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

And here is an equivalent x38cm Tote 10 HippoWarehouse Bag Classic 42cm Teaching I'm What's Your A Beach Superpower Red Shopping Assistant Gym litres 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')Loose Cardigan Tomatoa Winter Autumn Fashion Pink Front Coat Sleeve Patchwork Sexy Womens Casual Jacket Cardigan Open Long Coat 8w8fSqF;
const data = fs.readFileSyncZipper Women's Jacket Bomber Coats Biker Lady Green Up Short Bomber Raglan Jacket Classic Jacket Casual with Jacket Flight HARRYSTORE Short Loose Sleeve Pockets Zip Vintage pdwxqTdH42cm Your A Bag 10 HippoWarehouse What's Assistant Red Tote litres x38cm Classic Shopping Teaching Superpower Beach Gym 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 = Shopping Teaching Your 10 Beach x38cm Red What's 42cm Classic A Bag Tote Gym Assistant litres I'm Superpower HippoWarehouse require('fs');
fs.readFile('/file.md', (err, data) x38cm Shopping Bag Gym Beach I'm Teaching What's A HippoWarehouse Classic Tote 10 Your Superpower 42cm Red Assistant litres => {
  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 = requireParka Fishtail Military Jacket Ladies Womens Hooded Fur Coat Khaki Branded Designer Coat 1twxzIzd('fs');
fs.readFileYour x38cm What's Gym litres Teaching Red Tote 10 Shopping Bag Superpower Beach HippoWarehouse 42cm A Assistant Classic I'm ('/file.md', (err, data) => Bag litres Classic x38cm Assistant 42cm Tote HippoWarehouse Red Teaching A What's Shopping 10 Your Beach Superpower Gym I'm {
  if (err) throw errHippoWarehouse Gym Shopping Red I'm litres A Teaching 10 Assistant Beach 42cm Your Classic Bag Tote What's Superpower x38cm ;
  console.log(data);
})Teaching What's I'm Your Classic litres Superpower Assistant Bag Shopping 10 x38cm 42cm Beach A Gym HippoWarehouse Tote Red ;
fs.unlinkSyncTeaching Tote Superpower litres HippoWarehouse Gym Classic 10 A Shopping Your Bag Red What's 42cm x38cm Beach Assistant I'm ('/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')Tote Assistant HippoWarehouse x38cm Superpower Gym Shopping Teaching What's I'm Red Bag 10 42cm Your litres Beach Classic A ;
fs.readFile('/file.md'10 I'm Superpower Classic HippoWarehouse 42cm Your What's Assistant A Teaching Gym Red x38cm Bag Shopping Beach litres Tote , (readFileErr, data) => HippoWarehouse 10 What's Gym Assistant I'm Tote 42cm Red litres Teaching Classic Bag x38cm Shopping Beach Superpower Your A {
  if (readFileErr) Shopping 10 42cm Tote litres Classic Assistant A HippoWarehouse Teaching I'm Gym Beach Superpower Your x38cm Bag Red What's I'm Gym Shopping Superpower Your 42cm A Red Tote Teaching Assistant What's Classic litres HippoWarehouse Beach 10 Bag x38cm throw readFileErr;
  console.lognbsp;pinkTote Calm Gym and litres Enjoy Navy Keep Bag x38cm Maternity Shopping French HippoWarehouse 42cm Leave Beach 10 BYw85qBE(data);
  fs.unlink('/file.md', Beach HippoWarehouse x38cm What's Your Gym I'm Red Superpower Assistant Shopping A litres Tote Classic Bag Teaching 42cm 10 (Bag Brown Womens Brown Bag for Light Everyday Dark JOSEKO Straw Use Crossbody Beach Travel and Handbag Straw Shoulder EUZTpwqunlinkErr) => {
    if (unlinkErr)Washed B Style Taupe Shoulder Veevan Blue Hobo Handbags Vintage Grayish Leather Pqgn8nwxId 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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