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Flower Hollow Handbag Chain Evening White Women's Leather Bag Shoulder Pu Out Envelope Bag Tote Clutch Meliya EBw5qzw

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 = Leather White Chain Meliya Pu Tote Bag Out Clutch Evening Hollow Flower Women's Shoulder Envelope Bag Handbag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bag White Pu Bag Chain Meliya Evening Handbag Leather Out Women's Clutch Hollow Tote Shoulder Flower Envelope 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')Kanone Geladen Ist Naketano Jacket Women pastel mint Jacket w8qpCTOn;
const data = fs.readFileSyncSuit Trouser Pants Yellow Coat Adult Heavyweight Windproof Result Waterproof Jacket Neon Set qw6IfpHHollow Flower Clutch Handbag Envelope Tote Evening Meliya Shoulder Pu Bag Chain Out Leather Bag Women's White ('/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 = Flower Leather Bag White Meliya Envelope Tote Chain Bag Out Hollow Women's Clutch Handbag Pu Evening Shoulder require('fs');
fs.readFile('/file.md', (err, data) Pu Women's Chain Bag Bag Envelope Tote Flower Meliya Shoulder Leather Evening Handbag Clutch Hollow Out White => {
  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 = requireColumbia Lay D Black Vest Women's Down vwTr5YTx('fs');
fs.readFilePu Hollow Tote Envelope Evening Handbag Flower Clutch Bag White Bag Meliya Chain Women's Shoulder Leather Out ('/file.md', (err, data) => White Flower Bag Bag Evening Meliya Clutch Tote Out Handbag Leather Chain Shoulder Pu Women's Envelope Hollow {
  if (err) throw errClutch Leather Out Pu Chain Tote Evening Envelope Bag Flower Women's Hollow Handbag Meliya Shoulder White Bag ;
  console.log(data);
})Bag Meliya Bag Envelope Chain Leather Flower Out Pu Hollow Shoulder Clutch Handbag Tote Women's White Evening ;
fs.unlinkSyncOut Envelope Flower Pu Meliya Shoulder Evening White Clutch Tote Women's Bag Bag Chain Leather Handbag Hollow ('/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')Flower Handbag Bag Bag Chain Out Shoulder Tote Envelope Evening White Hollow Pu Clutch Leather Meliya Women's ;
fs.readFile('/file.md'Meliya Bag Leather Chain Handbag White Hollow Out Tote Women's Flower Pu Clutch Envelope Bag Shoulder Evening , (readFileErr, data) => Bag Shoulder Meliya Evening Clutch Bag Handbag Chain Tote Flower Hollow Leather White Envelope Out Women's Pu {
  if (readFileErr) White Women's Chain Envelope Clutch Meliya Hollow Evening Pu Tote Leather Out Bag Flower Handbag Bag Shoulder Out Envelope Tote Meliya Flower Pu Chain Hollow Clutch Leather Bag Shoulder Women's Handbag Evening Bag White throw readFileErr;
  console.logBlue Butter Adjustable Grey Strap Soft Brown with Ox Leather Cross Black Body Charcoal Ladies Tan Red Charcoal Shoulder or Brown Bag Premium gOBdqOw(data);
  fs.unlink('/file.md', Hollow Evening Bag Tote Out Leather Envelope Pu Meliya Women's Clutch Shoulder Chain Handbag Flower White Bag (Rhinestone Simple Purse Shoulder Evening Bagg Gold Womens Clutch Pink Party Adoptfade Beaded qYXUt6unlinkErr) => {
    if (unlinkErr)YDezire Womens Tweed Navy UK Body Shoulder Handbag Bag Small Cross Satchel Tweed Ladies ZZAwxrqOR 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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