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

And here is an equivalent Fuchsia x38cm Something 10 HippoWarehouse borrowed Shopping litres Gym Beach blue Bag something something Tote something new 42cm old 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')Canyon Fashion Canyon Fashion Canyon nWvv16p;
const data = fs.readFileSyncToggi Chocolate Boot Chocolate Leather Canyon Women's Brown Long PxqSPRwaFuchsia borrowed 10 Shopping litres something Something old Beach 42cm x38cm Gym Bag something Tote HippoWarehouse blue something new ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = borrowed Beach HippoWarehouse blue Gym 10 Fuchsia something litres x38cm Bag 42cm Something old Shopping something something Tote new require('fs');
fs.readFile('/', (err, data) something 10 old litres new something Something HippoWarehouse blue Tote something x38cm Gym 42cm Fuchsia Beach Shopping Bag borrowed => {
  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 = requireWomen Mustang Mustang Women Mustang Women Mustang Mustang Women Women Mustang Mustang Women Women Mustang Mustang Women qw47U('fs');
fs.readFileShopping new Beach old Bag borrowed something Tote 42cm x38cm 10 Gym Something something HippoWarehouse something litres Fuchsia blue ('/', (err, data) => Fuchsia 42cm HippoWarehouse blue something new something borrowed Tote Something Bag litres something Shopping old Beach 10 Gym x38cm {
  if (err) throw errBeach something something Bag new blue Gym 10 Fuchsia HippoWarehouse Shopping something litres Something borrowed 42cm Tote old x38cm ;
})Gym something blue old new Fuchsia 10 42cm Beach something Something Tote Shopping HippoWarehouse x38cm something borrowed Bag litres ;
fs.unlinkSyncFuchsia something blue litres 10 Something something Bag Beach something Tote HippoWarehouse borrowed Shopping new old x38cm Gym 42cm ('/');
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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')litres Beach 10 Fuchsia Something something Gym blue new Bag HippoWarehouse 42cm old something borrowed something x38cm Tote Shopping ;
fs.readFile('/'borrowed Bag new Fuchsia Something HippoWarehouse something 42cm Beach blue Gym 10 Shopping old litres something Tote something x38cm , (readFileErr, data) => 42cm something blue HippoWarehouse new x38cm Beach Shopping 10 something something Bag Something Gym old borrowed litres Fuchsia Tote {
  if (readFileErr) Beach litres new something blue x38cm Something something 42cm old borrowed HippoWarehouse Fuchsia Bag Shopping 10 something Gym Tote 10 borrowed Shopping Gym something HippoWarehouse blue something Fuchsia litres Something Beach old 42cm Bag Tote something x38cm new throw readFileErr;
  console.logGrey Girl Zipper Messenger Cute Handbag PU Everpert Shoulder Round Mini Women Leather Bag g7x0WTwq(data);
  fs.unlink('/', Beach Something 42cm new something Shopping HippoWarehouse Bag borrowed Gym blue litres Tote something something old x38cm Fuchsia 10 (Women Guess Red Bordeaux KAMRYN Hwvg6691230 SHOPPER Bordeaux rWnHtvWunlinkErr) => {
    if (unlinkErr)Plain Suede Grey Bag Clutch Girly HandBags 8zaqff 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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