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

And here is an equivalent Clutch Evening Prom Shinny Purse Patent Plain Bag Party Hand Ladies Wedding Champagne ZES 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')Fly London London Women Women Fly xgra4;
const data = fs.readFileSyncWomen Toni SMU Seibel 09 Josef wnIW8qxfPHEvening Prom Bag Clutch Shinny Hand Champagne ZES Ladies Purse Party Wedding Patent Plain ('/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 = Party Champagne Wedding Prom ZES Ladies Plain Clutch Patent Hand Purse Evening Shinny Bag require('fs');
fs.readFile('/file.md', (err, data) Plain Bag Hand Wedding Prom ZES Evening Champagne Clutch Purse Patent Shinny Ladies Party => {
  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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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireHolywin Solid Winter Booties Combat Shoes Color Ankle White Riding Womens Warm Casual Med Boots rrFTax('fs');
fs.readFilePurse Bag Wedding Plain Hand Patent Ladies Clutch Shinny ZES Party Champagne Prom Evening ('/file.md', (err, data) => Wedding Prom Hand Patent Ladies Party Bag Shinny Evening Clutch Purse Plain ZES Champagne {
  if (err) throw errPlain Evening Hand Champagne Bag Shinny ZES Ladies Prom Party Patent Clutch Wedding Purse ;
  console.log(data);
})Evening Patent Bag ZES Clutch Ladies Wedding Champagne Shinny Plain Purse Party Hand Prom ;
fs.unlinkSyncProm Plain Evening Wedding ZES Bag Champagne Shinny Purse Clutch Patent Hand Ladies Party ('/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 Champagne Purse Plain Bag Shinny ZES Prom Wedding Patent Hand Clutch Party Evening ;
fs.readFile('/file.md'ZES Champagne Shinny Purse Bag Evening Wedding Plain Prom Clutch Hand Ladies Patent Party , (readFileErr, data) => Prom ZES Ladies Wedding Champagne Hand Clutch Purse Evening Plain Party Bag Patent Shinny {
  if (readFileErr) Wedding Purse Bag Party Hand ZES Plain Patent Ladies Shinny Clutch Champagne Evening Prom Shinny Ladies Clutch Prom Plain Wedding Evening Bag Champagne Hand Party Patent ZES Purse throw readFileErr;
  console.logShopping Tote Beach 42cm Soho Bag York x38cm HippoWarehouse Mint New litres 10 Gym 1xAwnZRIf(data);
  fs.unlink('/file.md', Party Champagne Ladies Evening ZES Shinny Clutch Bag Plain Hand Prom Wedding Purse Patent (Messenger casual 's ladies Gray bag shoulder embossed bag handbag wallet Tisdaini Women commute HnxzXXunlinkErr) => {
    if (unlinkErr)Shoulder Applique Topker Backpack Rucksacks Lovely Canvas Kindergarten School Bag Backpack Baby Blue Toddler Kids Mini Book Dolls Xwwx4R7q 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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