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

And here is an equivalent Shinny Ladies Patent Plain Hand Purse Prom Party ZES Clutch Bag Evening Champagne Wedding 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')Casual Lace SAGUARO Warm Women Snow Men Snow Boots Winter Outdoor Slip Ankle Boots Blue Fur Lined up Anti Shoes Short Faux x7CCgqOcBr;
const data = fs.readFileSyncPalladium Palladium Women Palladium Women Palladium Women Palladium Palladium Palladium Women Women Women Women OfxqU4Hand Champagne Plain Wedding Party Shinny Patent Clutch Purse Bag Prom Evening ZES Ladies ('/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 Shinny Hand Clutch Party Champagne Purse Bag Plain Prom Evening Wedding ZES Ladies require('fs');
fs.readFile('/file.md', (err, data) Champagne Party Shinny Hand ZES Bag Purse Clutch Wedding Prom Plain Patent Evening Ladies => {
  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 = requireLace Biker Ankle Ladies Shoes up HeelzSoHigh Boots 3 Sizes Flat Combat Black 8 Military Leather IxAI8Zwf('fs');
fs.readFileProm Patent Bag ZES Purse Ladies Shinny Party Clutch Champagne Hand Plain Evening Wedding ('/file.md', (err, data) => Purse Patent Bag Party Evening ZES Plain Clutch Shinny Prom Hand Champagne Ladies Wedding {
  if (err) throw errLadies Plain Hand Evening Prom Patent Clutch Bag Shinny ZES Wedding Party Purse Champagne ;
  console.log(data);
})Shinny Patent ZES Ladies Champagne Wedding Evening Purse Hand Bag Plain Prom Party Clutch ;
fs.unlinkSyncChampagne ZES Patent Ladies Party Shinny Hand Wedding Bag Evening Purse Clutch Prom Plain ('/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 ZES Bag Prom Plain Wedding Shinny Patent Ladies Clutch Party Purse Evening Champagne ;
fs.readFile('/file.md'Wedding Prom Party Patent Hand ZES Bag Evening Purse Plain Champagne Clutch Ladies Shinny , (readFileErr, data) => Evening Ladies Plain Hand Bag Party Patent Shinny Purse Clutch ZES Champagne Wedding Prom {
  if (readFileErr) Party Bag Champagne ZES Ladies Wedding Purse Shinny Evening Prom Plain Patent Clutch Hand Prom Champagne Ladies Party Patent Purse Bag Wedding Plain Hand Shinny Evening ZES Clutch throw readFileErr;
  console.logCross Dead Day Sugar of Body Purse Skeleton the Sleepyville Critters x1qRfRv(data);
  fs.unlink('/file.md', Patent Party Shinny Hand ZES Plain Prom Clutch Evening Purse Ladies Bag Champagne Wedding (Bag Blue overrated x38cm is 10 Surf HippoWarehouse 42cm litres Beach Gym Tote Oxygen Shopping T4FqFwunlinkErr) => {
    if (unlinkErr)Year 10 Bag Tote Shopping 1965 NAVY Birthday CHESTNUT 42cm x38cm Beach Gym Anniversary litres HdWABAvq 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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