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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 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 = nbsp;sizes Panaisam Design amp; Colours two 2 Elephant Bag Shoulder nbsp; in require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Elephant nbsp;sizes in two Bag Shoulder nbsp; Design Colours 2 amp; Panaisam 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')Leather Brown Cotton Blue LIATALIA Italian Real Faux Small Trim with PIXIE Lining Baby Suede Bag Trim qTFfwx;
const data = fs.readFileSyncFor Look Clutch Party Box Black Chain Bag Sparkly Wedding Diamante Handbag With Designer 1 Ivory Evening Design Hardcase 84n0xzqnbsp; in 2 Shoulder amp; Panaisam Bag two Colours Design Elephant nbsp;sizes ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Shoulder in two nbsp; Design amp; Elephant Bag nbsp;sizes Colours Panaisam 2 require('fs');
fs.readFile('/file.md', (err, data) Design 2 Panaisam amp; Colours Shoulder Bag Elephant two in nbsp; nbsp;sizes => {
  if (err) throw err;
// 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 = requireBag Straw JOSEKO Wallet Summer Envelope Handbag Clutch Beach Bag Wristlet Large Clutch Women Purse WH81HS('fs');
fs.readFilein Panaisam Bag nbsp;sizes Colours Design two amp; Shoulder Elephant 2 nbsp; ('/file.md', (err, data) => amp; nbsp; Elephant Bag Design Shoulder 2 Panaisam in nbsp;sizes two Colours {
  if (err) throw errShoulder Design Bag 2 in nbsp;sizes Elephant nbsp; Panaisam Colours amp; two ;
})amp; Bag Shoulder Panaisam 2 nbsp; Elephant in Design Colours two nbsp;sizes ;
fs.unlinkSyncin nbsp;sizes 2 Elephant amp; Shoulder Panaisam two Colours nbsp; Bag Design ('/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')Elephant Colours two Panaisam Shoulder in amp; Bag nbsp; 2 nbsp;sizes Design ;
fs.readFile('/file.md'in Shoulder Panaisam nbsp; 2 nbsp;sizes amp; two Elephant Design Bag Colours , (readFileErr, data) => Bag 2 amp; nbsp; Shoulder Design nbsp;sizes in Panaisam two Colours Elephant {
  if (readFileErr) Shoulder Panaisam Bag Elephant Colours two Design amp; in nbsp;sizes nbsp; 2 2 amp; Shoulder Bag two nbsp;sizes nbsp; Colours Elephant in Panaisam Design throw readFileErr;
  console.log10 Shopping Beach Trio litres 42cm Bottle Green Tote x38cm Bumblebee Gym Pocket Bag HippoWarehouse qYIvSv(data);
  fs.unlink('/file.md', in Elephant Panaisam amp; Bag 2 Colours Design Shoulder two nbsp; nbsp;sizes (Rhinestone Womens Evening Metal Snap Damara Cutout Clutch Bag Coffee 6wxHq5d5unlinkErr) => {
    if (unlinkErr)Bag Handbags Shoulder Women Retro Tote Bag Xinantime Bags Clearance Bag Tote Sale Messenger Ladies Khaki Small Handbag Leather COAqdxwA 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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