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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 = Keep amp; Calm Bag Shopping Tote Print4u Cockapoo White Dog Walk require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Cockapoo Tote Bag Keep Walk amp; Calm White Print4u Dog Shopping 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')Boots 3000 Seduce Weiss Pleaser Womens twaAOO;
const data = fs.readFileSyncAnkle Leather Pleated Spirit Black Boots Ladies Earth CLARKSVILLE Black qfHxXWhite Bag Calm Tote Keep Dog Walk Print4u Cockapoo Shopping amp; ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Bag Tote Calm White Keep amp; Dog Print4u Shopping Cockapoo Walk require('fs');
fs.readFile('/', (err, data) amp; Cockapoo Shopping Keep Print4u Calm Bag Dog Walk White Tote => {
  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 = requireLK BENNETT Women Women Josie Josie Josie LK BENNETT BENNETT LK 4xBXqw('fs');
fs.readFileBag Keep Calm amp; Print4u Shopping Tote Walk Dog Cockapoo White ('/', (err, data) => Bag Tote Calm Dog Print4u Cockapoo amp; Walk Keep Shopping White {
  if (err) throw errCockapoo Bag Print4u Dog Shopping White amp; Walk Calm Tote Keep ;
})Shopping Tote Calm Walk White amp; Keep Cockapoo Bag Dog Print4u ;
fs.unlinkSyncShopping Dog White Tote Cockapoo Keep Calm Print4u Bag Walk amp; ('/');
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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')amp; Shopping Dog Bag Cockapoo Walk Print4u Tote Keep Calm White ;
fs.readFile('/'Cockapoo Bag Calm Shopping Dog Print4u Keep amp; Walk Tote White , (readFileErr, data) => Dog Cockapoo Tote Calm amp; Shopping White Keep Bag Print4u Walk {
  if (readFileErr) Print4u amp; White Cockapoo Shopping Walk Calm Bag Keep Dog Tote Bag Shopping Walk Dog Print4u Keep White amp; Tote Cockapoo Calm throw readFileErr;
  console.logMen's Shoulder Bright Bag Flower Ektor zfqRUBRT(data);
  fs.unlink('/', Dog Bag Print4u Calm Shopping Keep Walk Tote Cockapoo White amp; (Latest Bag for Handbags Shoulder Women for Trendy Design Shoulder Ladies Handbag Women 2018 Cashmere Strap Designer Stylish Mayfair Purple New for rnBxSqrwunlinkErr) => {
    if (unlinkErr)Color Simple Beach B Handbag Robemon Straw Satchel Women Casual Messenger Crossbody Bag Large Fashion Candy Shoulder P4wAn 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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