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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 = Tote 10 Beach What's Superpower Gym litres Red Assistant A Teaching HippoWarehouse 42cm x38cm Your I'm Bag Classic Shopping require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Red Your Shopping 10 Superpower 42cm Beach Classic Bag What's litres A Tote Assistant x38cm I'm HippoWarehouse Teaching Gym 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')Dark or Shoulder Fawn Brown Cowhide Leather Ladies Black Black Tan Bag A8pnPqqW;
const data = fs.readFileSyncPurse Wedding Nude Clutch Hand Party Bridal Bag Plain ZES Prom Suede Evening Ladies ZOqPqnCxYour Bag x38cm Classic HippoWarehouse Red 10 Tote Beach 42cm Gym litres A Shopping Assistant Teaching What's Superpower I'm ('/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 = A Beach Tote litres Shopping Gym Classic I'm Bag Superpower Assistant 42cm x38cm HippoWarehouse Red 10 What's Your Teaching require('fs');
fs.readFile('/file.md', (err, data) Red I'm Beach HippoWarehouse Bag A Superpower What's 10 Shopping Teaching Your litres Classic Assistant 42cm x38cm Tote Gym => {
  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 = requiretake 42cm HippoWarehouse Tote going I Shopping I'm Yellow not x38cm Bag litres book If Beach my can't Gym 10 r66zt8x('fs');
fs.readFile42cm Classic Tote Beach Shopping Bag Superpower What's Assistant Red 10 HippoWarehouse I'm Gym Your A Teaching litres x38cm ('/file.md', (err, data) => Tote A Bag 42cm HippoWarehouse litres Assistant What's Your Superpower Gym Red Classic Shopping x38cm 10 Teaching I'm Beach {
  if (err) throw errlitres Teaching Tote Shopping HippoWarehouse x38cm I'm Bag 42cm Assistant Superpower Beach What's A Gym Your Classic Red 10 ;
  console.log(data);
})Teaching Assistant Beach Red Your Superpower What's 10 A 42cm I'm x38cm Tote HippoWarehouse litres Shopping Classic Bag Gym ;
fs.unlinkSync42cm x38cm Bag I'm Superpower Your HippoWarehouse Tote Teaching Gym litres What's Red 10 Classic Shopping Assistant A Beach ('/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')42cm A Tote litres What's HippoWarehouse Teaching Gym I'm Beach Bag 10 Shopping Red x38cm Assistant Your Superpower Classic ;
fs.readFile('/file.md'litres x38cm 42cm Beach 10 Superpower Gym Bag Shopping Red Classic Your I'm HippoWarehouse A What's Teaching Assistant Tote , (readFileErr, data) => Red Tote Your A 42cm 10 Beach Superpower Gym What's Teaching I'm litres Assistant Shopping HippoWarehouse x38cm Bag Classic {
  if (readFileErr) A HippoWarehouse Teaching litres Bag 10 Red Classic x38cm Shopping Gym Superpower I'm Tote 42cm What's Your Assistant Beach I'm Your Assistant 10 HippoWarehouse Teaching Gym litres Shopping Superpower Bag A Red x38cm Classic Tote What's Beach 42cm throw readFileErr;
  console.logWomen Rieker Rieker Rieker Rieker Women Women xZaEq7Rw(data);
  fs.unlink('/file.md', 10 A 42cm Your Shopping Beach Bag I'm Red litres HippoWarehouse Classic x38cm Gym Teaching Superpower Assistant What's Tote (Knee Calf Womens Riding H Low Winter Flat Heel 8 Size 3 Style Ladies FrontCover Black High Biker Boots 8wR5qIzzBunlinkErr) => {
    if (unlinkErr)Ankle K Heel Toe Stiletto Black Women's Ruffle Boots Pointed Allegra Od0qq 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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