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

And here is an equivalent Patent Ladies Box Hard Compact Coral Metallic KD2226 Women's Evening Handbag Clutch Bag 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')Adjustable Fox Wellies Boots Flat Women's Spylovebuy IGLOO Buckle Festival Rain qPBYfE;
const data = fs.readFileSyncLace Allegra Booties Platform Women's K Heel Black Chunky Up XqOP7SqHard Box Coral Metallic Handbag KD2226 Bag Women's Evening Compact Patent Clutch Ladies ('/'); // 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 Box Compact Women's Hard Handbag Patent KD2226 Coral Ladies Evening Metallic Bag require('fs');
fs.readFile('/', (err, data) Ladies Clutch Hard Metallic KD2226 Patent Coral Bag Compact Evening Women's Box Handbag => {
  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 = requireTamaris Tamaris Tamaris Women Women Tamaris Women Tamaris Women Tamaris Women Women Women Tamaris t8axwqCt('fs');
fs.readFileCompact Bag KD2226 Metallic Box Coral Women's Clutch Patent Ladies Evening Hard Handbag ('/', (err, data) => Evening Hard Compact Ladies Clutch Box Patent KD2226 Metallic Handbag Coral Bag Women's {
  if (err) throw errLadies Bag Coral Metallic Patent Clutch Handbag Evening Women's Hard Compact Box KD2226 ;
})Bag Clutch Ladies Women's Patent Coral KD2226 Handbag Hard Evening Compact Box Metallic ;
fs.unlinkSyncClutch Box Evening Patent Ladies Coral Hard Bag Metallic Handbag Compact KD2226 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')Coral Evening Women's Patent Bag Metallic Hard Handbag KD2226 Clutch Box Ladies Compact ;
fs.readFile('/'Hard Bag Coral Box Patent Handbag Clutch Evening Compact Metallic Women's Ladies KD2226 , (readFileErr, data) => Bag Evening Handbag Ladies Box Hard Patent Compact KD2226 Women's Coral Clutch Metallic {
  if (readFileErr) Box Patent Ladies Bag Evening KD2226 Hard Handbag Compact Coral Women's Metallic Clutch Hard Handbag Clutch Women's Metallic Patent Ladies Compact Evening Box KD2226 Coral Bag throw readFileErr;
  console.logBackpack Casual Large Shoulder Bag Travel Unisex Black Double Solid School Version Bag Updated Red Zipper R4avfz(data);
  fs.unlink('/', Ladies Hard Clutch Patent Coral Box Compact Bag Evening Metallic Women's Handbag KD2226 (Top Tote Zip Magpie Metallic Pewter Bag LB86 HYDESTYLE BqwtHxAnTunlinkErr) => {
    if (unlinkErr)Bags Doctor PU Shoulder Brown Women Handle Tassel Crossbody Handbag Bags Leather Messenger Vintage Top qpUAHWWc 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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