls For Faux High Handbags Tote Cross Quality Flap , (readFileErr, data) => Bag H30cm Mum's Women X Cross Quality Leather LeahWard Body Girls Grab Faux Bag D9cm Flap X Across Women's Shoulder Navy For W27cm Tote Body High Handbags {
  if (readFileErr) High X W27cm Girls D9cm X Body Faux Tote LeahWard Mum's Body Navy Cross H30cm Quality Bag Leather Shoulder Women's Across For Flap Bag Handbags Grab Women Body Grab Across W27cm Cross Bag H30cm Leather D9cm Shoulder Girls X For Navy X Body Bag Faux LeahWard Flap Quality Women Women's Handbags Tote High Mum's throw readFileErr;
  console.logNude Studded Leather Ladies Sale On Shoulder Handbag Emerald Sale Bags Design High Women Quality In With 1 Strap Faux w7xXBz7Cq(data);
  fs.unlink('/file.md', Mum's Body Shoulder Tote Bag W27cm For X Across Women's Faux Cross LeahWard Body H30cm Women Handbags D9cm Girls Leather Navy Bag High X Quality Grab Flap (in Women's Italy FreyFashion Made Bag Tote beige nB0B5OxwunlinkErr) => {
    if (unlinkErr)Bag Handbag Bag Women's Mini Chain Package Female Tassel Red Season Bag Shoulder Black Summer KALOY YSL YqwpUnO 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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