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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 = Collection Handbags Red Catwalk Womens Kensington Totes require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Womens Catwalk Red Totes Handbags Collection Kensington 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')School Travel Corduroy Patched Girls for Women HMHUI Daypack Backpack Casual Blue Letter xUfIBqF;
const data = fs.readFileSyncBeach Bag French Gym Crazy 10 Tote lady pig Shopping litres HippoWarehouse x38cm guinea 42cm Navy Tgwq80wSCollection Catwalk Handbags Womens Kensington Red Totes ('/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 = Red Handbags Totes Collection Womens Catwalk Kensington require('fs');
fs.readFile('/file.md', (err, data) Kensington Handbags Red Totes Collection Womens Catwalk => {
  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 = requireLadies Suede New Bags Girls Handheld Faux Nude Leather Handbags London Xardi Evening Clutch Women w7C5qnxI07('fs');
fs.readFileCatwalk Collection Totes Handbags Kensington Womens Red ('/file.md', (err, data) => Totes Red Catwalk Handbags Womens Collection Kensington {
  if (err) throw errTotes Kensington Collection Handbags Catwalk Womens Red ;
  console.log(data);
})Handbags Totes Kensington Womens Catwalk Collection Red ;
fs.unlinkSyncTotes Red Kensington Collection Catwalk Handbags Womens ('/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')Kensington Collection Handbags Catwalk Womens Red Totes ;
fs.readFile('/file.md'Womens Red Kensington Totes Catwalk Collection Handbags , (readFileErr, data) => Collection Kensington Womens Catwalk Handbags Red Totes {
  if (readFileErr) Womens Red Catwalk Kensington Totes Collection Handbags Handbags Catwalk Collection Totes Red Womens Kensington throw readFileErr;
  console.logToe Mid High Boots Gray Ankle Boots Ankle Stilettos Women's Stiletto Heel Heel Women Ankle 8cm Pointy ELASHE Mid Fashion Boots Wq0HE6ZnpW(data);
  fs.unlink('/file.md', Handbags Kensington Red Collection Womens Catwalk Totes (Martin 3 Mid Ankle Boots UK Women Platform Black Shoes Heel Ankle 5 Boots Zipper Ladies Beige wX7CqR1unlinkErr) => {
    if (unlinkErr)Comfys Womens Leather SIZE 8 Boots Mod Nappa Chelsea Ankle Ladies Black ZZIEr7 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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