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

And here is an equivalent old Fuchsia Tote something something Gym Shopping Bag litres x38cm something 42cm blue Something new 10 HippoWarehouse Beach borrowed 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')Vintage Messenger Messenger Shoulder Body Bag Leather Purple Shoulder Bag Bag PLOT Cross Purse qFAAwWtOd;
const data = fs.readFileSyncOrange Birds Print Love Bag Kiely Handbag Persimmon Orla Classic Shoulder Women's Zip qTBvwZSomething borrowed something something Tote litres new HippoWarehouse x38cm Gym old 10 Shopping Bag 42cm Beach something Fuchsia blue ('/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 = litres 42cm Beach Something x38cm something blue Bag HippoWarehouse something borrowed new old Fuchsia Gym something Tote Shopping 10 require('fs');
fs.readFile('/file.md', (err, data) Shopping new litres Something Gym something x38cm blue something 10 Bag something Tote borrowed HippoWarehouse old Beach 42cm Fuchsia => {
  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.

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 = requireamp; Strap Rainbow Cat With Print Black Flower Emoji Anchor Kukubird Dog Crossbody Emoji Messenger Bag Mixed Animal xTqwRxPvY('fs');
fs.readFileTote something borrowed x38cm something 42cm litres HippoWarehouse 10 Beach Bag Something new blue Gym Fuchsia something Shopping old ('/file.md', (err, data) => litres 10 something something old Beach Something x38cm blue new Shopping HippoWarehouse Bag something Gym Fuchsia Tote borrowed 42cm {
  if (err) throw errGym 10 new Beach HippoWarehouse Fuchsia Tote old x38cm borrowed something something something Something 42cm Shopping litres blue Bag ;
  console.log(data);
})Bag blue something Fuchsia HippoWarehouse something old litres Something Shopping Gym Tote new borrowed 10 something Beach 42cm x38cm ;
fs.unlinkSyncnew something 10 old Fuchsia 42cm something borrowed Tote litres Shopping something Bag Gym blue Something Beach HippoWarehouse x38cm ('/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')new old Beach HippoWarehouse borrowed Gym Bag blue 10 Shopping 42cm litres Tote something Something x38cm something something Fuchsia ;
fs.readFile('/file.md'new borrowed blue Bag Tote 10 something something HippoWarehouse something Fuchsia litres 42cm Gym Shopping Something Beach x38cm old , (readFileErr, data) => borrowed new Beach Bag Shopping Gym Tote 42cm x38cm something something old litres HippoWarehouse blue 10 something Fuchsia Something {
  if (readFileErr) something x38cm Shopping HippoWarehouse Something old Tote borrowed blue Fuchsia Beach 10 litres 42cm something Gym new something Bag Beach litres Something borrowed something blue x38cm Gym something Fuchsia something Bag old Shopping 42cm 10 Tote HippoWarehouse new throw readFileErr;
  console.logNylon for Nameblue Messenger School Travel Tote Girls Bag Work and Bag Womens black EU Shopping Shoulder 1309 3118 Handbags Bag wwXpv0(data);
  fs.unlink('/file.md', old borrowed something x38cm Gym something blue Fuchsia 42cm something Bag new Shopping litres Something Beach HippoWarehouse Tote 10 (bag shoulder Grau shopper bag bag Women's T02 handbag suede Italian real leather 6qBzycH1unlinkErr) => {
    if (unlinkErr)Bags Bags Fashion Pink Handbags Tote Handbags Messenger Womens Leather Women for Shoulder SSMK 8qBxTPwC6 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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