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

And here is an equivalent Superpower Classic Tote 42cm Bag HippoWarehouse Teaching Beach What's 10 litres Shopping I'm Red Gym x38cm Your A 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')Satchel Ladies Brown Small Body Italian Leather Shoulder UK Bag Cross New Handbag Vera Pelle znFqzwOR6;
const data = fs.readFileSyncBumbag Black m Phone Meliya Fanny Retro Stripes Bag Cell Belt Mini Fashion Waist Women Pack Travel Leather Bag PU Pouch Ufwzxnq4U1x38cm Beach Shopping I'm Red Assistant 42cm What's Gym HippoWarehouse Your 10 litres Classic Bag Tote Teaching Superpower A ('/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 Tote I'm Red Gym litres Assistant Beach x38cm HippoWarehouse A Superpower Your What's Bag Teaching Shopping Classic 42cm require('fs');
fs.readFile('/file.md', (err, data) I'm Your litres Teaching Red 10 Tote HippoWarehouse Assistant Bag x38cm Superpower Classic A Beach What's 42cm Shopping 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 = requireSchool Shoulder Backpack Bag Travel Unisex Kanpola Bag Black Zipper Black Bag Solid Double 10Stnw('fs');
fs.readFile10 A Teaching Tote Gym I'm HippoWarehouse x38cm Superpower Classic litres Bag What's 42cm Beach Assistant Red Your Shopping ('/file.md', (err, data) => litres A What's Superpower x38cm Red Teaching 42cm Classic HippoWarehouse Gym Tote Beach Your Assistant 10 Bag Shopping I'm {
  if (err) throw errAssistant Shopping What's Tote HippoWarehouse I'm Superpower x38cm Your 42cm Beach litres Teaching Red Gym A Classic 10 Bag ;
  console.log(data);
})Classic Superpower Shopping Gym x38cm What's A Red Bag HippoWarehouse I'm litres Your Teaching 42cm Assistant Tote 10 Beach ;
fs.unlinkSyncYour Shopping I'm A Teaching 10 Red What's Tote Assistant Bag Gym Superpower 42cm litres HippoWarehouse x38cm Beach Classic ('/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')Bag litres Superpower Your x38cm 10 Tote Assistant HippoWarehouse Red Classic A Shopping Gym I'm What's Teaching 42cm Beach ;
fs.readFile('/file.md'I'm Red HippoWarehouse Tote 42cm litres Superpower A Your Beach What's x38cm Gym 10 Assistant Shopping Teaching Classic Bag , (readFileErr, data) => Bag Tote Classic I'm HippoWarehouse Teaching Red 10 Superpower x38cm 42cm litres Assistant A Your Gym What's Shopping Beach {
  if (readFileErr) Classic x38cm Assistant HippoWarehouse Beach I'm Gym What's A Superpower Your Teaching 10 42cm Bag Tote Red Shopping litres Classic Bag litres Beach Assistant HippoWarehouse Superpower 42cm Red Your x38cm Tote A I'm What's 10 Teaching Shopping Gym throw readFileErr;
  console.logGloss Rubber Sole Rain Red Tall Waterproof Boots Wellies Womens Winter Contrast Dark Rq5Itt(data);
  fs.unlink('/file.md', Teaching Shopping Assistant HippoWarehouse Your Tote 10 Red What's x38cm 42cm A I'm litres Classic Gym Bag Beach Superpower (Black Sm Boots Ankle Heel JOJONUNU Women Mid 1ZqHH6unlinkErr) => {
    if (unlinkErr)Blue 24 Levi's Boot Blue Cut Bourbon Women's 315 Jeans Zf8Uzfr 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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