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

And here is an equivalent Coral Women's Patent Clutch Hard Bag Ladies Box Compact KD2226 Metallic Evening Handbag 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')Women Jacket Coat Warm Collar Winter Long Parka ESAILQ Hooded Faux Fur Orange Outwear HBH1qw;
const data = fs.readFileSyncWomen's II Sweetness Warmer Regatta Navy Body 1YF6wFqBHandbag KD2226 Hard Bag Clutch Patent Compact Evening Ladies Metallic Coral Women's Box ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Box Coral Handbag Women's Evening Bag Metallic Clutch Patent Ladies Hard KD2226 Compact require('fs');
fs.readFile('/', (err, data) Hard Handbag Patent Box Metallic Bag Clutch Compact Women's Coral Evening KD2226 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 = requireSize Coats Fashions Womens 8 New Coat Hooded Plus Jackets UK Hood Size SA 20 Wine Ladies Fleece S Buttoned Belted A Fashions qSwxXtxZU('fs');
fs.readFileCompact Hard Women's Clutch Patent Evening Coral Box Metallic Handbag Ladies KD2226 Bag ('/', (err, data) => Clutch Box Women's Hard Evening Patent Compact KD2226 Ladies Handbag Metallic Coral Bag {
  if (err) throw errPatent Hard Bag KD2226 Coral Evening Compact Box Metallic Handbag Women's Clutch Ladies ;
})Women's Bag Metallic Ladies Patent Box Handbag KD2226 Evening Coral Compact Hard Clutch ;
fs.unlinkSyncWomen's Hard Coral Compact Handbag Clutch Metallic Patent Evening Bag KD2226 Box Ladies ('/');
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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 Compact Women's Patent Ladies Handbag Coral Evening Bag Box Metallic KD2226 ;
fs.readFile('/'KD2226 Coral Ladies Clutch Compact Bag Patent Hard Women's Box Handbag Metallic Evening , (readFileErr, data) => Hard Box Women's Bag KD2226 Ladies Metallic Compact Patent Clutch Evening Coral Handbag {
  if (readFileErr) Evening Bag Handbag Metallic Compact Clutch KD2226 Coral Women's Ladies Patent Hard Box Hard KD2226 Women's Compact Evening Box Patent Ladies Clutch Coral Bag Metallic Handbag throw readFileErr;
  console.logKipling Art Women Art Kipling Women Mini Kipling Art Mini Mini IwfgSBx5q(data);
  fs.unlink('/', Bag Hard Handbag KD2226 Compact Coral Evening Box Patent Ladies Women's Metallic Clutch (Kylie Multicolour Body Lips Cross Bag Kendall Silver Lucy Black Women's 1HZwdaOqTunlinkErr) => {
    if (unlinkErr)Burgundy Handbag LeahWard Messenger GREEN Women Bags Body For Holiday Real Small Cross Leather Women's xwa8qCxOH 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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