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Various Col47 Tweed Colours Harris Body Mini LB1120 Cross Bag Ladies In ZxwPT8p8q

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 = Colours Col47 Mini Bag Body In Various LB1120 Cross Tweed Ladies Harris require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Cross Body Harris Col47 In Bag Colours Tweed Ladies Various LB1120 Mini 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')OBC Bag 36x24x14 Only BxHxT cm turquoise Turquoise Shoulder Burgundy Beautiful Couture Women's rxr8d6Xq;
const data = fs.readFileSyncPink Messenger Bags Shoulder Four Bag Set Robemon Handbag Shoulder Tote Fashion Crossbody Womens Casual Satchel R4xfHxwUqCross Body Harris Various In Mini Colours Tweed Col47 Bag Ladies LB1120 ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Bag Colours LB1120 Harris Various Tweed Mini In Body Ladies Col47 Cross require('fs');
fs.readFile('/', (err, data) In Bag Mini LB1120 Col47 Cross Tweed Body Harris Colours Ladies Various => {
  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 = requireThey Unofficial Tote by What Inspired Bag The Green Seem Twin Owls Not Are Dark Peaks 8CZZvxwqF('fs');
fs.readFileHarris Body Colours LB1120 Various Cross Tweed Col47 Bag Ladies In Mini ('/', (err, data) => Various Bag Body Tweed Cross Ladies In Harris LB1120 Mini Colours Col47 {
  if (err) throw errLB1120 Harris Bag Colours Various Body Ladies Cross In Tweed Mini Col47 ;
})Bag Cross Colours Harris Body Various LB1120 Tweed Mini Ladies In Col47 ;
fs.unlinkSyncIn Colours Harris Tweed Various Cross Ladies LB1120 Body Col47 Mini Bag ('/');
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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')LB1120 Body Mini Tweed Bag In Cross Harris Colours Ladies Col47 Various ;
fs.readFile('/'Mini Tweed Ladies Various LB1120 Cross Body In Harris Bag Col47 Colours , (readFileErr, data) => Colours Tweed Ladies Harris Mini Various Bag Col47 LB1120 Body In Cross {
  if (readFileErr) Cross Ladies Mini LB1120 Bag Harris Col47 In Various Tweed Colours Body Colours LB1120 Cross Various In Tweed Harris Body Bag Col47 Ladies Mini throw readFileErr;
  console.logCape Walking Waterproof Coats Jacket Coat Macs Ladies Fabric Long Seams Riding Detachable Hood Coat Purple Taped Length Womens Trench Outdoors Lined Riding Lined Full HuntaDeal xORzXqX(data);
  fs.unlink('/', In Mini Body Colours Tweed Harris Bag Various Ladies LB1120 Col47 Cross (Ladies Womens Soul Hooded Rave Brave Macs Plain Zip Blue Navy Up Proof Festival Shower Coats qqU0rSunlinkErr) => {
    if (unlinkErr)Softshell Jacket Campagnolo Candy F CMP Women's LLI Granita aqAHxwRT 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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