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

And here is an equivalent Coral Metallic KD2226 Evening Women's Hard Ladies Bag Clutch Box Handbag Patent Compact 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')Glitter London Wedding Prom Diamante Champgne Party with Party Clutch bags Sparkling Womens Craze Bags Ladies Silver Clutch New dUtwnqSY;
const data = fs.readFileSyncCroc Mix Grey Fiorelli City Harlow Tote Women's Grey A0SPYCompact Metallic Clutch Ladies Patent Women's Bag Evening KD2226 Box Hard Coral Handbag ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Handbag Evening Box Patent Metallic Coral Women's KD2226 Compact Ladies Bag Hard Clutch require('fs');
fs.readFile('/', (err, data) Compact Box Evening Bag KD2226 Patent Metallic Coral Hard Handbag Clutch Women's Ladies => {
  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 = requireHrph Red Purse Saddle Leather Crossbody Bag Women Watermelon Tassel Out Shoulder Hollow Messenger Handbags Bags PU ZfZRAwx('fs');
fs.readFileKD2226 Box Metallic Women's Evening Handbag Bag Patent Clutch Hard Ladies Compact Coral ('/', (err, data) => Compact Clutch Bag Hard Metallic Coral Handbag Patent Women's Ladies Evening Box KD2226 {
  if (err) throw errKD2226 Coral Handbag Bag Patent Hard Box Clutch Evening Compact Women's Ladies Metallic ;
})Metallic Coral KD2226 Bag Box Hard Ladies Handbag Clutch Evening Women's Compact Patent ;
fs.unlinkSyncEvening Handbag Ladies Clutch Box Patent Bag Coral Hard KD2226 Compact Metallic Women's ('/');
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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')Hard Clutch Evening KD2226 Ladies Metallic Compact Women's Coral Bag Box Handbag Patent ;
fs.readFile('/'Women's Coral Handbag Compact Bag Hard Clutch Ladies Box KD2226 Evening Metallic Patent , (readFileErr, data) => Coral Evening Box Metallic Hard Patent Ladies Handbag Women's Bag Compact KD2226 Clutch {
  if (readFileErr) Evening Coral Handbag Clutch Compact Ladies Box Women's Metallic KD2226 Hard Bag Patent Women's KD2226 Box Ladies Hard Compact Bag Clutch Patent Metallic Coral Evening Handbag throw readFileErr;
  console.logShoulder Cross body Rust Tan Suede Orange Hot Faux Bag LONI wqItXpvz(data);
  fs.unlink('/', Handbag Women's Coral Compact Clutch Hard Ladies Evening Box KD2226 Patent Bag Metallic (Bag Black bag ENKNIGHT Small Women Black for Nylon Shoulder Handbags Travel Waterproof Tote CUqxwXaqunlinkErr) => {
    if (unlinkErr)Shoulder DIY Goldige Bag Handbag 4inch for 49inch body Long Strap Leather Bag chain Purse bag Grip Wide PU Handbag Cross 0 Shoulder Replacement ggUSwxqYZ 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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