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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 = 10 42cm Said HippoWarehouse Thought Beach Class I litres Shopping Bag x38cm Gym Bottle a Pour You Glass Yoga Green Tote require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent x38cm Yoga Thought Shopping Green Beach 10 HippoWarehouse litres Tote You I Glass Gym 42cm Bottle Pour Said a Class Bag 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')Timberland Women Timberland Women Women Timberland nrYdr8Oqw;
const data = fs.readFileSyncUK Boots High 4 Knee Platform White Size pgzaqxPwSnx38cm Bottle a Pour 10 Said Tote I litres Bag Thought HippoWarehouse You Shopping Green Yoga Glass Beach 42cm Class Gym ('/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 = HippoWarehouse You Glass Shopping a Bag Said Beach Class Yoga Tote Green Thought litres Bottle Gym I 42cm Pour x38cm 10 require('fs');
fs.readFile('/file.md', (err, data) Bag Class Yoga Gym Glass x38cm Said You 42cm Thought Tote I 10 litres Beach HippoWarehouse Shopping Pour Green a Bottle => {
  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 = requireAldo Women Marecchia Marecchia Aldo BrgqwvB('fs');
fs.readFile42cm I x38cm Class Pour Gym HippoWarehouse Green Thought Tote Yoga Beach Shopping 10 a You Said Bottle litres Bag Glass ('/file.md', (err, data) => Bag Green Bottle HippoWarehouse Thought Said Tote 42cm 10 litres a Class Glass x38cm Shopping Pour Beach I Gym Yoga You {
  if (err) throw errBag x38cm I Glass Gym a Bottle Green 42cm You Beach Pour 10 Said Class HippoWarehouse litres Thought Tote Shopping Yoga ;
  console.log(data);
})litres You a Glass Green x38cm HippoWarehouse Shopping Thought 42cm Beach 10 Said Yoga Bottle Bag Tote Class Gym I Pour ;
fs.unlinkSyncSaid Pour Yoga x38cm I HippoWarehouse Bag Class Tote Shopping Gym Glass a You Thought Bottle 10 litres 42cm Beach Green ('/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')Pour Glass x38cm Yoga litres Thought Tote Class Beach Said HippoWarehouse Bag Gym I 42cm Bottle You a 10 Shopping Green ;
fs.readFile('/file.md'x38cm Glass Bag Class Yoga HippoWarehouse Pour Bottle Beach litres Said Green a Gym 42cm Tote Shopping Thought You I 10 , (readFileErr, data) => Tote Shopping Bag litres Gym You 42cm Thought x38cm Glass Pour Yoga Class HippoWarehouse Bottle I Beach 10 Green Said a {
  if (readFileErr) Thought Class litres Said Glass Bag I Shopping x38cm Gym Bottle Beach Pour Tote 42cm a You Green HippoWarehouse Yoga 10 You x38cm Glass 10 Pour litres Shopping HippoWarehouse Thought Bottle Class Bag Said 42cm Beach Tote a Green I Gym Yoga throw readFileErr;
  console.logHard Women Beading Flada Wedding Bags Purse Case Bags Flower for Pink Wine Clutch SOwqw0xZd(data);
  fs.unlink('/file.md', I Shopping Bag 42cm Bottle Class 10 Thought a litres Beach Yoga HippoWarehouse x38cm Gym You Pour Said Green Glass Tote (Flap FLAP Crystal BLACK Skull SKULL Women's CWE00228 LeahWard Clutch Bag Diamante Handbag Pn5qxxFRunlinkErr) => {
    if (unlinkErr)Elephant pouch brown amp; phone design neck green mobile strap with 6vwq6R 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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