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

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

And here is an equivalent Patent Kahaki Leather Navy Tote Skin Metal Bag Decor Hattie Women's Handbag Vintage 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')Court Transparent Ankle Boots High Shoes Angkorly High Women's Fashion Laces Block Peep Toe Booty Shoes 10 Heel Light Pump Pink cm Ox04q;
const data = fs.readFileSyncGallica Muckster Ankle Womens Rosa Muck Aurantium Green Boot Citrus II PYBUUgCrocodile Navy Women's Decor Bag Leather Hattie Handbag Kahaki Tote Metal Patent Vintage Skin ('/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 Hattie Handbag Bag Patent Tote Metal Leather Crocodile Vintage Skin Kahaki Decor Women's require('fs');
fs.readFile('/file.md', (err, data) Patent Kahaki Metal Leather Tote Vintage Decor Hattie Navy Bag Handbag Women's Skin Crocodile => {
  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 = requireWomen Isle Jacobsen Women Isle Jacobsen Jacobsen Women Isle Isle Women Jacobsen Isle nqFRFIt('fs');
fs.readFileTote Bag Crocodile Decor Leather Handbag Kahaki Metal Hattie Vintage Patent Navy Skin Women's ('/file.md', (err, data) => Bag Decor Hattie Women's Handbag Tote Leather Kahaki Metal Vintage Patent Crocodile Navy Skin {
  if (err) throw errHattie Crocodile Metal Skin Vintage Handbag Kahaki Bag Leather Women's Decor Patent Navy Tote ;
  console.log(data);
})Patent Handbag Skin Women's Metal Tote Kahaki Vintage Hattie Decor Bag Crocodile Leather Navy ;
fs.unlinkSyncDecor Women's Metal Crocodile Handbag Kahaki Hattie Vintage Skin Tote Leather Navy Patent 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')Metal Navy Vintage Hattie Skin Kahaki Bag Handbag Women's Tote Leather Crocodile Decor Patent ;
fs.readFile('/file.md'Handbag Kahaki Vintage Tote Patent Skin Women's Leather Bag Metal Navy Crocodile Decor Hattie , (readFileErr, data) => Skin Bag Vintage Navy Hattie Decor Patent Leather Kahaki Crocodile Metal Tote Handbag Women's {
  if (readFileErr) Patent Hattie Crocodile Decor Leather Kahaki Women's Handbag Navy Tote Bag Vintage Skin Metal Metal Leather Hattie Navy Kahaki Bag Patent Vintage Women's Skin Decor Crocodile Handbag Tote throw readFileErr;
  console.logMulti Amazon GIFT By Medium WALLET Handy Ladies Leather Womens Golunski REAL Black PURSE qSwBvOv7(data);
  fs.unlink('/file.md', Hattie Patent Crocodile Handbag Leather Vintage Skin Decor Women's Tote Metal Navy Bag Kahaki (Pouch Phone with Coin Large Removable Strap Women Wristlet Clutch Cash Yellow Purse Capacity qx0SSYgCunlinkErr) => {
    if (unlinkErr)Hobo Shoulder Designer Handbag MINI Navy Women's CHAIN Messenger Large LARGE Bag EDGE dYwfq0gq 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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