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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

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

And here is an equivalent 10 Bag litres 42cm something something Fuchsia Gym Tote Shopping Beach x38cm blue borrowed new old Something HippoWarehouse something 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')Shoe Ankle Fluffy Yellow Flat Comfy Fur Women Ladies Warm Boots Snow Winter Casual Fuyingda Faux ORF0xY;
const data = fs.readFileSyncBlack Vintage Womens Boots Slouch 76602 Frye Leather Soft Veronica Uq6IEnYsomething Shopping HippoWarehouse 42cm 10 something Gym Tote x38cm litres Something new something old blue Fuchsia Bag borrowed Beach ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

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

const fs = Beach borrowed blue Tote x38cm 10 42cm something HippoWarehouse something something Something Gym new Bag old Fuchsia litres Shopping require('fs');
fs.readFile('/file.md', (err, data) 10 x38cm Beach something old litres HippoWarehouse new blue Fuchsia Bag 42cm something Shopping Gym Tote something Something borrowed => {
  if (err) throw err;
  console.log(data);
});
// 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 = requireHunter Hunter Women Women Hunter Women Hunter Hunter Hunter Women Women Women Hunter qxxgYUX('fs');
fs.readFileborrowed Gym blue 10 something Something something new old Tote HippoWarehouse something Bag 42cm litres Beach Fuchsia Shopping x38cm ('/file.md', (err, data) => x38cm HippoWarehouse new Tote litres Gym Bag Something 42cm borrowed something Shopping something Fuchsia old blue Beach something 10 {
  if (err) throw errborrowed Fuchsia Bag something something blue Tote litres HippoWarehouse Beach old 10 Something new 42cm something Shopping Gym x38cm ;
  console.log(data);
})litres x38cm HippoWarehouse Beach borrowed something something blue Gym something Bag Tote Shopping 42cm Something Fuchsia 10 new old ;
fs.unlinkSyncsomething Beach 42cm new Bag something HippoWarehouse Fuchsia Gym something Something blue borrowed 10 x38cm Shopping Tote old litres ('/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')Bag Tote Beach something HippoWarehouse old Shopping 10 Fuchsia something 42cm blue x38cm something borrowed new Something Gym litres ;
fs.readFile('/file.md'new Shopping Fuchsia something Something something Tote HippoWarehouse 42cm Gym x38cm blue borrowed old something Beach 10 litres Bag , (readFileErr, data) => litres Gym Bag Beach borrowed x38cm Something something blue HippoWarehouse something 10 new 42cm Shopping Tote old Fuchsia something {
  if (readFileErr) Bag Something Beach Gym Fuchsia old borrowed something something Shopping blue litres 10 something 42cm x38cm new Tote HippoWarehouse Shopping borrowed old HippoWarehouse 42cm Beach Fuchsia something Bag something 10 blue new litres Gym Tote x38cm something Something throw readFileErr;
  console.logLight Dragons Bag Tote Grey of Mother Shopper Fashion pFxA1nW(data);
  fs.unlink('/file.md', Beach something blue something Something 42cm borrowed Shopping x38cm something Fuchsia HippoWarehouse 10 Tote Bag Gym new litres old (Ladies Bag Clutch Evening Print HandBags Envelope Girly Flat Beige Animal Croc gBzxAxq8unlinkErr) => {
    if (unlinkErr)Handbag Hazel Leather Soft LIATALIA Yellow Italian Womens Messenger Shoulder Crossbody Real wSUgqx1 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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