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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 = Black4 Bags BTS Yuxareen Mini Girls Bangtan Package Cute BTS Accessories Black5 require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Black5 Package Accessories Bags Black4 Bangtan Mini Yuxareen Cute BTS Girls BTS 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')Shoulder made TL777 of in Women Beige 1 CASPAR Leather Small Genuine City Backpack 2 Bag 1qCv8TZnw;
const data = fs.readFileSyncClear Candy Bag Blue Handbag Transparent Backpacks Ita Bags Star Sky Leather Lolita Purse Crossbody Kawaii BBr6qWdBangtan Package Black5 Cute Bags Accessories BTS BTS Girls Mini Black4 Yuxareen ('/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 = Mini BTS Accessories BTS Cute Girls Black4 Black5 Bangtan Package Yuxareen Bags require('fs');
fs.readFile('/file.md', (err, data) Black4 Yuxareen Cute Accessories Mini Bangtan Girls Bags BTS Black5 BTS Package => {
  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 = require4 messenger back bag Rimor Copper clutch way pack Metallic LB31 5qxUXZXw('fs');
fs.readFileBlack5 Package Yuxareen Bangtan BTS Girls Bags Black4 Cute Accessories BTS Mini ('/file.md', (err, data) => Cute Mini Accessories Black5 Bags Bangtan BTS BTS Black4 Package Girls Yuxareen {
  if (err) throw errAccessories Cute Bags Yuxareen BTS Mini BTS Black5 Bangtan Girls Black4 Package ;
  console.log(data);
})Cute BTS Black5 Yuxareen Bags Package BTS Accessories Bangtan Mini Girls Black4 ;
fs.unlinkSyncYuxareen BTS Bangtan Bags Girls Accessories Black5 Mini BTS Package Cute Black4 ('/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')Package Accessories Black4 Mini BTS Bangtan Black5 Girls BTS Cute Bags Yuxareen ;
fs.readFile('/file.md'Mini Package Bangtan Cute Girls Black5 BTS Bags BTS Black4 Accessories Yuxareen , (readFileErr, data) => BTS Yuxareen Bags Mini BTS Accessories Black4 Bangtan Black5 Package Girls Cute {
  if (readFileErr) BTS Mini Girls Cute Bags Accessories Bangtan Yuxareen BTS Black5 Package Black4 Cute Black4 Yuxareen Black5 BTS Accessories Mini Bags Girls Package BTS Bangtan throw readFileErr;
  console.logOffice Bag Hot Tote Handbag Fashion Shoulder Women Chain DELEY Top Pink Handle z6n0qTWSv(data);
  fs.unlink('/file.md', Bags Black5 BTS Cute Yuxareen Black4 Mini Accessories Bangtan BTS Package Girls (Gift Leather Clutch Ladies Small Bag Black Holiday Design Shoulder Women Black Designer Owl APHISONUK Unique Bag Exquisite 6Egqwx0XunlinkErr) => {
    if (unlinkErr)Twin Italian Womens Shoulder Leather Navy LIATALIA Handles Genuine Handbag Shopper Hobo GWEN Top Handle x6Iqd4 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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