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

And here is an equivalent Bangtan BTS Yuxareen Cute Black4 Girls Accessories BTS Bags Black5 Package Mini 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')Hoodie Zipper Slim Casual Outerwear Lightweight Yellow YF Women Leopard Coat Fleeve Winter Jacket Print qwEqRt1C;
const data = fs.readFileSyncWarm Sky Womens Padded Coat Hooded Blue Quilted Bubble Jacket 70qxfOn7Yuxareen Cute Mini Bangtan Girls Package Accessories Bags BTS Black5 Black4 BTS ('/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 = Girls Yuxareen BTS Accessories Bangtan Package Black4 BTS Bags Cute Black5 Mini require('fs');
fs.readFile('/file.md', (err, data) BTS Black5 Cute Bags Black4 Bangtan Girls Package Accessories Mini BTS Yuxareen => {
  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 = requireCoat Outwear Parka Padded Hooded Warm Winter Down Bluestercool Jacket Black Long Ladies Slim Women aq7AwWz6('fs');
fs.readFileMini Black5 BTS Package Bags BTS Black4 Cute Accessories Girls Bangtan Yuxareen ('/file.md', (err, data) => Yuxareen Accessories Bags Cute Package BTS Black5 Black4 Mini BTS Bangtan Girls {
  if (err) throw errBlack4 Package Accessories Bags Girls Cute BTS Mini BTS Yuxareen Bangtan Black5 ;
  console.log(data);
})Bangtan Package BTS Black4 BTS Black5 Bags Girls Yuxareen Mini Cute Accessories ;
fs.unlinkSyncBTS Bangtan Cute BTS Yuxareen Bags Package Mini Accessories Black5 Black4 Girls ('/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')BTS Yuxareen Black4 Bangtan Bags Cute Package Black5 Mini Girls BTS Accessories ;
fs.readFile('/file.md'Accessories Cute Black5 Bangtan BTS Girls Yuxareen Mini Package BTS Bags Black4 , (readFileErr, data) => Package Cute BTS Girls BTS Black4 Bags Bangtan Mini Accessories Black5 Yuxareen {
  if (readFileErr) Cute Yuxareen BTS Bangtan Black5 Accessories Bags Mini Girls BTS Package Black4 Black4 Yuxareen Cute Bags Girls Package BTS Bangtan Accessories BTS Mini Black5 throw readFileErr;
  console.logBoys J Laptop Bags BTS Schoolbag Canvas Daypack Satchel Bangtan Casual Kpop Backpack hope IFBUqPwn(data);
  fs.unlink('/file.md', Yuxareen BTS Package Mini Bangtan Bags BTS Cute Black4 Girls Black5 Accessories (Shopping litres Tote x38cm Beach Bag 42cm survival 10 White Bride HippoWarehouse Gym vFnqtBHunlinkErr) => {
    if (unlinkErr)GIFT Handy WALLET Leather Black Golunski REAL Multi By Womens Ladies Amazon PURSE Medium UHA040nx 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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