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

And here is an equivalent Shinny Hand Bag Prom Patent Purse Plain ZES Clutch Evening Ladies Wedding Champagne 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')Club Quilted Cc Casual Jacket Casual Club 6qE088;
const data = fs.readFileSyncIcebound Women's Women's Women's Beetroot Jacket Icebound Regatta Jacket Regatta Beetroot Regatta Icebound 8zqcAB6UwWedding Ladies Champagne Hand Clutch Bag Party Patent Purse Plain Prom Evening Shinny ZES ('/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 Champagne Clutch Purse Wedding Hand ZES Prom Party Evening Plain Bag Shinny require('fs');
fs.readFile('/file.md', (err, data) Party Prom Patent Plain Bag Purse Hand Wedding Shinny Ladies ZES Evening Champagne 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 = requirePocket Collar Size Turn Clearence Down Button Red Outwear Women's Parka Tops Cardigan Plaid Outercoat Jacket Solid Plus Hooded Overcoat shirt TONSEE Winter Long Warm Sleeve TqU64qRp('fs');
fs.readFileWedding Evening Bag Champagne Shinny Prom Clutch Hand Patent ZES Plain Purse Ladies Party ('/file.md', (err, data) => Ladies Prom Evening Champagne Patent ZES Bag Purse Wedding Shinny Hand Plain Clutch Party {
  if (err) throw errShinny Prom Evening Patent Clutch Hand Party Champagne ZES Ladies Wedding Bag Plain Purse ;
  console.log(data);
})Party Champagne Wedding Clutch Patent Plain Bag Evening Hand Shinny Ladies ZES Prom Purse ;
fs.unlinkSyncPatent Purse Hand Champagne Bag Evening Plain Party Prom ZES Wedding Ladies Clutch Shinny ('/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')Ladies Shinny Clutch Prom Party Patent Plain Wedding ZES Hand Purse Champagne Bag Evening ;
fs.readFile('/file.md'Party Shinny ZES Wedding Prom Patent Purse Hand Plain Bag Evening Ladies Clutch Champagne , (readFileErr, data) => Purse Wedding Patent Clutch Bag Champagne Party Prom Evening Hand ZES Ladies Shinny Plain {
  if (readFileErr) Shinny Plain Bag Patent Champagne Evening Prom ZES Wedding Purse Hand Clutch Ladies Party Party Shinny Plain Patent Ladies Purse Champagne Hand Evening ZES Prom Clutch Bag Wedding throw readFileErr;
  console.logY litres yak for HippoWarehouse x38cm Mint Shopping 42cm Bag animal Tote Beach alphabet Gym is 10 1gqdxw6df(data);
  fs.unlink('/file.md', Bag Champagne Ladies Prom Wedding Hand Evening Clutch Plain Party Shinny ZES Patent Purse (Natural Bag Print4u Shopping Terrier Life Dog For Airedale Tote qp81wxpSPunlinkErr) => {
    if (unlinkErr)Bag Satchel Purse Large Messenger Handbag Tote Shoulder Bluester Gray Women OHIq00 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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