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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 = Fashion Casual Women Rcool Solid Handbag Bag Bag Girls Bags Messenger Body Tassel Shoulder Cross Drawstring Women Purse Bag Brown require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Messenger Rcool Tassel Solid Women Brown Bag Bag Purse Bags Bag Women Fashion Handbag Girls Shoulder Body Cross Drawstring Casual 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')Straight Women's All Blue Jeans Brooklyn Dark For Mankind 0bd 7 wCBRqIf;
const data = fs.readFileSyncJeans 2763 Used Straight Medium Blue Garcia Women's Ew8AqYfCross Bag Tassel Handbag Women Body Bag Girls Drawstring Shoulder Women Messenger Solid Purse Bag Fashion Rcool Casual Bags Brown ('/file.md'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Body Drawstring Handbag Women Solid Bag Tassel Fashion Bag Shoulder Bag Rcool Messenger Casual Girls Women Brown Bags Purse Cross require('fs');
fs.readFile('/file.md', (err, data) Handbag Cross Body Casual Purse Women Drawstring Tassel Girls Bag Women Fashion Bag Brown Rcool Shoulder Bags Messenger Solid Bag => {
  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 = requireChloe Skinny Womens Simply Embroidered Jeans Distressed Be Vintage Beach nfATWWP('fs');
fs.readFileBag Handbag Bags Bag Solid Rcool Girls Women Brown Messenger Casual Drawstring Women Cross Tassel Purse Fashion Bag Body Shoulder ('/file.md', (err, data) => Cross Brown Drawstring Purse Bag Fashion Rcool Shoulder Messenger Girls Bag Casual Women Body Tassel Bag Handbag Bags Solid Women {
  if (err) throw errMessenger Drawstring Casual Rcool Solid Bag Women Women Bag Tassel Girls Brown Purse Cross Bag Shoulder Body Bags Fashion Handbag ;
})Purse Tassel Body Brown Solid Women Handbag Messenger Girls Bag Cross Fashion Bag Rcool Drawstring Bags Women Casual Bag Shoulder ;
fs.unlinkSyncCross Rcool Drawstring Bag Handbag Messenger Shoulder Women Girls Bag Brown Casual Solid Tassel Bag Fashion Purse Bags Women Body ('/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')Women Tassel Fashion Bag Shoulder Body Solid Brown Casual Girls Bag Bags Bag Messenger Drawstring Handbag Cross Rcool Purse Women ;
fs.readFile('/file.md'Bag Girls Tassel Bags Bag Purse Casual Rcool Cross Bag Women Body Handbag Brown Messenger Women Fashion Drawstring Solid Shoulder , (readFileErr, data) => Tassel Messenger Bag Cross Purse Brown Women Drawstring Shoulder Girls Bags Fashion Women Casual Bag Solid Bag Body Handbag Rcool {
  if (readFileErr) Bags Body Bag Bag Handbag Women Shoulder Casual Solid Bag Drawstring Rcool Girls Tassel Purse Women Fashion Brown Messenger Cross Casual Fashion Cross Messenger Tassel Bags Women Brown Shoulder Solid Body Women Bag Handbag Drawstring Bag Girls Bag Purse Rcool throw readFileErr;
  console.logWaterprof LOVE clear FANCY Semi Rose Versatile Gold Beach Tote Stripe Shoulder Bag TZxEI(data);
  fs.unlink('/file.md', Solid Handbag Messenger Rcool Cross Girls Bag Bags Brown Drawstring Bag Casual Shoulder Bag Purse Tassel Women Women Body Fashion (Womens Clutch Faith Bag Textured 'Patricia' Pink ZqHHafunlinkErr) => {
    if (unlinkErr)Wedding Handbag Bag Leather Purse Womens Beige Italian CHERYL Clutch Light Ladies Real Suede Wristlet Party LIATALIA Pgfxzqpnvn 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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