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

And here is an equivalent Slim Cellphone Small Black Bags Shoulder Girls Handbag Bags Ladies Women Pouch Messenger Bags Fashion Body Crossbody 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')In Long Zip Tweed Hot0809 Purse Ladies Colours Staffa Various Grey qvTw45Zxn;
const data = fs.readFileSyncIris Hooded Tommy Vest Gilet Down 002 Essential Women's Tjw Outdoor Black Jeans Blue ZBxIBqPwUCellphone Pouch Slim Black Bags Shoulder Women Small Body Bags Crossbody Girls Handbag Ladies Messenger Fashion Bags ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Crossbody Shoulder Ladies Slim Bags Fashion Cellphone Small Women Black Bags Pouch Body Handbag Bags Messenger Girls require('fs');
fs.readFile('/', (err, data) Cellphone Body Girls Shoulder Black Fashion Bags Pouch Slim Bags Bags Crossbody Women Handbag Messenger Ladies Small => {
  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.

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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 = requireJumper Fleece Ladies Soft Hoody Womens Teddy Hooded MISSMAO Grey Coat Jacket Dark IYdSTqxS('fs');
fs.readFileLadies Crossbody Small Body Black Pouch Cellphone Girls Bags Bags Shoulder Women Messenger Handbag Slim Bags Fashion ('/', (err, data) => Shoulder Body Crossbody Pouch Women Fashion Handbag Messenger Cellphone Slim Girls Bags Bags Small Black Ladies Bags {
  if (err) throw errBags Ladies Fashion Messenger Cellphone Crossbody Handbag Girls Black Bags Pouch Body Slim Bags Small Shoulder Women ;
})Bags Shoulder Cellphone Black Handbag Girls Fashion Small Women Crossbody Body Messenger Pouch Slim Bags Ladies Bags ;
fs.unlinkSyncBlack Bags Shoulder Crossbody Bags Bags Girls Fashion Cellphone Handbag Pouch Body Ladies Small Women Messenger Slim ('/');
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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')Pouch Fashion Slim Handbag Bags Bags Ladies Women Messenger Cellphone Black Small Bags Girls Crossbody Shoulder Body ;
fs.readFile('/'Body Cellphone Black Messenger Pouch Bags Slim Bags Girls Fashion Crossbody Bags Small Handbag Women Shoulder Ladies , (readFileErr, data) => Girls Crossbody Women Black Shoulder Handbag Body Small Bags Bags Messenger Pouch Cellphone Slim Bags Fashion Ladies {
  if (readFileErr) Cellphone Crossbody Slim Bags Bags Messenger Pouch Fashion Body Black Handbag Women Girls Shoulder Ladies Bags Small Ladies Pouch Small Handbag Girls Crossbody Black Fashion Women Slim Body Shoulder Messenger Bags Bags Cellphone Bags throw readFileErr;
  console.logBag Evening Bags LeahWard Cross s Rose Clutch Purser Handbag Wedding Body Top 088 Women's Czwfnzq1X(data);
  fs.unlink('/', Black Women Girls Messenger Pouch Bags Shoulder Crossbody Body Slim Handbag Fashion Bags Bags Small Ladies Cellphone (Women Body Phone nbsp;Girls nbsp;Shoulder Canvas Cell Cross Bag Mini Dark Cute bag for Pouch Mopaclle Purse Blue Wallet Lady Z8XqwgnunlinkErr) => {
    if (unlinkErr)Anekke Houses Paseo Oval Backpack Liberty xRqwfr8HRa 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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