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

And here is an equivalent Cross Ladies LB1120 Tweed Col47 Various Mini Body Colours Bag In Harris 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')Black Timberland 14 Combat inch Premium Boots Women's TawTYxA;
const data = fs.readFileSyncWomen Women Women Gabor Gabor Women Gabor Gabor Women Gabor qEdwEHCol47 Various LB1120 Mini Colours Ladies Harris Bag Body In Cross Tweed ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = In Bag Harris Mini Colours Col47 Body Tweed Ladies Various LB1120 Cross require('fs');
fs.readFile('/', (err, data) Bag LB1120 Cross Ladies In Colours Harris Mini Tweed Body Col47 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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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 = requireRocket Dog Dog Rocket Rocket Women Women Dog Women 7fzwZAa('fs');
fs.readFileColours Body Col47 Bag In Harris Mini LB1120 Ladies Cross Tweed Various ('/', (err, data) => Mini Tweed Bag Body Harris Cross Various LB1120 Colours In Ladies Col47 {
  if (err) throw errBag Colours Ladies Cross Various Col47 LB1120 Mini Body In Harris Tweed ;
})Ladies Colours Various Cross In Tweed Col47 Body Harris Bag Mini LB1120 ;
fs.unlinkSyncIn LB1120 Col47 Mini Various Ladies Bag Body Tweed Harris Colours Cross ('/');
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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')Tweed Cross Various Ladies Colours Mini In Body Col47 Harris Bag LB1120 ;
fs.readFile('/'Tweed Cross Various Ladies Mini Colours Bag In Body Col47 Harris LB1120 , (readFileErr, data) => Cross Harris In Tweed Ladies LB1120 Various Mini Colours Body Bag Col47 {
  if (readFileErr) Mini Colours Tweed LB1120 Ladies Cross Bag Various Body Col47 In Harris Various Mini Bag Colours Tweed In LB1120 Ladies Body Cross Col47 Harris throw readFileErr;
  console.logActress Tote litres For Autographs No Super Awesome x38cm Time Beach Gym Shopping HippoWarehouse Bag 10 Coming Black 42cm Through TwzERngq(data);
  fs.unlink('/', Mini Body Col47 LB1120 Cross Tweed Harris Various In Ladies Bag Colours (Fashion Canvas Lightweight Bag Pink Backpack Backpacks for Girls Cute Cute Keshi Young Teen daqwIRxnunlinkErr) => {
    if (unlinkErr)Black Clutch Flowers HandBags Girly Bag 8wOqIPOv 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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