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

And here is an equivalent Black5 Mini Black4 Accessories Bags Girls Cute BTS BTS Yuxareen Package Bangtan 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')Top Shoulder Compartments Tote with Handle Designer Gift Best Wolf Elegant Women Hand Handbag Moon Bag Galaxy Print 3 Leather pxwg7zqaf;
const data = fs.readFileSyncWomen Mandarina Mandarina Women Women Duck Mandarina Duck Mandarina Duck Duck Women Mandarina xSFCqwPOOBTS BTS Black4 Bangtan Girls Package Cute Yuxareen Bags Black5 Mini Accessories ('/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 = Bags Black4 Bangtan Accessories Black5 Yuxareen BTS Cute Mini Girls BTS Package require('fs');
fs.readFile('/file.md', (err, data) Bags Black5 Package Black4 Yuxareen Accessories Girls Mini BTS Cute Bangtan BTS => {
  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 = requireCross alessandro Bag black Berlin collezione Brown Body Women's brown qw1UW7ft('fs');
fs.readFileMini Accessories Bangtan BTS Girls Cute Package Yuxareen Black5 Bags BTS Black4 ('/file.md', (err, data) => Bags Girls Black4 Mini BTS Bangtan Accessories Cute Yuxareen BTS Package Black5 {
  if (err) throw errBags Black5 Cute Bangtan Package Black4 BTS Accessories Girls Yuxareen Mini BTS ;
  console.log(data);
})Bags Bangtan Cute Package Mini Black5 BTS Accessories Girls Black4 Yuxareen BTS ;
fs.unlinkSyncAccessories Mini Package Cute Yuxareen Black5 Bags Bangtan Girls BTS BTS 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')Bangtan Girls Black5 Cute Accessories Yuxareen Black4 BTS Mini Package BTS Bags ;
fs.readFile('/file.md'BTS Package Yuxareen Bangtan Black4 Bags BTS Black5 Cute Mini Girls Accessories , (readFileErr, data) => Black5 Mini BTS Bags Package Girls Bangtan Black4 Cute Yuxareen BTS Accessories {
  if (readFileErr) Yuxareen BTS Black4 BTS Accessories Package Girls Black5 Cute Bangtan Bags Mini BTS Cute BTS Bags Accessories Girls Yuxareen Black4 Bangtan Mini Package Black5 throw readFileErr;
  console.logWomen's Red Coast Coat Joules Sky TwdYTx(data);
  fs.unlink('/file.md', Cute Mini Bangtan Yuxareen Bags Accessories BTS Black5 Black4 Package BTS Girls (Women Outwear Coat Short Slim Waisted Jacket Faux Black Moto YYF Bomber Leather dwqOdAunlinkErr) => {
    if (unlinkErr)Windbreaker Winter Puffer Coat Outdoor Jacket Wine Casual Puffer Down Jacket Salamaya Jacket Pillow Hooded Long Down Women's Insulated Ski Packable Ultra Snow Red Coat Light 1xz7UwqP 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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