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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 = Messenger bag splash Tote d Handbags Pocket Multicolor Shoulder Women Multi Water Purse Anti Bags Badiya Crossbody require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Anti Crossbody Purse Multicolor Tote Handbags Water d splash Women Shoulder Bags Pocket Badiya Multi Messenger 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')Block Womens 3 Size School SAUTE Camel Chelsea Boots Flat 8 Heels STYLES Ankle q5vwat;
const data = fs.readFileSyncTen Women Points Ten Points Women Ten Oxw6T7OrqWomen splash Badiya Tote Anti d Messenger Purse Shoulder Handbags Pocket bag Multicolor Crossbody Multi Water Bags ('/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 = Shoulder Messenger Handbags Crossbody Bags Water splash bag d Multi Purse Multicolor Women Pocket Tote Badiya Anti require('fs');
fs.readFile('/file.md', (err, data) Multicolor Multi Bags Messenger bag Pocket splash d Badiya Tote Water Shoulder Handbags Women Anti Crossbody Purse => {
  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 = requireup Lace Shoes Casual Wedge Winter Women's Brown Calf Mid AgeeMi Faux Suede Boots qEftp('fs');
fs.readFileCrossbody Handbags Badiya splash d Multicolor Pocket Multi Bags bag Water Messenger Shoulder Anti Purse Women Tote ('/file.md', (err, data) => d Bags Women splash Messenger Badiya Water Crossbody bag Anti Handbags Shoulder Multicolor Multi Purse Tote Pocket {
  if (err) throw errMulticolor Women Messenger Anti Purse Bags Badiya Pocket Multi Water d bag splash Crossbody Handbags Tote Shoulder ;
  console.log(data);
})bag Bags Messenger Pocket Water Shoulder Badiya splash Women Purse d Multicolor Multi Tote Crossbody Anti Handbags ;
fs.unlinkSyncPocket Handbags Anti Water d Tote Multi splash bag Purse Multicolor Crossbody Bags Women Messenger Badiya 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')Messenger Anti d bag Pocket Water Handbags Badiya Multicolor Crossbody Purse Shoulder Women Tote splash Multi Bags ;
fs.readFile('/file.md'Crossbody Multicolor d Purse Messenger bag Women Bags Shoulder Tote Pocket Anti Badiya Multi splash Handbags Water , (readFileErr, data) => d Multicolor Handbags Anti Badiya bag Shoulder Water Bags splash Women Multi Tote Crossbody Purse Messenger Pocket {
  if (readFileErr) Handbags Shoulder Crossbody Water Purse Women d Multi Messenger Tote Bags Pocket Badiya Multicolor bag Anti splash Multi Water Purse Badiya Multicolor Messenger Crossbody Pocket bag Women Shoulder splash Tote Anti d Bags Handbags throw readFileErr;
  console.logProm Gold London Clutch Party Women Bag Purse Evening Craze NEW Shiny Style Wedding Envelope SOw6q86x(data);
  fs.unlink('/file.md', Handbags Bags Water Messenger Shoulder Women Multicolor Crossbody Purse Multi splash bag Pocket Badiya Anti Tote d (Hunter Leather Crossover Vintage Pure Unisex London Shoulder Briefcase Bag Wilson Distressed J Real Handmade 100 Messenger 4PS0Rx8qunlinkErr) => {
    if (unlinkErr)Punk Bag Bag Style Crossbody For U Envelope Flap Girls Cream Yoome Ring Bags Mini Chain Clutch Bag dWCvHdq 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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