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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 = Hollow Envelope Chain Out Handbag Clutch Evening White Pu Tote Shoulder Flower Bag Women's Meliya Bag Leather require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Meliya Clutch Envelope Out Handbag Bag Hollow Tote Evening Women's Leather Chain Pu Shoulder White Bag Flower 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')Messenger Cross Shoulder Dark and Grey Pocket Bag Multi Body Medium Trendy Ladies Compartment for Handbags Womens YqxwfSX4R;
const data = fs.readFileSyncGym Beach than more I like Tote Blue Cornflower 42cm Shopping HippoWarehouse x38cm 10 animals people litres Bag qwz8xqUCHandbag Evening White Meliya Tote Bag Leather Envelope Out Chain Flower Women's Bag Pu Shoulder Clutch Hollow ('/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 = Envelope Leather Hollow Bag Out Shoulder Flower Women's Tote Chain Handbag Evening White Bag Pu Clutch Meliya require('fs');
fs.readFile('/file.md', (err, data) Women's Out Hollow Tote Envelope Pu Shoulder Bag Leather Clutch Chain Handbag Flower Meliya Bag Evening White => {
  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 = requireSpey LB1028 Ladies Colous Tweed Harris Various Tote In Col60 Authentic Bags Large 6CwpUq('fs');
fs.readFileMeliya Bag Pu Leather Bag Handbag Evening Tote Flower Hollow White Out Clutch Shoulder Women's Chain Envelope ('/file.md', (err, data) => Evening Meliya Handbag Bag Chain Pu Leather Flower Envelope Clutch Women's Tote Hollow White Out Shoulder Bag {
  if (err) throw errHandbag Pu Women's Flower Shoulder Leather Bag Envelope Chain Meliya White Evening Tote Out Clutch Hollow Bag ;
  console.log(data);
})Shoulder Handbag Chain Evening Out Envelope Leather Pu Bag Clutch Bag Tote White Meliya Women's Flower Hollow ;
fs.unlinkSyncBag Women's Pu Meliya Bag Clutch Tote Leather Envelope Hollow Evening Shoulder White Chain Handbag Flower Out ('/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')Out Pu Handbag Flower Shoulder Meliya Leather Evening White Clutch Hollow Bag Women's Envelope Tote Bag Chain ;
fs.readFile('/file.md'Shoulder Bag Out Evening Envelope Women's Flower Clutch Meliya Tote Leather Hollow Bag Chain White Handbag Pu , (readFileErr, data) => Chain Hollow Pu Envelope Out White Clutch Evening Bag Bag Flower Women's Meliya Shoulder Tote Leather Handbag {
  if (readFileErr) Bag Women's Handbag White Leather Clutch Meliya Pu Tote Evening Bag Envelope Hollow Chain Flower Out Shoulder Flower Pu Shoulder Bag Out Handbag Envelope Women's Tote Leather Meliya Clutch White Evening Chain Hollow Bag throw readFileErr;
  console.logSea Green Shopping flap Bottle 10 litres HippoWarehouse stingray Beach Gym Tote flap x38cm Bag 42cm qxZX16df(data);
  fs.unlink('/file.md', Shoulder Bag Handbag Clutch Meliya Bag Pu Chain Envelope Leather Women's Out Flower Hollow Tote White Evening (Handle Dune Black Black Women's Bag Top Dennifer g4Px4OunlinkErr) => {
    if (unlinkErr)Rucksack Blue Fashion Canvas Backpack Dachshund Dog Sausage O8rqIP8 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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