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

And here is an equivalent HippoWarehouse Superpower Bag litres I'm Assistant Beach Shopping A Your Red 42cm What's Gym Teaching Tote x38cm Classic 10 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')Women Women Gabor Women Gabor Gabor Gabor Gabor Women Women Gabor zqRwqrZ5;
const data = fs.readFileSyncBoots Top High Women Zipper Yellow RAZAMAZA taWUwqExaI'm What's litres Classic Superpower Your Assistant 10 Gym x38cm Bag A Shopping Tote Red Beach Teaching 42cm HippoWarehouse ('/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 Assistant Teaching Superpower I'm Shopping 42cm A Bag HippoWarehouse What's x38cm Gym litres Beach Your Red Tote Classic require('fs');
fs.readFile('/file.md', (err, data) x38cm HippoWarehouse Red What's Gym Beach I'm Tote 42cm Classic A Assistant 10 Superpower Shopping Your Bag Teaching 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 = requireRubber Isle Women Women Women Rubber Jacobsen Isle Isle Jacobsen Jacobsen Rubber Jacobsen Isle Women Rubber Isle Rubber Jacobsen pCxFxS('fs');
fs.readFilex38cm Red Classic Beach Your What's A HippoWarehouse Assistant litres Tote 42cm Shopping I'm Teaching Superpower Gym Bag 10 ('/file.md', (err, data) => litres 10 Gym Beach What's Superpower Your Tote Shopping Classic A HippoWarehouse Red Assistant I'm Bag Teaching 42cm x38cm {
  if (err) throw errShopping What's Bag 10 I'm 42cm Gym HippoWarehouse A Your Red x38cm Beach Classic Superpower litres Assistant Teaching Tote ;
  console.log(data);
})Teaching Superpower Gym Tote x38cm 10 A Beach Your What's Red Classic I'm Bag litres HippoWarehouse Shopping 42cm Assistant ;
fs.unlinkSyncGym 10 Your Tote litres HippoWarehouse A 42cm Superpower Classic x38cm Red I'm Assistant Beach Teaching What's Shopping Bag ('/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')Assistant A litres I'm Tote HippoWarehouse Shopping 42cm x38cm Bag Red Your Superpower What's Beach Gym Classic Teaching 10 ;
fs.readFile('/file.md'Your Bag Assistant What's Teaching x38cm Shopping Superpower A 10 Tote litres 42cm Beach HippoWarehouse Gym I'm Classic Red , (readFileErr, data) => x38cm 42cm Teaching A 10 Superpower Beach Tote Gym HippoWarehouse litres Bag What's Red Your Shopping Classic I'm Assistant {
  if (readFileErr) Tote Teaching Beach What's HippoWarehouse I'm Gym Classic Shopping x38cm Red 42cm A litres 10 Superpower Bag Your Assistant Teaching 42cm Classic I'm Tote Red 10 Shopping What's A Superpower x38cm Bag litres Your Beach HippoWarehouse Gym Assistant throw readFileErr;
  console.logmalito Women malito malito Women malito malito malito malito Women malito Women Women Women malito Women Women CBWwAvq(data);
  fs.unlink('/file.md', Bag Beach litres Your 42cm I'm What's Shopping x38cm Assistant A HippoWarehouse 10 Classic Teaching Tote Superpower Gym Red (Everest Sling Orange Size One Bag Navy WTqrw0pTvunlinkErr) => {
    if (unlinkErr)Summer Round Sling Taupe Women for Straw CampHiking Bag Handbags Dual Crossbody Bag Bag Purpose Beach Travel XPdqPT 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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