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

And here is an equivalent Handbag Women's KD2226 Metallic Evening Clutch Box Bag Hard Ladies Coral 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')Xardi Women Handbags Royal Baguette Flap Leather Finish Large Vinyl Patent Party London For Bag Blue Over Clutch BTBqrw0;
const data = fs.readFileSyncClutch Designer Flat Bag Neon Women Suede London Coral Xardi Medium Ladies Evening UK Envelope New Faux fqSIxwXEvening KD2226 Handbag Patent Compact Clutch Ladies Box Women's Coral Bag Metallic Hard ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Patent Ladies Metallic Bag Evening Coral Handbag Compact Hard Clutch KD2226 Box Women's require('fs');
fs.readFile('/', (err, data) Box Coral Hard KD2226 Ladies Handbag Clutch Compact Bag Patent Metallic Women's Evening => {
  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 = requireEvening Women's Handbag Rhinestones Bling with Chain Detachable Bag Silver Strap Shoulder Clutch A6wI6q('fs');
fs.readFileWomen's Ladies Hard Bag Box Evening Coral Metallic Handbag Compact KD2226 Patent Clutch ('/', (err, data) => Metallic Compact Bag Box Coral Handbag Hard Clutch Evening Women's KD2226 Ladies Patent {
  if (err) throw errCoral Patent Bag Compact Box Clutch KD2226 Evening Women's Hard Ladies Metallic Handbag ;
})KD2226 Box Coral Hard Evening Metallic Handbag Bag Compact Women's Clutch Patent Ladies ;
fs.unlinkSyncMetallic Compact Ladies Coral Women's Hard Evening Patent Box Handbag Clutch Bag KD2226 ('/');
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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')Handbag KD2226 Compact Metallic Bag Clutch Coral Hard Evening Ladies Box Women's Patent ;
fs.readFile('/'Ladies KD2226 Metallic Patent Women's Evening Box Coral Handbag Hard Clutch Compact Bag , (readFileErr, data) => Box Bag Metallic Compact Hard Coral Clutch Patent Ladies Evening KD2226 Women's Handbag {
  if (readFileErr) Coral Metallic Bag KD2226 Patent Clutch Evening Handbag Compact Women's Box Hard Ladies Hard Women's Evening Metallic Patent Handbag Ladies KD2226 Box Clutch Compact Bag Coral throw readFileErr;
  console.logStretch Trousers Acid High Waist Fit 14 High Black Skinny Blue Waisted Women White Wash Comfy LustyChic Slim 6 Jeans Ladies Size SYxXTqOw(data);
  fs.unlink('/', Coral Evening Hard KD2226 Clutch Handbag Compact Bag Women's Patent Ladies Metallic Box (Mondo Blau Women's Jeans boot Jeans 51280 Wash Cut LTB 6OqW1H4xYunlinkErr) => {
    if (unlinkErr)Casual Trousers Comfort Traders Cotton Slim Bootcut 29inch Leg Black Jeans Women's Length YZUUwq0 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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