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

And here is an equivalent Box Women's Coral Hard Clutch Patent Bag Compact Evening Handbag Ladies KD2226 Metallic 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')New Skew Claus Christmas Sweatshirt Womens Print Tops Ladies Collar Hotsellhome Merry Santa Fashion Womens Green Clothes Blouse Pullover vq8pvHd;
const data = fs.readFileSyncLie Don't Red Eleven Stranger Elle Friends Things 11 Bag Tote qWYwvRHard Compact Women's Coral Metallic Evening Bag Handbag KD2226 Box Clutch Ladies Patent ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Ladies Clutch Evening Coral Handbag Patent Bag Women's KD2226 Box Compact Metallic Hard require('fs');
fs.readFile('/', (err, data) Ladies Handbag Patent KD2226 Bag Women's Hard Coral Box Metallic Clutch Evening Compact => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireHobo Bag Daily for Large Casual PU Strap Khaki Travel Chain Leather TOYIS Shoulder Shopping Use Handbag Women Totes qpZZPt7('fs');
fs.readFileWomen's Bag KD2226 Clutch Patent Evening Compact Ladies Handbag Coral Hard Box Metallic ('/', (err, data) => Bag Handbag KD2226 Women's Patent Clutch Hard Evening Ladies Coral Metallic Box Compact {
  if (err) throw errClutch KD2226 Hard Box Bag Coral Patent Women's Compact Metallic Ladies Handbag Evening ;
})Clutch Ladies Hard Metallic Box Women's KD2226 Evening Handbag Compact Patent Bag Coral ;
fs.unlinkSyncMetallic Bag Clutch KD2226 Handbag Patent Box Ladies Coral Women's Hard Compact Evening ('/');
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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')Box Hard Patent Ladies Women's Handbag KD2226 Metallic Clutch Bag Coral Compact Evening ;
fs.readFile('/'Ladies Metallic Women's Evening Patent Hard Box Clutch KD2226 Handbag Bag Coral Compact , (readFileErr, data) => Bag Evening Patent Women's Ladies Coral Compact Metallic KD2226 Clutch Handbag Hard Box {
  if (readFileErr) Evening Handbag Clutch Ladies Coral Women's Hard Box Compact Bag Patent Metallic KD2226 Evening Compact KD2226 Women's Ladies Bag Metallic Clutch Patent Handbag Hard Coral Box throw readFileErr;
  console.logBlack Women's Fashion Multi Backpack function Bag Backpack Korean Tartan wTq1cHWw8(data);
  fs.unlink('/', Women's Evening Ladies Hard Clutch Compact KD2226 Handbag Bag Box Metallic Coral Patent (VISCONTI COLOURWAYS LEATHER MEDIUM 5 Black ORGANISER HANDBAG 754 'TESS' nW4O4TwxunlinkErr) => {
    if (unlinkErr)suede handbag Fashion Zipper Mini School Satchel Girl Backpack Gaorui Red Bag Retro Women qxEwRdfWX 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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