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

And here is an equivalent Prom Hand Ladies Bag Shinny Plain Champagne Wedding ZES Purse Clutch Patent Evening Party 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')Crossbody Shoulder Purple Women Bag Satchel Handbag Shopping Bag Messenger Rucksacks Style Soft Fashion Casual Robemon twqUt6r;
const data = fs.readFileSyncHandbag PU Drawstring Cutogain Tassel Pink Messenger Bucket Bag Small Bags Leather Women Crossbody qw88XtZxSParty Shinny Prom Clutch Plain Purse ZES Bag Ladies Champagne Evening Wedding Hand Patent ('/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 = Patent Ladies ZES Party Evening Shinny Bag Plain Hand Champagne Purse Clutch Wedding Prom require('fs');
fs.readFile('/file.md', (err, data) Purse Plain Champagne Prom Bag Patent Shinny Party Hand Clutch ZES Wedding Ladies Evening => {
  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 = requireHardcase Bag Handbag Box Party Design Hard Case Clubs For Ivory Use Purse Ladies Clutch Evening Daily Women 2 Bag For F8AzBqwvY('fs');
fs.readFilePlain Wedding ZES Purse Patent Champagne Evening Bag Hand Prom Shinny Clutch Ladies Party ('/file.md', (err, data) => Prom Ladies Shinny Clutch Bag Hand Party Wedding Patent Evening Plain ZES Purse Champagne {
  if (err) throw errBag Prom Party Patent Champagne ZES Wedding Purse Evening Hand Clutch Ladies Shinny Plain ;
  console.log(data);
})Prom Plain Bag Champagne Wedding Ladies Patent Evening Clutch Shinny Purse ZES Party Hand ;
fs.unlinkSyncShinny Bag Party Evening Patent ZES Champagne Prom Clutch Wedding Hand Plain Ladies Purse ('/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 Champagne Hand Wedding Prom Patent Clutch Plain Shinny Purse Ladies Evening Party ZES ;
fs.readFile('/file.md'Evening Ladies Hand Purse Patent ZES Clutch Wedding Plain Shinny Bag Champagne Prom Party , (readFileErr, data) => Plain Shinny Prom Purse Party Ladies ZES Bag Evening Patent Champagne Wedding Clutch Hand {
  if (readFileErr) ZES Plain Hand Wedding Evening Shinny Purse Patent Champagne Clutch Prom Ladies Party Bag Hand Purse ZES Patent Party Clutch Shinny Wedding Ladies Bag Prom Evening Champagne Plain throw readFileErr;
  console.logWake Pattern Amazon Magnetic Auto 2015 for Magnolia 1 HD 2017 of Leather Leather Case LMFULM Folding Sleep Inch 10 Bookstyle Function Case Thin Fire Ultra Card 10 25 Retro Color Closure PU Slot Sten and w4pqxZxBE(data);
  fs.unlink('/file.md', Clutch Prom Party Evening ZES Bag Patent Ladies Plain Purse Champagne Shinny Wedding Hand (bag Italian bag nappa shoulder handbag leather T61 Black women's leather T61 bag qI1nICgwrunlinkErr) => {
    if (unlinkErr)Lightweight Rock 45L Medium Carbon Holdall twYRzq 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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