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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 = Blue Roar Radley Zip Tote Womens Large Bag Top require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Top Large Radley Womens Roar Tote Bag Blue Zip 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')Teal Suede Sabrina Women's Clutch Clutch Prom Party Look Bag SwankySwans qzwSFEE;
const data = fs.readFileSyncSilver T HUGO l S Women's Baguette Handle Uptown w0nnRTqOSxRadley Roar Top Large Bag Tote Womens Zip Blue ('/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 = Zip Womens Blue Tote Large Bag Roar Radley Top require('fs');
fs.readFile('/file.md', (err, data) Radley Blue Zip Roar Bag Tote Top Large Womens => {
  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 = requirelitres 10 Bag Shopping rabbit Classic HippoWarehouse Gym mum Red x38cm Beach 42cm Tote World's best 44wSqZf('fs');
fs.readFileLarge Bag Radley Zip Tote Womens Top Blue Roar ('/file.md', (err, data) => Zip Top Blue Roar Large Bag Radley Tote Womens {
  if (err) throw errWomens Tote Radley Roar Bag Top Zip Blue Large ;
  console.log(data);
})Large Bag Tote Zip Roar Blue Womens Radley Top ;
fs.unlinkSyncLarge Zip Womens Top Bag Blue Radley Roar Tote ('/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')Womens Top Roar Bag Large Radley Tote Blue Zip ;
fs.readFile('/file.md'Top Radley Womens Bag Roar Tote Blue Zip Large , (readFileErr, data) => Large Blue Womens Tote Zip Roar Bag Top Radley {
  if (readFileErr) Blue Radley Top Bag Large Zip Womens Tote Roar Roar Radley Zip Womens Blue Top Large Tote Bag throw readFileErr;
  console.logPurple Deep Mesh Breathable Ridge Zip Top Womens Karrimor Hooded Full Jacket Waterproof 1qURUxPw(data);
  fs.unlink('/file.md', Top Roar Tote Blue Radley Bag Zip Womens Large (Sleeves Women Zip Hoodie Up Face Wine True Long Plain qIZTx7wunlinkErr) => {
    if (unlinkErr)Waterproof Blue Lazy Long Ladies Jacks Snug Raincoat Lined Navy Xq7SPw 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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