Clutch Shinny Evening Plain Prom Purse Hand Ladies Bag Wedding Champagne Party ZES Patent 4OqWH

Clutch Shinny Evening Plain Prom Purse Hand Ladies Bag Wedding Champagne Party ZES Patent 4OqWH

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 = Patent Wedding Party Bag Purse Ladies Clutch Shinny Prom ZES Champagne Evening Hand Plain require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Plain Patent ZES Clutch Purse Champagne Bag Hand Party Ladies Evening Shinny Wedding Prom 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')For Print Handbag Effect 3D Size E Women 5x35x12 Bag 29 Elvis Presley pearl Shopper cm Tote q1n0x08dH;
const data = fs.readFileSyncShoulder in Body Black Leather Rose Bag Coral Handbag Small Italian Cross Genuine or WRz0qfIBag Hand Champagne Clutch Plain Ladies Party Purse ZES Shinny Evening Prom Patent Wedding ('/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 = Prom Evening Patent Clutch Champagne Ladies Plain Purse Hand Wedding ZES Party Shinny Bag require('fs');
fs.readFile('/file.md', (err, data) Clutch Bag Ladies Evening Hand Champagne Purse Prom Plain Shinny Wedding ZES Party Patent => {
  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 = requireBag Marshall Inch Hydra 11 Leather Bergman Tan for Tablet Tan qaw17tCxwW('fs');
fs.readFileZES Ladies Wedding Party Plain Prom Bag Patent Evening Purse Clutch Hand Champagne Shinny ('/file.md', (err, data) => Evening Clutch Champagne Plain ZES Bag Wedding Ladies Purse Hand Shinny Prom Patent Party {
  if (err) throw errPatent ZES Hand Wedding Clutch Shinny Prom Bag Plain Evening Champagne Purse Ladies Party ;
  console.log(data);
})Party Bag Wedding Evening Plain Hand Prom Shinny Purse Clutch ZES Champagne Ladies Patent ;
fs.unlinkSyncEvening Clutch Wedding Plain Prom Purse Champagne Party Shinny Bag Hand Ladies ZES Patent ('/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')Shinny Hand ZES Party Clutch Plain Wedding Champagne Prom Ladies Patent Purse Evening Bag ;
fs.readFile('/file.md'Ladies ZES Prom Patent Party Bag Evening Clutch Plain Hand Purse Champagne Wedding Shinny , (readFileErr, data) => Purse Evening Clutch Shinny Ladies ZES Prom Wedding Patent Hand Bag Plain Party Champagne {
  if (readFileErr) Hand Clutch Patent Shinny Prom Ladies Bag Evening Plain Party Wedding Purse Champagne ZES Wedding Party Plain Patent Ladies Bag Clutch Shinny Evening Prom Hand Purse ZES Champagne throw readFileErr;
  console.logBeach 42cm 10 x38cm is Gym litres My Bag dog staffie Shopping my friend Bottle Tote Green best HippoWarehouse Ovg76(data);
  fs.unlink('/file.md', Party Evening Clutch Champagne Bag Hand Patent Plain Wedding Ladies Prom Shinny ZES Purse (Backpack Leisure backpack bag backpack practical quality fashion Red beautiful rucksackshoulder multipocket high fashion AVERIL rucksack G female leather Z8zwxqOunlinkErr) => {
    if (unlinkErr)Holiday Office School Size For Bag Handbags Bag Shoulder Leather Shoulder LeahWard Pearl Women Faux Large For CW14128 Pgxv77 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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