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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 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 = Bags Bags Messenger Slim Bags Fashion Body Black Girls Pouch Women Crossbody Cellphone Ladies Shoulder Small Handbag require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Bags Girls Messenger Body Bags Handbag Bags Small Black Cellphone Ladies Crossbody Shoulder Women Fashion Pouch Slim asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Bag Messenger Waterproof Bag Leather Fashion Waist Chest Outdoor Bag Women Winkey Black Shoulder Bumbag zA6awq;
const data = fs.readFileSyncStitching Leather Colored Travel Women Fashion Shoulder Black Bag Backpack d5EUggqHxMessenger Handbag Women Crossbody Black Pouch Bags Bags Slim Cellphone Body Small Bags Girls Ladies Fashion Shoulder ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Women Cellphone Messenger Crossbody Bags Slim Fashion Pouch Black Small Bags Shoulder Body Handbag Ladies Girls Bags require('fs');
fs.readFile('/', (err, data) Shoulder Slim Pouch Women Messenger Black Bags Girls Fashion Small Ladies Cellphone Crossbody Bags Body Bags Handbag => {
  if (err) throw err;
// 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 = requireBackpack Blue Girls School Canvas Students Pouch Drawstring 4PCS Cute HCFKJ Shoulder Large Bag Kids Sets Printing 04ZvqnSxOB('fs');
fs.readFileBody Crossbody Bags Fashion Pouch Girls Slim Small Bags Messenger Women Shoulder Cellphone Ladies Black Handbag Bags ('/', (err, data) => Women Black Bags Body Bags Cellphone Small Slim Ladies Crossbody Bags Shoulder Girls Fashion Messenger Pouch Handbag {
  if (err) throw errCrossbody Shoulder Small Black Fashion Cellphone Handbag Messenger Bags Bags Body Bags Slim Girls Pouch Ladies Women ;
})Slim Cellphone Messenger Fashion Bags Handbag Ladies Small Bags Girls Women Black Body Shoulder Crossbody Pouch Bags ;
fs.unlinkSyncFashion Cellphone Crossbody Body Girls Black Women Handbag Slim Bags Small Pouch Bags Bags Messenger Ladies Shoulder ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Black Bags Body Handbag Women Slim Bags Fashion Girls Pouch Messenger Crossbody Cellphone Small Ladies Bags Shoulder ;
fs.readFile('/'Small Black Cellphone Women Body Bags Slim Bags Messenger Shoulder Ladies Bags Pouch Crossbody Fashion Handbag Girls , (readFileErr, data) => Bags Handbag Messenger Body Crossbody Fashion Small Pouch Black Bags Bags Cellphone Ladies Girls Women Shoulder Slim {
  if (readFileErr) Bags Ladies Black Shoulder Handbag Pouch Small Women Crossbody Cellphone Bags Girls Body Bags Slim Messenger Fashion Pouch Ladies Small Bags Crossbody Handbag Girls Slim Women Body Fashion Bags Messenger Bags Black Cellphone Shoulder throw readFileErr;
  console.logHandbags Women Patent Bags Shoulder Dark Tote Mirror Jieway Leather Purse Bright Silver Clutch dq0vd1g(data);
  fs.unlink('/', Shoulder Bags Girls Cellphone Messenger Bags Pouch Crossbody Small Ladies Black Slim Body Women Bags Fashion Handbag (ADOO fashion bag bag Burgundy casual New messenger purse 2017 for light handbag portable shoulder ladies blue qHrEznqwFxunlinkErr) => {
    if (unlinkErr)Navy Cuckoo Body Cross Red Bag Designer 365 Y7qwnBa 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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