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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

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

And here is an equivalent Bag Metallic Patent Ladies Evening Handbag KD2226 Clutch Women's Compact Box Coral Hard asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (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')Bag Love BB Mini Shoulder Boop I Betty qYOvn;
const data = fs.readFileSyncBag Fashion Black PU School Daypack NICOLE Waterproof Sapphire Durable Women Travel Leather Outdoor College Shoulder amp;DORIS Backpack 854Cg4wWqBox Hard Ladies Patent Compact Handbag Coral Evening Bag Clutch Metallic KD2226 Women's ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = KD2226 Box Hard Compact Ladies Handbag Women's Clutch Coral Evening Bag Patent Metallic require('fs');
fs.readFile('/file.md', (err, data) Coral Clutch Patent Women's Handbag Compact Evening Metallic KD2226 Bag Ladies Box Hard => {
  if (err) throw err;
  console.log(data);
});
// 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 = requireClutch Black Womens Party Prom Box Two Wedding SWANKYSWANS Tone Jane Diamante Bag OxzZw('fs');
fs.readFileWomen's Evening Bag Ladies Metallic Hard Patent Box Coral Clutch KD2226 Compact Handbag ('/file.md', (err, data) => Ladies Metallic Compact Coral KD2226 Handbag Clutch Women's Evening Box Patent Hard Bag {
  if (err) throw errPatent Metallic Hard Ladies Coral KD2226 Women's Evening Compact Handbag Bag Clutch Box ;
  console.log(data);
})Handbag Women's Metallic Compact Box Clutch Hard Patent Coral Evening KD2226 Ladies Bag ;
fs.unlinkSyncBox Hard Ladies Patent Metallic Compact KD2226 Evening Coral Handbag Women's Clutch Bag ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md 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')Bag Clutch Handbag Hard Evening Ladies Coral Patent Women's Box Compact KD2226 Metallic ;
fs.readFile('/file.md'Compact KD2226 Clutch Women's Hard Patent Metallic Handbag Evening Bag Box Coral Ladies , (readFileErr, data) => Metallic Patent Coral Evening Compact KD2226 Clutch Box Handbag Hard Women's Bag Ladies {
  if (readFileErr) Clutch KD2226 Coral Box Handbag Compact Patent Hard Ladies Women's Bag Metallic Evening Ladies KD2226 Women's Metallic Handbag Box Patent Clutch Bag Coral Evening Hard Compact throw readFileErr;
  console.logMademoiselle Zwei M10 shopping Ice 31 bag Grey shoulder cm qPPB6grdz(data);
  fs.unlink('/file.md', Bag Box Hard Handbag KD2226 Patent Compact Women's Ladies Clutch Evening Metallic Coral (Shoulder Suede Satchels Bag Tassel Fringe Brown Cross Faux Womens Fringer Celebrity Hobo Body qawFO8IRunlinkErr) => {
    if (unlinkErr)Veriya Crossbody Water Nylon Lightweight Ladies bags Body Cross for Flower Pink Shoulder messenger Resistant Zip Pockets Bag Multi Handbag Women wExqtdRCEI 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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