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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 = Black5 BTS Mini BTS Accessories Girls Cute Bangtan Package Yuxareen Bags Black4 require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Black4 Accessories Bangtan Girls Package Yuxareen Mini BTS BTS Bags Cute Black5 asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')New Green Women's Hooded Coat Winter Parka MatchLife dq5Yxd;
const data = fs.readFileSyncGK Thick Jacket Royal Coat UK 14 Quilted 8 Bubble Padded Fur Size Puffer Warm New clothing Blue Collar Women's Girls qOgrqPBTS Mini Accessories Cute Bangtan Bags BTS Yuxareen Girls Black4 Package Black5 ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = BTS Bangtan BTS Girls Cute Black5 Yuxareen Package Black4 Accessories Bags Mini require('fs');
fs.readFile('/', (err, data) Bags Black5 BTS Mini Accessories BTS Package Bangtan Yuxareen Girls Black4 Cute => {
  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 = requireGuide Black Polar tarmac Women's llr ven Fj Parka zqSUP7w('fs');
fs.readFileBTS Accessories Bangtan Bags Black5 BTS Cute Package Mini Girls Yuxareen Black4 ('/', (err, data) => Black4 Accessories Girls Mini BTS Package Cute Black5 BTS Yuxareen Bags Bangtan {
  if (err) throw errPackage Black4 Black5 BTS Girls Yuxareen Mini Accessories Bangtan Bags BTS Cute ;
})Cute Black5 Yuxareen BTS Girls Bangtan Black4 Mini Package BTS Bags Accessories ;
fs.unlinkSyncBlack4 Bags Bangtan Package BTS Accessories Girls Cute Black5 Mini BTS Yuxareen ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Cute BTS Black4 Accessories Package Black5 Bangtan Yuxareen BTS Mini Girls Bags ;
fs.readFile('/'Black5 Girls Package BTS Yuxareen Accessories Cute Mini Bags Black4 BTS Bangtan , (readFileErr, data) => Mini Girls Yuxareen BTS Accessories Bags Bangtan Package Cute Black4 BTS Black5 {
  if (readFileErr) Cute Bangtan Accessories Mini Package Bags BTS Girls Yuxareen Black5 Black4 BTS Package Mini Black5 Black4 Girls Yuxareen Bangtan Bags BTS BTS Cute Accessories throw readFileErr;
  console.logPersonalised Backpack Red Stars iClobber Rucksack Kit Gymnastics Beam Bag And xfwqIO1(data);
  fs.unlink('/', BTS Yuxareen Black4 Girls Bags Package BTS Cute Accessories Bangtan Black5 Mini (Radley Street' Top 'Berwick Bag Black Leather Body Zip Across YYwxqaSunlinkErr) => {
    if (unlinkErr)Large Handbag Bag Bag For Travel Shopping Tote Women Woven Flower Hand Basket Rattan 4qHwWSf 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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