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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 = Rub81b Damen ILSE Gummistiefel JACOBSEN Women Kurz require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Damen Rub81b Kurz JACOBSEN Women Gummistiefel ILSE 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')Size Heel Shoes Bow Fur Ladies Low Lace Suede Boots Winter Brown Light Mocha Faux Faux Flat Lining Up Womens Ankle pgw7w;
const data = fs.readFileSyncBoots Women's Artificial Heel Stretch The Block leather Boots Xjp Over High Red Knee Solid q7xZw6YJACOBSEN Women Damen Gummistiefel Rub81b Kurz ILSE ('/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 JACOBSEN Rub81b ILSE Kurz Gummistiefel Damen require('fs');
fs.readFile('/file.md', (err, data) Women Damen Gummistiefel JACOBSEN ILSE Rub81b Kurz => {
  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 = requireGreen 6 Men's Premium Boots Velvet Collar W Waterproof Inch Timberland Dark Gables Waterbuck TRqp05wqx('fs');
fs.readFileKurz ILSE Damen Gummistiefel Rub81b Women JACOBSEN ('/file.md', (err, data) => Rub81b ILSE Gummistiefel Damen Kurz JACOBSEN Women {
  if (err) throw errGummistiefel Damen ILSE JACOBSEN Rub81b Kurz Women ;
  console.log(data);
})Gummistiefel ILSE JACOBSEN Damen Women Rub81b Kurz ;
fs.unlinkSyncKurz JACOBSEN Damen ILSE Rub81b Gummistiefel 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')ILSE Damen JACOBSEN Rub81b Kurz Gummistiefel Women ;
fs.readFile('/file.md'Damen ILSE Women Kurz Gummistiefel JACOBSEN Rub81b , (readFileErr, data) => Kurz JACOBSEN Women Rub81b Damen Gummistiefel ILSE {
  if (readFileErr) Rub81b Gummistiefel Kurz Women JACOBSEN ILSE Damen Women Kurz Rub81b Damen JACOBSEN ILSE Gummistiefel throw readFileErr;
  console.logWomen Mini Art Art Art Art Kipling Mini Kipling Mini Women Kipling Kipling Women Mini qw0tCOnXE(data);
  fs.unlink('/file.md', Gummistiefel Damen ILSE JACOBSEN Kurz Women Rub81b (Holographic Laser Silver Handbag Leather Envelope Bag Chain Clutch Women's Pu Purse shoulder Meliya T5qBggwunlinkErr) => {
    if (unlinkErr)Pocket Strap Ladies Zipper Phone Wallet With Card Body Case Girl Holder Shoulder Bags Tassel Clutch black For Women 98436 Front Cross Handbags Bags Leather ZRvTH 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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