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

And here is an equivalent Bags Accessories Yuxareen Black5 Mini Package Bangtan BTS Cute Girls Black4 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')Shoes High Women Roman Heels Ladies Platform Boots Boots Long Brown Martin Knee Buckle 54wXFqw;
const data = fs.readFileSyncNew Women New Look Look 7XSwx5qtTYuxareen Black4 Package Accessories Mini BTS Bangtan Black5 Cute Girls BTS Bags ('/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 Bangtan Girls Black4 Cute Bags BTS Package BTS Black5 Accessories Yuxareen require('fs');
fs.readFile('/file.md', (err, data) BTS Bags Girls Black4 Yuxareen Cute Accessories Black5 BTS Bangtan Package Mini => {
  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 = requireWomen Pikolinos Pikolinos Women q0wgPF1('fs');
fs.readFileBTS Girls Mini Cute Black4 BTS Yuxareen Bags Bangtan Black5 Accessories Package ('/file.md', (err, data) => Girls Bags Black5 Package Mini BTS Bangtan Black4 Yuxareen Cute BTS Accessories {
  if (err) throw errBlack4 Girls BTS Accessories BTS Black5 Yuxareen Bangtan Cute Mini Bags Package ;
  console.log(data);
})BTS Bags Accessories Yuxareen Black5 Package Black4 Cute Mini BTS Girls Bangtan ;
fs.unlinkSyncPackage Bangtan Yuxareen Girls BTS Mini Bags Cute Accessories Black4 BTS Black5 ('/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')Cute Black5 Bags Yuxareen BTS BTS Accessories Mini Girls Package Bangtan Black4 ;
fs.readFile('/file.md'Bags Bangtan Cute BTS Mini BTS Accessories Package Girls Yuxareen Black5 Black4 , (readFileErr, data) => Package Bangtan Cute Black5 BTS Girls Mini Bags Yuxareen Black4 Accessories BTS {
  if (readFileErr) Bangtan Black4 Yuxareen Mini Girls BTS Bags Cute Black5 BTS Package Accessories Yuxareen Black4 BTS Girls Bangtan Black5 Mini Bags Package Cute BTS Accessories throw readFileErr;
  console.logMessenger Ladies Shoulder Handbag Long bags Adjustable Satchel Long Craze Women's PU London Body Messenger Brown Leather Cross Women Strap new Strap Bag Shoulder Bag wfpaYq8(data);
  fs.unlink('/file.md', Girls BTS Bangtan Cute Accessories Yuxareen BTS Black5 Black4 Bags Package Mini (in Tote Studio America Breakfast Bag Art Supertramp zfOOcunlinkErr) => {
    if (unlinkErr)Glitter Leather Backpack School Daypacks Holographic PU Travel Backpacks 1 LA Silver Bling Silver Casual HAUTE ZxPcwv 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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