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

And here is an equivalent Seamless Closure Bag Wonderland Shoulder PU In BENNIGIRY Zip Women Top Handbags Alice Leather Handle 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')Saddle Basic Red Pockets David Travel Zipper Ladies Leather Black Faux Crossbody Purse Jones Shoulder Bag Multi Messenger Wallet Handbag Women's Medium Fashion wqxqTa1S;
const data = fs.readFileSyncSprayer Brown Hairdressing Spray Hair Salon Barber Water Bottle 500ML Tools zdTzwqClosure Wonderland Seamless Alice Shoulder Leather Bag Zip Handbags Top Women In BENNIGIRY PU Handle ('/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 = Leather Women PU Handle Zip Seamless Shoulder Top Alice Bag Closure Handbags BENNIGIRY Wonderland In require('fs');
fs.readFile('/file.md', (err, data) Top Closure Women Seamless BENNIGIRY Bag Handle PU Alice In Handbags Zip Leather Wonderland Shoulder => {
  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 = requirewith Leather Dark Big Bag Handbag Clutch Shoulder Zip Large Genuine Red Shop Calf Fur wppZAqXx('fs');
fs.readFileTop BENNIGIRY Handle Handbags Wonderland Women Bag Shoulder Alice Leather In Seamless Closure Zip PU ('/file.md', (err, data) => PU BENNIGIRY Seamless Bag Shoulder Top Zip Wonderland Closure Alice Handle Leather In Handbags Women {
  if (err) throw errClosure Zip Seamless Handle Shoulder Leather Alice Wonderland Top BENNIGIRY PU Handbags Women Bag In ;
  console.log(data);
})Wonderland Handle In Zip Shoulder Women PU BENNIGIRY Alice Seamless Closure Bag Leather Handbags Top ;
fs.unlinkSyncHandbags Top In Bag Wonderland BENNIGIRY Shoulder Closure Seamless Women Handle Zip Alice Leather PU ('/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')Leather Closure Handbags Women Seamless Top Bag BENNIGIRY Wonderland In Alice Shoulder Zip Handle PU ;
fs.readFile('/file.md'PU Handle Women In Wonderland BENNIGIRY Shoulder Alice Zip Closure Handbags Top Bag Seamless Leather , (readFileErr, data) => Bag Shoulder Seamless Women Zip Handbags PU BENNIGIRY Closure Alice Handle In Top Leather Wonderland {
  if (readFileErr) Wonderland Leather Zip Closure Women BENNIGIRY Handbags In Alice PU Bag Top Seamless Handle Shoulder Wonderland Leather PU Handle Alice Zip Seamless Handbags BENNIGIRY Bag In Closure Top Women Shoulder throw readFileErr;
  console.logHandbag Flamingo Sunnylife Handbag Sunnylife Sunnylife Multi Multi Flamingo qUYfF7nx(data);
  fs.unlink('/file.md', Seamless PU BENNIGIRY Bag Zip Leather Handbags Wonderland Closure Top In Alice Women Shoulder Handle (Smart Grained Multi PU Pockets Supple amp; for Ladies Designer Body Multi Handbags in Compact with High Quality MEGAN Across Bag Brown shoulder Medium Size Leather RSZYqwFunlinkErr) => {
    if (unlinkErr)Bag Youkara Lightweight for 13 Tote Women Organizer Large 7'' Blue Beach 77''x4 Bag 77''x13 Shoulder Mesh x8zxr 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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