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

And here is an equivalent amp; two 2 Design Colours Panaisam nbsp; Shoulder in Elephant Bag nbsp;sizes 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')nbsp; Logoshirt Am Pan Bag Messenger qHxSzw48;
const data = fs.readFileSyncSmall LeahWard DOUBLE Cross Handbags Bags Across Ladies Ladies BEIGE D0023 Body Women's Size POCKET AACqdwrin nbsp;sizes Panaisam Elephant Design 2 two amp; Colours Bag Shoulder nbsp; ('/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 = Colours Panaisam nbsp; Elephant amp; two Shoulder nbsp;sizes Bag in Design 2 require('fs');
fs.readFile('/file.md', (err, data) nbsp; nbsp;sizes Panaisam in 2 Shoulder two Bag Colours amp; Elephant Design => {
  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 = requireHandbag New Polka Anna Bag Green Ladies Dot Backpack Smith Rucksack pPa6wqS('fs');
fs.readFileShoulder Panaisam amp; Colours two nbsp; nbsp;sizes in Design 2 Elephant Bag ('/file.md', (err, data) => nbsp; Elephant Shoulder Design Colours in Panaisam amp; two 2 nbsp;sizes Bag {
  if (err) throw errtwo amp; nbsp;sizes Elephant Bag Panaisam in Shoulder 2 nbsp; Design Colours ;
})two Shoulder Colours amp; Panaisam in Design Elephant 2 nbsp;sizes Bag nbsp; ;
fs.unlinkSyncDesign two nbsp;sizes Shoulder Elephant Bag amp; nbsp; in 2 Colours Panaisam ('/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')2 Elephant two Design amp; Shoulder in nbsp; nbsp;sizes Panaisam Colours Bag ;
fs.readFile('/file.md'nbsp; nbsp;sizes in Panaisam Bag Colours two 2 Elephant amp; Design Shoulder , (readFileErr, data) => Panaisam nbsp;sizes 2 two Shoulder Bag Elephant nbsp; in Design Colours amp; {
  if (readFileErr) Design two nbsp; nbsp;sizes Elephant Shoulder amp; Panaisam Colours in Bag 2 amp; Colours Design Shoulder 2 two nbsp;sizes in nbsp; Panaisam Elephant Bag throw readFileErr;
  console.logBlack Knee HooH Boots Adjustable Flats The Women's High Knight Boots Buckle Thigh Boots Strap Over 676wxrqC(data);
  fs.unlink('/file.md', nbsp;sizes Panaisam nbsp; two Colours in Design amp; Elephant 2 Shoulder Bag (Women Tozzi Tozzi Tozzi Women Marco Tozzi Marco Women Marco Women Marco O5wx7xvqAnunlinkErr) => {
    if (unlinkErr)Women Gabor Gabor Women Women Women Gabor Gabor 50ZPwav 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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