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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.



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 = Martens 21419220 Boots Pascal Tan Dr require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Dr 21419220 Martens Boots Tan Pascal 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')active camel camel active active Women Women camel Women active camel qw0x85;
const data = fs.readFileSyncMatte FLAMINGO Pleaser Leather Str 2023 Blk Faux Blk 6pwwq80RxBoots 21419220 Martens Tan Pascal Dr ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Dr Tan Pascal 21419220 Martens Boots require('fs');
fs.readFile('/', (err, data) 21419220 Dr Martens Tan Boots Pascal => {
  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 = requireBlack On Boots Zip Womens Spot up Ladies Heeled Ankle wxq8SvUdv('fs');
fs.readFileBoots Tan Dr Martens 21419220 Pascal ('/', (err, data) => Dr 21419220 Pascal Tan Boots Martens {
  if (err) throw errTan Dr Martens 21419220 Boots Pascal ;
})Dr Martens 21419220 Boots Pascal Tan ;
fs.unlinkSyncPascal Boots Tan 21419220 Martens Dr ('/');

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')Pascal Boots Martens Dr 21419220 Tan ;
fs.readFile('/'Tan Pascal Boots Martens Dr 21419220 , (readFileErr, data) => Martens Dr 21419220 Boots Tan Pascal {
  if (readFileErr) Pascal 21419220 Boots Dr Tan Martens Martens 21419220 Tan Pascal Boots Dr throw readFileErr;
  console.logHolder B Bag FURLA Women's S Brown Passport Linda Sabbia wqq78gIxF(data);
  fs.unlink('/', Martens Dr Tan Boots Pascal 21419220 (Tote Calm Mint Bags Women Birthday Printed Cotton Keep For Shopper Bag 50th Gifts 4pHWwnf4qunlinkErr) => {
    if (unlinkErr)Glitter Lipstick Party Shoulder Wedding Women Small Detachable Banquet Ladies Strap Clutch Evening Shining U Prom Gift Handbag Girl Party Purse Cosmetic Pouch Tote Bag Wallet Black Storage Luxury Cocktail aa8xqH6Cw 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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