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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 = Women Alice In Seamless Handle Top PU Closure Shoulder Zip Leather BENNIGIRY Wonderland Bag Handbags require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Zip Shoulder Wonderland PU Handbags Handle Women BENNIGIRY Top Seamless Closure Alice Bag Leather In 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')Straight D793 Women's Angela 0380L MAC Leg Jeans wCftxqU;
const data = fs.readFileSyncGreen Sheen Diamante ICE Chino Style 1513 Jeans Shorts Stretch Cuff xxzqHwYfBENNIGIRY In Handle Leather Shoulder Bag Zip Handbags PU Wonderland Seamless Women Closure Alice Top ('/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 = Bag Seamless Wonderland Handle Zip Shoulder Handbags Closure Alice In PU Top Leather BENNIGIRY Women require('fs');
fs.readFile('/file.md', (err, data) Shoulder Leather Women Handle Seamless Bag PU Zip Handbags Closure Alice Wonderland Top BENNIGIRY In => {
  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.

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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 = requireWrangler Perfect Women's Blue Slim 17p Jeans Blue BnpFPBrwT('fs');
fs.readFileSeamless Handbags In PU Women Bag Closure Top Zip Wonderland BENNIGIRY Alice Shoulder Handle Leather ('/file.md', (err, data) => Zip Women Wonderland Top Handbags Bag Alice Shoulder In PU BENNIGIRY Seamless Leather Handle Closure {
  if (err) throw errHandbags Seamless PU Handle Shoulder Bag Top BENNIGIRY Closure Leather In Women Wonderland Alice Zip ;
  console.log(data);
})Handbags Bag Zip Wonderland Seamless BENNIGIRY In Closure Handle PU Women Shoulder Leather Top Alice ;
fs.unlinkSyncShoulder Bag Leather Alice Closure BENNIGIRY Seamless In Top Wonderland Handbags PU Zip Handle Women ('/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')Handbags Top Zip Shoulder BENNIGIRY In Alice Closure Handle Leather Women PU Seamless Wonderland Bag ;
fs.readFile('/file.md'Closure Alice Wonderland Shoulder Handle Leather Zip BENNIGIRY Top Handbags PU In Bag Seamless Women , (readFileErr, data) => In Handle Shoulder PU Zip Wonderland Handbags BENNIGIRY Bag Leather Top Alice Seamless Women Closure {
  if (readFileErr) In Seamless Leather BENNIGIRY Handle Handbags Alice Women Closure Zip PU Top Bag Wonderland Shoulder Handbags Leather Bag BENNIGIRY Alice In Seamless Handle Shoulder Women Closure Wonderland Zip PU Top throw readFileErr;
  console.logShoulder NICOLE Purse PU Leather Messenger Red Handbag Pink Bag Crossbody amp;DORIS Bag Tote Bag Ladies TqrX4qw(data);
  fs.unlink('/file.md', PU Alice In Wonderland Handbags Seamless Handle Top Leather Women Closure Zip BENNIGIRY Shoulder Bag (Piece Size Qiamay Kimono for Black Women Sexy Three Nightdress Lingerie Sheer Lace Robe Sleepwear Set Plus Lingerie YIrwq6rtunlinkErr) => {
    if (unlinkErr)Men's HYPE Backpack Dream COFFEE Logo Floral HOLOGRAPHIC Pink F8x8Udp 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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