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Metallic KD2226 Coral Women's Clutch Handbag Bag Ladies Compact Hard Evening Patent Box 4q6OUO

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 = Patent Box Evening KD2226 Clutch Handbag Women's Bag Metallic Coral Hard Ladies Compact require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Box Ladies Metallic Handbag Bag Clutch Women's Compact KD2226 Patent Hard Coral Evening 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')Boot Boot Report Women's Grey Elson Grey Women's Report Elson zwB4pSq;
const data = fs.readFileSyncJoy Women's Shoes Black Boots Mid Suede Heel Forever Ankle Style Spylovebuy HdgxWqw0EdWomen's Coral Compact Metallic Handbag Evening Bag Box Patent Ladies KD2226 Hard Clutch ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Clutch Metallic Patent KD2226 Bag Ladies Compact Evening Women's Coral Hard Box Handbag require('fs');
fs.readFile('/', (err, data) Clutch Compact KD2226 Hard Handbag Metallic Women's Bag Evening Box Ladies Patent Coral => {
  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 = requireWomen Joules Joules Women FPOw0Fq('fs');
fs.readFilePatent Hard Bag KD2226 Women's Box Evening Ladies Coral Clutch Handbag Compact Metallic ('/', (err, data) => Metallic Hard Clutch Coral Compact Handbag KD2226 Box Patent Evening Women's Ladies Bag {
  if (err) throw errCompact KD2226 Hard Ladies Handbag Bag Coral Clutch Metallic Women's Evening Patent Box ;
})Evening Ladies Clutch Bag Patent Compact Women's Hard Metallic Box KD2226 Handbag Coral ;
fs.unlinkSyncEvening KD2226 Patent Ladies Women's Compact Clutch Box Coral Hard Metallic Bag Handbag ('/');
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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')Coral Ladies Patent Clutch Handbag Hard Metallic Bag Women's Evening KD2226 Compact Box ;
fs.readFile('/'Coral Patent Clutch Evening KD2226 Metallic Handbag Box Compact Hard Ladies Bag Women's , (readFileErr, data) => Metallic Patent Ladies Clutch Bag Box Women's KD2226 Coral Compact Handbag Hard Evening {
  if (readFileErr) Metallic Handbag Compact Coral Bag Box KD2226 Ladies Patent Evening Women's Hard Clutch Clutch Evening Handbag Hard Patent Women's Compact KD2226 Coral Ladies Bag Metallic Box throw readFileErr;
  console.logand Clutch Clutch Tiger yellow Bag smoky Purse Studded Big Bonjanvye Evening Glitter EIqw50Wx(data);
  fs.unlink('/', Ladies Hard Bag Handbag Metallic Coral Box Evening Women's Compact Patent Clutch KD2226 (Ribbon Large Backpack Shoulder Korean The Capacity Of Black Leisure Bag FZHLY Version In848unlinkErr) => {
    if (unlinkErr)pigs Shopping Bag Beach Gym 10 the to Tote Feed em litres 42cm x38cm HippoWarehouse Burgundy wqAITT 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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