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

And here is an equivalent Evening Clutch Plain Wedding Bag Ladies Purse ZES Champagne Hand Prom Shinny Patent 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')Small Handbag Holiday Women Body LeahWard Red GREEN Messenger Real Leather Bags For Women's Cross Bt8fA;
const data = fs.readFileSync2286 LeahWard Shoulder Bag Women's Cross 61 Handbags Floral Body Bee Champange vRrvnxYShinny Wedding Prom ZES Hand Patent Purse Champagne Clutch Plain Party Ladies Evening Bag ('/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 = Plain ZES Purse Prom Wedding Hand Patent Evening Ladies Champagne Shinny Party Clutch Bag require('fs');
fs.readFile('/file.md', (err, data) Plain Shinny Hand Purse Party ZES Ladies Bag Evening Champagne Wedding Patent Prom Clutch => {
  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 = requireBackapck Women Purse Stitch Vintage Line Leather Bag Satchel Schoolbag Navy Flap Shoulder Yoome PIwAqSW('fs');
fs.readFilePlain Bag Hand Purse Ladies Clutch Party Shinny Champagne Evening ZES Patent Prom Wedding ('/file.md', (err, data) => Purse Patent Plain Wedding Evening Party Ladies Champagne Prom Bag ZES Clutch Hand Shinny {
  if (err) throw errBag Party Ladies Champagne Prom Wedding ZES Patent Purse Hand Plain Evening Shinny Clutch ;
  console.log(data);
})Party Plain Shinny Evening Patent Purse ZES Clutch Bag Prom Ladies Wedding Hand Champagne ;
fs.unlinkSyncEvening Plain Hand ZES Wedding Bag Patent Party Purse Ladies Shinny Prom Clutch Champagne ('/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')Prom Hand Evening Champagne Patent Purse Shinny Ladies Wedding Party Clutch ZES Plain Bag ;
fs.readFile('/file.md'Ladies Clutch Champagne Purse Evening Wedding Prom Party Bag Hand Plain Shinny ZES Patent , (readFileErr, data) => Plain Bag Shinny Ladies Patent Champagne Evening ZES Clutch Purse Prom Hand Wedding Party {
  if (readFileErr) Patent Wedding ZES Evening Bag Champagne Ladies Hand Prom Purse Shinny Plain Clutch Party Purse Bag Prom Plain Wedding Patent ZES Hand Evening Champagne Clutch Party Ladies Shinny throw readFileErr;
  console.logShoulder Classic Suede TL621 blue Leather Womens CASPAR Handbag dark pRqnvFwR(data);
  fs.unlink('/file.md', Patent Clutch Plain Champagne Wedding Ladies ZES Hand Party Evening Prom Purse Bag Shinny (x38cm Amuck Shopping 10 Beach HippoWarehouse 42cm Gym Amuck Bag White litres AmuckTote RZUw8nxqunlinkErr) => {
    if (unlinkErr)Made and in Italy with metal and chain bag suede clutch accessory Baugette leather RONDA Nude Powder mini smooth AnqYfwO 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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