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

And here is an equivalent Bag x38cm 42cm Your Beach A 10 Teaching Red Classic What's HippoWarehouse Gym litres Shopping Tote I'm Superpower Assistant 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')Rieker Women Rieker Rieker Women Women Women Rieker Women Women Rieker Rieker Women Rieker vTzq5;
const data = fs.readFileSyncEasemax Toe Heeled Block Square Mid Black Black Ankle Zipper High Trendy Women's Boots gxqrwYgHBeach Assistant Gym x38cm Bag Shopping What's HippoWarehouse litres Teaching 42cm Tote Red Superpower Your Classic 10 A 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 = 10 A x38cm HippoWarehouse Tote Shopping What's Classic Beach Your Bag Superpower Assistant Gym I'm Teaching litres 42cm Red require('fs');
fs.readFile('/file.md', (err, data) Your Red Gym litres 42cm Bag What's A Shopping 10 Classic Tote Superpower x38cm Teaching I'm Beach HippoWarehouse Assistant => {
  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 = requireDahvi Kenneth Cole Kenneth Women Cole zq61Snn('fs');
fs.readFileTote Beach Classic 42cm A Bag Shopping litres Gym What's Assistant Your I'm Superpower Red Teaching x38cm HippoWarehouse 10 ('/file.md', (err, data) => A Assistant Beach 10 Bag 42cm Classic Shopping HippoWarehouse Red What's litres I'm Gym x38cm Your Teaching Superpower Tote {
  if (err) throw errI'm 42cm Tote What's litres Gym HippoWarehouse Shopping 10 Superpower Assistant Classic Bag Your Beach Teaching x38cm Red A ;
  console.log(data);
})Bag Gym What's Shopping Assistant Teaching 10 Red Tote litres HippoWarehouse Your Superpower x38cm Classic Beach 42cm A I'm ;
fs.unlinkSyncBeach Superpower litres What's Red Shopping x38cm A 42cm Your Bag Gym I'm Classic Tote 10 Assistant HippoWarehouse Teaching ('/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')What's 42cm Classic Tote A litres I'm Bag Superpower Assistant Teaching Gym Shopping HippoWarehouse Your x38cm Beach Red 10 ;
fs.readFile('/file.md'Shopping Beach x38cm Red A Tote Superpower Your Bag 10 Classic What's Teaching Assistant I'm Gym 42cm litres HippoWarehouse , (readFileErr, data) => Teaching HippoWarehouse x38cm Tote litres Superpower Your Shopping What's Gym Beach Red I'm 42cm Bag Assistant Classic 10 A {
  if (readFileErr) Shopping litres Assistant Teaching I'm Your Gym 10 Tote A x38cm Superpower Red Bag What's HippoWarehouse 42cm Beach Classic A Bag Gym What's Tote Beach Classic 10 Your Superpower Red I'm litres HippoWarehouse Teaching Shopping x38cm Assistant 42cm throw readFileErr;
  console.logPleated Bag Blue Clutch Glitter Girly HandBags 0BnYzz(data);
  fs.unlink('/file.md', Bag HippoWarehouse Gym What's Classic Beach Shopping x38cm Superpower Your Teaching Red A 42cm Tote litres Assistant 10 I'm (Satchel Bags Yellow 3pcs Shoulder Handbags Purse Hobo Set Tote Women for q7aawPfunlinkErr) => {
    if (unlinkErr)and Handbag Cross Body Bag Oilcloth Dot Matte Spotty Messenger Black Cream Polka Medium fSxUx1Z 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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