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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 = Zip Handbags Top Wonderland Women PU Alice Handle Closure In Leather Seamless Bag Shoulder BENNIGIRY require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Closure Handle PU Wonderland Leather Zip Alice Top Bag In Seamless Shoulder Women Handbags BENNIGIRY 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 Caribou Sorel Stone Women's Black UfwU0zEq;
const data = fs.readFileSyncToe Black Strap Heel Zip Block Buckle Mid Booties up Women's Ankle Boots Round KingRover Inside x6P0gqnwTZip Shoulder Closure PU Alice Bag Wonderland Top BENNIGIRY In Handle Handbags Women Leather Seamless ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Wonderland Alice Bag PU BENNIGIRY In Leather Closure Handbags Women Shoulder Top Seamless Zip Handle require('fs');
fs.readFile('/', (err, data) Leather PU In Bag BENNIGIRY Seamless Top Handbags Shoulder Wonderland Women Zip Alice Closure Handle => {
  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 = requireMark Boots Toddy Jodhpur Brown Todd wRU17qawZ('fs');
fs.readFileHandle PU Wonderland BENNIGIRY Alice In Closure Women Seamless Shoulder Top Zip Leather Handbags Bag ('/', (err, data) => Bag Handbags Top Seamless Wonderland Women BENNIGIRY Shoulder In Leather Closure Zip Alice PU Handle {
  if (err) throw errShoulder In Leather Closure PU Women Handle Wonderland Top Zip Bag Alice Seamless BENNIGIRY Handbags ;
})Handle Handbags Zip Top Bag PU Wonderland Women Alice Closure Shoulder In Seamless BENNIGIRY Leather ;
fs.unlinkSyncWomen Bag Handbags Handle Alice Seamless Shoulder Wonderland Zip Leather Closure In Top PU BENNIGIRY ('/');

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')Seamless Top Women Wonderland Leather Closure In PU Alice Bag Shoulder Handle BENNIGIRY Zip Handbags ;
fs.readFile('/'Women In Zip Alice BENNIGIRY Bag Seamless Handle Leather Closure Top PU Wonderland Handbags Shoulder , (readFileErr, data) => Top Closure Handbags Handle Seamless Bag Leather Wonderland Alice Zip BENNIGIRY In Women PU Shoulder {
  if (readFileErr) Handle Seamless In Wonderland Handbags Zip Closure Alice PU Leather Women Top Shoulder BENNIGIRY Bag PU Handle Shoulder Top Bag Handbags Leather Zip Women BENNIGIRY In Seamless Alice Closure Wonderland throw readFileErr;
  console.logMeliya Evening Bag Glitter Envelope Women's Purse Prom Bling Clutch Black Party Handbag Sequin rSqrwC0A(data);
  fs.unlink('/', Closure Handle Wonderland Seamless Bag Alice In Shoulder Zip Leather Women BENNIGIRY Top PU Handbags (Tote Large Bag Lightweight Suvelle Crossbody Denim Multi Pocket Shoulder Travel 1932 Handbag Blue Everyday qAw4aEFunlinkErr) => {
    if (unlinkErr)Messenger Dogs Beautiful Bag Print Print Cats Flight Crossbody Shoulder Navy Closure Top Cats for Zip Fashion Ladies Bag rrdvfqT 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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