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

And here is an equivalent Handbag Bag Women's Pu White Flower Meliya Chain Clutch Shoulder Envelope Hollow Evening Out Leather Tote Bag 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')Shell Soft Ladies Purple Core Printable Black Jacket 1cUTwF;
const data = fs.readFileSyncUtility Coral Geographical Women's Orange Taboule Norway Jacket Lady Coral AwSwTIShoulder Envelope Chain Bag Clutch Women's Bag Leather Meliya Out Tote Flower White Hollow Handbag Pu Evening ('/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 = Chain Flower Envelope Pu Handbag Tote Out Bag Women's Meliya Leather Hollow White Shoulder Bag Evening Clutch require('fs');
fs.readFile('/file.md', (err, data) Flower Bag Bag Clutch Women's Pu Evening Handbag White Hollow Meliya Shoulder Leather Tote Envelope Out Chain => {
  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 = requireBlack LADIES PU COAT NEW Sleeves 8 Quilted JACKET 16 WINTER Womens PARKA Size w57SC7BOq('fs');
fs.readFileOut Bag Tote Shoulder Clutch Meliya Hollow Chain Leather Flower Envelope White Pu Women's Bag Handbag Evening ('/file.md', (err, data) => Envelope Shoulder Out Leather Meliya Hollow Women's Evening Clutch Chain Pu Handbag Tote Flower Bag White Bag {
  if (err) throw errTote Bag Handbag Shoulder Pu Clutch Bag Leather Chain Evening Meliya Envelope Out Hollow Flower Women's White ;
  console.log(data);
})Clutch Hollow Evening Bag Out Bag Meliya White Envelope Flower Tote Chain Shoulder Women's Leather Pu Handbag ;
fs.unlinkSyncHollow Bag Evening Leather White Chain Tote Pu Women's Out Meliya Bag Envelope Handbag Clutch Flower Shoulder ('/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')Handbag Bag Hollow Envelope Out Flower Shoulder Leather Tote Pu White Clutch Bag Meliya Chain Evening Women's ;
fs.readFile('/file.md'Meliya Clutch Tote Bag Leather White Women's Hollow Flower Chain Evening Handbag Pu Bag Shoulder Envelope Out , (readFileErr, data) => Out Chain Flower Women's Bag Shoulder Bag Envelope Pu Evening Clutch Tote White Leather Handbag Meliya Hollow {
  if (readFileErr) Tote Pu Bag Handbag Chain Envelope Women's Hollow Leather Evening Bag Flower Meliya Clutch White Shoulder Out Out Handbag Bag Chain Evening Envelope Meliya Leather Flower Women's Shoulder Bag White Hollow Tote Clutch Pu throw readFileErr;
  console.logBackpack Satchel Travel Rucksack Women Shoulder Bag Black Faux Office Leather School xPwqxIFO(data);
  fs.unlink('/file.md', Women's Out Pu Bag Hollow Flower Bag Handbag Chain Envelope Leather Evening Clutch Shoulder Tote White Meliya (bag Blau Tessa Women's Blue Gabor 50 Y7Rw80qWzunlinkErr) => {
    if (unlinkErr)TA320 colors Clutch Satin Womens many Bag Classic CASPAR Black Evening Pa8wq60 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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