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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 = something Shopping Beach Tote Bag x38cm HippoWarehouse 42cm old Gym new 10 something Fuchsia something blue litres borrowed Something require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent blue borrowed new Shopping Gym 10 42cm something something Tote old litres Something Bag something x38cm Beach HippoWarehouse Fuchsia 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')110 Bootie Laced Punk New Black Women's Spikes Metal Rock S1 Goth wqIxFgnPE;
const data = fs.readFileSyncGTX Clarks Women Mint Clarks Aqua Mint wfdqxnapIfborrowed Gym 10 Tote blue something Bag Fuchsia something litres new old Beach x38cm 42cm Shopping Something something HippoWarehouse ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = x38cm Tote something 42cm Beach borrowed litres Bag Gym Something 10 Fuchsia new Shopping something old something blue HippoWarehouse require('fs');
fs.readFile('/', (err, data) something Gym blue old Tote HippoWarehouse borrowed x38cm Beach new something 10 Bag Something litres Shopping something 42cm Fuchsia => {
  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 = requireHigh Denim Shoes Buckle Sleeve Mid Heel Womens Boots Xianshu Warm Calf Belt Blue 5IOqSKw('fs');
fs.readFileSomething litres Shopping 10 new 42cm old something Tote HippoWarehouse x38cm Gym Bag something borrowed blue Beach something Fuchsia ('/', (err, data) => old something something borrowed Bag Gym Beach something blue x38cm 42cm litres 10 Something new Shopping Tote HippoWarehouse Fuchsia {
  if (err) throw errold blue 10 new something Fuchsia something HippoWarehouse Tote Shopping x38cm Bag something litres Something borrowed Gym Beach 42cm ;
})HippoWarehouse Gym Fuchsia x38cm Bag 10 Something something something something new old 42cm Beach Shopping borrowed litres blue Tote ;
fs.unlinkSyncSomething Shopping borrowed Gym old Tote new x38cm 42cm something something Fuchsia blue HippoWarehouse Bag something 10 litres Beach ('/');
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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')something something Beach 10 Something HippoWarehouse old borrowed Tote something litres 42cm new Bag Shopping Fuchsia Gym blue x38cm ;
fs.readFile('/'blue Beach Gym Fuchsia something new something 10 Tote old Bag borrowed Shopping Something something litres 42cm HippoWarehouse x38cm , (readFileErr, data) => old blue something litres something something borrowed new Something Gym Tote HippoWarehouse Beach 10 x38cm Shopping 42cm Bag Fuchsia {
  if (readFileErr) Fuchsia 42cm litres borrowed something Beach old Gym Shopping new Bag blue 10 Something something something HippoWarehouse Tote x38cm something x38cm litres Gym 42cm HippoWarehouse new Tote Something 10 borrowed something Fuchsia something Shopping old Bag Beach blue throw readFileErr;
  console.logCroc Bag Brown Bag Suede Aossta Envelope Bag Clutch Shoulder Occasion Wedding Real Leather RPR7tT(data);
  fs.unlink('/', 10 Fuchsia Bag x38cm Beach blue borrowed HippoWarehouse something Something old Shopping Gym something litres 42cm something new Tote (Large Color Fashion H Robemon Handbag Shoulder Beach Straw Women Satchel Bag Simple Casual Candy Crossbody Messenger UxwS7nTunlinkErr) => {
    if (unlinkErr)737 Shoulder Cross Collection 79 Multi Mala Multi Black Plum Leather Bag BURCHELL Leather Body Yq8CSTxw 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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