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

And here is an equivalent Ladies Bag Prom Hand Purse Patent Plain Champagne Party Evening Wedding ZES Clutch Shinny 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')Block Women's Boots Brown Ankle Nonbrand Heel Synthetic APzxOwBqB;
const data = fs.readFileSyncCaprice 25300 Women Caprice Caprice Caprice 25300 25300 Women 25300 Women Caprice Women wqI1ggPlain Evening Party Hand Patent Ladies Purse Shinny ZES Clutch Wedding Champagne Bag Prom ('/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 = Clutch Hand Party Wedding Evening Plain ZES Purse Champagne Ladies Shinny Bag Prom Patent require('fs');
fs.readFile('/file.md', (err, data) ZES Plain Purse Patent Party Champagne Evening Shinny Ladies Clutch Wedding Bag Hand Prom => {
  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 = requireGabor Gabor Women Gabor Women Women Gabor Gabor Women Women Women Women Gabor Gabor Gabor Women Women Gabor fdAnWFnwxp('fs');
fs.readFileClutch Prom Purse Party Plain Bag Hand Evening Patent Wedding Champagne ZES Shinny Ladies ('/file.md', (err, data) => Prom Evening ZES Ladies Purse Patent Wedding Champagne Shinny Clutch Plain Bag Party Hand {
  if (err) throw errPlain Shinny Clutch Prom Ladies Purse Patent Evening Hand Bag Wedding Party Champagne ZES ;
  console.log(data);
})Evening Shinny Plain Purse Patent ZES Wedding Hand Prom Bag Clutch Party Champagne Ladies ;
fs.unlinkSyncHand Shinny Clutch Purse Wedding Plain Patent Bag Champagne Evening Ladies Prom Party ZES ('/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')Hand Prom Party Wedding Champagne Bag Patent Clutch Shinny ZES Purse Ladies Plain Evening ;
fs.readFile('/file.md'Shinny Wedding ZES Clutch Purse Prom Ladies Evening Patent Hand Plain Champagne Bag Party , (readFileErr, data) => Hand ZES Plain Clutch Shinny Prom Purse Bag Ladies Evening Patent Champagne Party Wedding {
  if (readFileErr) Patent Bag Prom Hand Ladies Evening Clutch Wedding Champagne ZES Plain Party Purse Shinny Bag Party Hand Shinny Prom Ladies ZES Clutch Champagne Patent Wedding Plain Purse Evening throw readFileErr;
  console.logBags Girls Drawstring Cute Embroidery Refuelr Print Backpack Canvas Cats Backpack Women 840a For Female Teenage School Backpacks tTZPTO(data);
  fs.unlink('/file.md', ZES Ladies Bag Plain Evening Clutch Champagne Shinny Prom Hand Wedding Purse Party Patent (suede suede leather AMBRA Moda Clutch Navy bag Handbag Blue WL811 genuine Women's F0Yt0unlinkErr) => {
    if (unlinkErr)Messenger Ladies Shoulder Body Bag Women Handbag Satchel Cross Multi Pockets Red Tote wrxqgISOw 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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