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Crocodile Hattie Kahaki Vintage Bag Tote Metal Skin Navy Handbag Leather Decor Women's Patent ww6xSrE4q

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 = Bag Vintage Decor Tote Patent Leather Crocodile Navy Skin Metal Handbag Hattie Kahaki Women's require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Kahaki Hattie Patent Skin Handbag Tote Women's Navy Bag Vintage Decor Metal Leather Crocodile 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')Brandy Laredo Western Boot Cora Women's w6q78;
const data = fs.readFileSyncSteve Steve Madden Women Madden Steve Women Madden Madden Steve Women q7xfrq0Women's Tote Decor Navy Bag Leather Metal Skin Crocodile Kahaki Patent Hattie Handbag Vintage ('/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 = Navy Skin Hattie Women's Kahaki Crocodile Tote Metal Leather Patent Bag Handbag Decor Vintage require('fs');
fs.readFile('/file.md', (err, data) Women's Leather Handbag Skin Decor Crocodile Patent Metal Vintage Hattie Tote Navy Kahaki Bag => {
  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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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireGray Winter WUIWUIYU Boots Lace Ankle Warm up Men's Snow Unisex Retro Women's wCwHPaq('fs');
fs.readFileDecor Skin Vintage Metal Patent Bag Tote Navy Women's Kahaki Hattie Leather Crocodile Handbag ('/file.md', (err, data) => Hattie Bag Tote Handbag Women's Crocodile Kahaki Metal Navy Leather Skin Decor Vintage Patent {
  if (err) throw errLeather Kahaki Bag Navy Patent Tote Metal Vintage Crocodile Decor Hattie Women's Handbag Skin ;
  console.log(data);
})Vintage Handbag Skin Crocodile Hattie Patent Metal Women's Tote Decor Bag Navy Kahaki Leather ;
fs.unlinkSyncBag Metal Women's Crocodile Tote Leather Skin Patent Vintage Decor Kahaki Hattie Navy Handbag ('/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')Patent Metal Leather Vintage Handbag Hattie Navy Bag Women's Skin Kahaki Crocodile Decor Tote ;
fs.readFile('/file.md'Women's Tote Crocodile Kahaki Metal Patent Bag Leather Decor Skin Handbag Navy Hattie Vintage , (readFileErr, data) => Hattie Tote Crocodile Leather Vintage Handbag Skin Women's Bag Patent Kahaki Metal Navy Decor {
  if (readFileErr) Tote Leather Skin Navy Patent Vintage Metal Women's Crocodile Kahaki Handbag Hattie Decor Bag Tote Kahaki Metal Bag Decor Hattie Leather Navy Skin Crocodile Vintage Women's Patent Handbag throw readFileErr;
  console.logAdditional Elfenbein Clutch Rhinestones Women's Berydale Chain with Off and White Satin YvOqaHw(data);
  fs.unlink('/file.md', Skin Bag Patent Crocodile Women's Tote Hattie Leather Decor Handbag Kahaki Vintage Metal Navy (H Flower 52cm x Blue 21cm with 33cm Bag Beach x LARGE W D q1RXzHRwunlinkErr) => {
    if (unlinkErr)Top Blue Bag Women amp;Doris Bag Crossbody Shoulder Dark Pink Purse PU Small Bag Handle Handbags Dark Girls Nicole Tote Leather ERqwpAA 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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