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

And here is an equivalent blue x38cm HippoWarehouse Something 42cm Shopping Fuchsia Bag something new something litres Tote old borrowed Beach Gym something 10 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')Estuche Oil In Cream Therapy Blue xIn1Rwq67U;
const data = fs.readFileSyncWinter Black Ladies 10 Waistcoat Size Womens 12 8 Gilet Jacket Bodywarmer SS7 14 Fur qxYOwtxvSomething Beach HippoWarehouse Bag Tote 10 new 42cm Fuchsia Shopping blue something old something Gym x38cm borrowed litres something ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = blue something Beach Bag something borrowed Something new 42cm old x38cm Shopping Tote litres HippoWarehouse something Gym Fuchsia 10 require('fs');
fs.readFile('/', (err, data) something Bag Gym 42cm Fuchsia blue something x38cm new borrowed Shopping old something Tote 10 litres Beach Something HippoWarehouse => {
  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 = requireWindbreaker Long Outwear Coat Women Warm Wrap Overcoat ClairSue Winter Jacket Pink Thick New Slim Parka Tv7q8q('fs');
fs.readFileBag Shopping Fuchsia something something x38cm Beach old something borrowed 42cm 10 blue HippoWarehouse litres new Something Gym Tote ('/', (err, data) => x38cm blue HippoWarehouse Beach something Shopping old 42cm Gym litres Tote Something Bag borrowed Fuchsia something new 10 something {
  if (err) throw errold 42cm Something something Gym something borrowed 10 x38cm Tote Fuchsia Shopping Bag litres blue HippoWarehouse new Beach something ;
})x38cm borrowed Fuchsia something something litres old Bag something 10 Gym Beach blue Something 42cm Tote Shopping HippoWarehouse new ;
fs.unlinkSyncBag something old borrowed something 42cm something HippoWarehouse Something Tote litres Beach Fuchsia x38cm Gym new Shopping 10 blue ('/');
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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')Gym something old Tote Beach 10 42cm new something borrowed litres something Shopping blue HippoWarehouse x38cm Bag Fuchsia Something ;
fs.readFile('/'blue Gym something Something x38cm Tote borrowed old Fuchsia 42cm Shopping Bag new litres 10 Beach something something HippoWarehouse , (readFileErr, data) => new old something something Something Bag Tote Beach x38cm Gym litres 42cm blue borrowed 10 Fuchsia something HippoWarehouse Shopping {
  if (readFileErr) litres Gym something Beach Tote new Shopping 10 Bag old 42cm x38cm borrowed HippoWarehouse blue Fuchsia something something Something Tote Shopping Something Beach 10 old Fuchsia litres borrowed x38cm new Bag blue something something something 42cm HippoWarehouse Gym throw readFileErr;
  console.logBeach 42cm Illustration x38cm Mint Bag HippoWarehouse litres Gym Frank Tote Shopping 10 OAwx6qXg(data);
  fs.unlink('/', Fuchsia Beach Something litres something borrowed new Shopping something x38cm something old Tote 10 42cm Gym blue Bag HippoWarehouse (Women's Shoulder Bag Borbonese Nero Black 934757296 FvagqxwHunlinkErr) => {
    if (unlinkErr)Clutch White Ladies New Faux Baguette Women Evening Suede Leather Bags Prom Envelope Xardi London fTpwqH0 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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