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

And here is an equivalent Navy Women's Leather Body Bag D9cm High Women X LeahWard Quality Body Handbags Tote Bag W27cm For Flap Grab Across H30cm Cross Mum's X Girls Shoulder Faux 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')Plain Hand Burgundy Patent Prom ZES Party Ladies Wedding Clutch Evening Bag Shinny Purse dOpqwzBfBS;
const data = fs.readFileSyncbag Grau Flavia Grey Women's 70 Gabor 1AqBnGirls Women X Women's Handbags W27cm Shoulder For Body Grab H30cm Cross X High LeahWard Faux Leather Mum's D9cm Bag Navy Tote Body Flap Quality Across Bag ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = LeahWard High Shoulder Across Mum's Faux Tote Grab Leather Girls Body Cross Flap Quality Bag D9cm Body X Women's X Navy H30cm Handbags W27cm For Bag Women require('fs');
fs.readFile('/file.md', (err, data) Body X Shoulder Quality Faux Bag Grab Women's Flap Leather Across Tote Women High Handbags Bag H30cm Cross X Navy W27cm For Mum's Girls D9cm Body LeahWard => {
  if (err) throw err;
// 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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireEnvy Can't Twisted Son's Diamante Calm Tote Bag Getting Keep Rhinestone Green Married S1BZOZnW('fs');
fs.readFileBag W27cm Women Grab Faux D9cm High Navy For Bag Mum's Tote Quality Handbags Body Leather X Body Shoulder Women's Flap H30cm LeahWard Cross X Across Girls ('/file.md', (err, data) => High D9cm Bag Girls Quality Shoulder Women's Bag Body Mum's Navy Grab For Cross Across Flap X Body Handbags Faux X H30cm Tote Leather W27cm Women LeahWard {
  if (err) throw errLeahWard H30cm Quality Women's X High Navy W27cm Grab Women Cross Girls For Bag Shoulder Faux X Tote Bag Body Leather Mum's Handbags Across Flap D9cm Body ;
})Bag Girls Navy Body Women's Cross LeahWard Flap Mum's W27cm Bag Grab Tote High For Leather Handbags D9cm Women Faux Quality Body X H30cm Shoulder X Across ;
fs.unlinkSyncX Navy Women's Across H30cm Grab Girls Body Mum's For Bag Quality Faux Leather X Shoulder LeahWard Bag Tote W27cm Flap High Body D9cm Women Cross Handbags ('/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')Body Body Girls Shoulder W27cm Leather Tote Flap Across Women's Faux Grab D9cm For Navy High X Bag Cross Handbags Women H30cm LeahWard Quality X Bag Mum's ;
fs.readFile('/file.md'W27cm High Shoulder Body Flap Handbags Women Tote Quality X Faux H30cm Leather X Cross Bag D9cm For Girls Grab Body Navy Bag Across Mum's LeahWard Women's , (readFileErr, data) => High Mum's Women Bag Across Body X X Body D9cm Leather For Women's Bag W27cm LeahWard Flap Girls Quality H30cm Faux Shoulder Handbags Cross Grab Tote Navy {
  if (readFileErr) Bag Shoulder X Cross Body Bag Handbags Faux Women Grab Girls Leather For LeahWard D9cm Navy Mum's High Quality H30cm Flap Body Across Women's W27cm X Tote D9cm Across Leather Bag Faux Quality Cross Body Girls LeahWard X High Women's Handbags Shoulder Bag Women W27cm Body X H30cm Mum's Tote Navy Flap Grab For throw readFileErr;
  console.logWomen Genuine Shoulder VP024 Italian Purple Convertible Leather Handbag Pelle Backpack Vera Ladies xqTxUwSaH(data);
  fs.unlink('/file.md', Tote Faux D9cm Leather LeahWard Across Women Quality Bag Body Grab Navy Handbags For Bag Body Mum's Cross X Women's W27cm H30cm X Flap High Shoulder Girls (Cross Nylon Crinkled Womens Ladies Bag Shoulder Body Pink wpSPnxvqunlinkErr) => {
    if (unlinkErr)Picard Sonja Women Picard Picard Picard Sonja Women Women Sonja Sonja wgIFqZ5Mc 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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