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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 = Bag Over Navy Messenger Crossbody Bag Bag Waterproof Shoulder Ladies Womens Work require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Navy Crossbody Work Ladies Waterproof Shoulder Womens Messenger Bag Bag Over Bag 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')Haute Camel for Ladies Black Envelope 3D Clutch Diva Bag qHqw86rP;
const data = fs.readFileSync4Colors Tote Purple Bag Zipper Canvas Boat Hjuns Cozy Blue Shoulder shopper g8XSqCrossbody Bag Work Womens Bag Messenger Ladies Bag Navy Over Waterproof Shoulder ('/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 Crossbody Shoulder Womens Over Ladies Waterproof Navy Messenger Bag Bag Work require('fs');
fs.readFile('/file.md', (err, data) Shoulder Over Navy Womens Bag Bag Crossbody Bag Work Ladies Messenger Waterproof => {
  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 = requireBeauty Clutch Clutch Bow Womens White Irregular Choice White q48vw1vt('fs');
fs.readFileWaterproof Womens Shoulder Bag Messenger Bag Over Navy Work Ladies Crossbody Bag ('/file.md', (err, data) => Messenger Waterproof Work Bag Shoulder Bag Navy Crossbody Womens Ladies Over Bag {
  if (err) throw errNavy Ladies Womens Crossbody Bag Bag Over Work Bag Messenger Waterproof Shoulder ;
  console.log(data);
})Bag Bag Ladies Navy Crossbody Womens Over Bag Waterproof Shoulder Messenger Work ;
fs.unlinkSyncWork Shoulder Bag Waterproof Crossbody Over Messenger Navy Bag Ladies Womens 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')Messenger Over Bag Navy Bag Work Shoulder Crossbody Ladies Waterproof Womens Bag ;
fs.readFile('/file.md'Messenger Womens Navy Bag Waterproof Ladies Shoulder Crossbody Bag Over Work Bag , (readFileErr, data) => Ladies Over Navy Shoulder Work Bag Messenger Bag Waterproof Womens Crossbody Bag {
  if (readFileErr) Waterproof Bag Womens Bag Ladies Over Messenger Navy Bag Crossbody Shoulder Work Waterproof Navy Bag Bag Work Over Shoulder Womens Crossbody Ladies Bag Messenger throw readFileErr;
  console.logParfois Cross Bag Camel Jessy Women YSYnRqr(data);
  fs.unlink('/file.md', Womens Ladies Bag Crossbody Over Bag Waterproof Messenger Work Bag Navy Shoulder (W01 Day BF BH Dot Polka Floral Skull Oilcloth OWL Shoes Patterns Cream Bag Butterfly Tote Butterfly Shopper GFM Birds aqt6nfZwxunlinkErr) => {
    if (unlinkErr)Vintage Exquisite Party Handbag Turquoise Handbag Peacock Sequin Clutch Purse Peacock Retro Teal ibella Evening Eye Wedding Blue Beaded Peacock Catching p6cnWqdqyS 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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