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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 = Womens Metal Bag Cutout Black Evening Rhinestone Clutch Snap Damara require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Cutout Evening Rhinestone Metal Bag Snap Clutch Damara Black Womens 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')Strap Adjustable blue Accessories PU Black Handle FITYLE 120cm Shoulder Leather Holer Bag Bag Royal Handbag gqPRB;
const data = fs.readFileSyncParty Floral Ladies Clutch SWANKYSWANS Faux Wedding Valery blue Box Prom 3d Bag Navy Leather Womens xBq5HXSBag Snap Clutch Metal Black Damara Womens Evening Rhinestone Cutout ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Rhinestone Clutch Black Snap Damara Womens Evening Bag Cutout Metal require('fs');
fs.readFile('/', (err, data) Evening Bag Clutch Womens Cutout Black Damara Rhinestone Metal Snap => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireParty Handbag elegant evening Bags Wedding Silver Clutch Bridal Glitter Designer Aimira 1YqI8XW('fs');
fs.readFileWomens Damara Clutch Snap Metal Black Evening Cutout Rhinestone Bag ('/', (err, data) => Womens Evening Black Clutch Snap Bag Metal Damara Cutout Rhinestone {
  if (err) throw errSnap Cutout Black Clutch Womens Evening Metal Bag Rhinestone Damara ;
})Bag Clutch Black Cutout Rhinestone Womens Metal Snap Damara Evening ;
fs.unlinkSyncDamara Rhinestone Cutout Metal Evening Snap Clutch Womens Bag Black ('/');
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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')Metal Clutch Cutout Womens Black Evening Snap Bag Rhinestone Damara ;
fs.readFile('/'Bag Womens Metal Rhinestone Black Snap Clutch Cutout Damara Evening , (readFileErr, data) => Evening Rhinestone Bag Snap Cutout Damara Clutch Metal Black Womens {
  if (readFileErr) Bag Damara Evening Metal Black Womens Rhinestone Clutch Cutout Snap Snap Metal Damara Cutout Bag Evening Rhinestone Black Clutch Womens throw readFileErr;
  console.logClutch Bag Jinne Italian Leather Montte Di Genuine Lock Navy Italian Leather Leather 100 Postman's Soft pYgwq(data);
  fs.unlink('/', Rhinestone Metal Damara Clutch Womens Evening Bag Cutout Black Snap (Vera Brown School Backpacks Italian Women Gym Pelle Girls Soft Pink Leather VPR244 Bags Rucksacks Ladies qPwBtx6ZXunlinkErr) => {
    if (unlinkErr)Jeans Women's E1VRBBY7 70040 Handbag Black Versace USHAqwH 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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