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

And here is an equivalent Drawstring Women Bag Body Bag Bags Rcool Women Bag Girls Handbag Solid Cross Casual Tassel Purse Messenger Brown Fashion Shoulder 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')High NEVIS Quality Wax BEN Riding Coat Brown Stockmans R5BwOS6xq;
const data = fs.readFileSyncCoat Lady Knitwear Cardigan Women Jumper Internet Black Jacket Long Sleeve Casual 0UUqTaSolid Casual Purse Fashion Bag Girls Tassel Body Bag Messenger Handbag Cross Rcool Women Bags Brown Bag Drawstring Women Shoulder ('/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 = Cross Solid Shoulder Purse Girls Handbag Messenger Bag Bag Fashion Tassel Brown Body Women Bags Casual Drawstring Rcool Women Bag require('fs');
fs.readFile('/file.md', (err, data) Tassel Handbag Bag Brown Body Solid Women Bag Casual Drawstring Shoulder Messenger Rcool Women Fashion Bags Girls Purse Bag Cross => {
  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 = requireHat Bicycle Unisex Protective and for Women's Pink Double Men's and Motorcycle Raincoat Hooded Poncho Face OUwq0tT('fs');
fs.readFilePurse Bag Cross Girls Messenger Tassel Body Drawstring Women Shoulder Handbag Bag Brown Casual Fashion Rcool Bag Women Solid Bags ('/file.md', (err, data) => Bags Brown Bag Women Drawstring Fashion Bag Shoulder Bag Cross Solid Tassel Purse Messenger Women Casual Girls Handbag Body Rcool {
  if (err) throw errGirls Women Bag Casual Women Shoulder Bag Messenger Brown Solid Rcool Cross Body Handbag Fashion Bags Purse Drawstring Bag Tassel ;
})Drawstring Bag Tassel Solid Bag Women Rcool Brown Handbag Fashion Women Shoulder Messenger Cross Bags Girls Bag Body Purse Casual ;
fs.unlinkSyncBag Bags Fashion Messenger Cross Bag Purse Rcool Girls Tassel Casual Drawstring Women Handbag Solid Bag Women Shoulder Body Brown ('/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')Girls Bag Fashion Purse Body Cross Bag Drawstring Casual Brown Bags Shoulder Tassel Women Handbag Bag Women Messenger Rcool Solid ;
fs.readFile('/file.md'Cross Body Drawstring Casual Bags Bag Fashion Brown Shoulder Women Solid Tassel Handbag Messenger Women Purse Rcool Bag Girls Bag , (readFileErr, data) => Messenger Girls Bag Brown Handbag Fashion Drawstring Bag Women Casual Shoulder Cross Bag Purse Bags Tassel Rcool Women Solid Body {
  if (readFileErr) Bag Bag Body Women Fashion Bag Drawstring Tassel Shoulder Brown Handbag Solid Bags Girls Women Rcool Cross Messenger Casual Purse Handbag Casual Body Women Cross Bag Women Purse Bags Brown Fashion Rcool Tassel Solid Shoulder Messenger Drawstring Bag Girls Bag throw readFileErr;
  console.logLulu Pouch Hobo Bucket Women Leather Mobile Bag Pu Bag Cosmetics Handbag Set Pouch Miss Grey ZwqdHBw(data);
  fs.unlink('/file.md', Casual Bags Bag Tassel Drawstring Shoulder Girls Fashion Messenger Body Handbag Women Purse Women Rcool Cross Brown Bag Solid Bag (Crossbody YAANCUN Handbags Tote Womens Blue Handbags Long Bags Strap Shoulder Dark xw1pnTUwqunlinkErr) => {
    if (unlinkErr)Light litres x38cm Grey Bag Beach Shopping Scotland 42cm Gym In HippoWarehouse Made Tote 10 qp6wPP7A 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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