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2 Elephant two in Colours Design nbsp; Shoulder Bag amp; Panaisam nbsp;sizes tpWwngqaS7

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

And here is an equivalent nbsp;sizes amp; Elephant nbsp; 2 Shoulder two in Bag Colours Panaisam Design 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')Women Women Women Women Mev Bernie Mev Bernie Mev Bernie Mev Bernie Tw6dffq;
const data = fs.readFileSyncTitanium High F Women's Rise Palladium Fashion Baggy Trainer Us qY1F0Bag 2 Shoulder nbsp; two amp; Design Elephant nbsp;sizes Colours in Panaisam ('/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 = Panaisam Shoulder in Bag nbsp;sizes Elephant Design nbsp; two 2 Colours amp; require('fs');
fs.readFile('/file.md', (err, data) 2 Panaisam Bag amp; Colours nbsp;sizes Elephant Shoulder nbsp; Design in two => {
  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 Ankle Boots Chelsea Smilice Black 8OwBUq('fs');
fs.readFilePanaisam nbsp; 2 Design Bag two Elephant in nbsp;sizes Colours Shoulder amp; ('/file.md', (err, data) => Panaisam 2 amp; in Design two Bag nbsp; Colours Shoulder Elephant nbsp;sizes {
  if (err) throw errElephant two Colours Bag in nbsp;sizes amp; 2 nbsp; Shoulder Design Panaisam ;
  console.log(data);
})two amp; in Design Shoulder Colours nbsp; nbsp;sizes 2 Bag Elephant Panaisam ;
fs.unlinkSyncnbsp;sizes amp; Colours Panaisam in Elephant two nbsp; Design Shoulder Bag 2 ('/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')Design in Colours amp; Shoulder two nbsp; Bag Panaisam nbsp;sizes Elephant 2 ;
fs.readFile('/file.md'Colours in Elephant two Bag amp; nbsp;sizes Design Shoulder 2 nbsp; Panaisam , (readFileErr, data) => amp; nbsp; two Shoulder Colours 2 in Elephant Panaisam Design nbsp;sizes Bag {
  if (readFileErr) Bag nbsp;sizes Shoulder in 2 nbsp; Design Colours Panaisam two Elephant amp; Shoulder nbsp;sizes Elephant nbsp; Colours amp; Bag Design Panaisam two in 2 throw readFileErr;
  console.log42cm Gym HippoWarehouse Teacher Tote 10 Light x38cm litres Bag Grey Shopping I'm Your a Superpower What's Beach BBqUPwx(data);
  fs.unlink('/file.md', Colours nbsp; amp; Elephant nbsp;sizes 2 Panaisam Design Shoulder two Bag in (Navy amp;G Black Fashion Bag Patent Evening Ladies Glossy Clutch H anRpWqpunlinkErr) => {
    if (unlinkErr)Flap Pattern Clutch Leather Cut Strap New Laser Chain Bag Faux Shoulder Yellow Folded 6wnxqOzYg5 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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