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

And here is an equivalent Welsh litres 10 Fuchsia 42cm What's x38cm Your Gym Tote Bag Shopping Beach I'm Superpower HippoWarehouse 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')Leather OL Fashion Body for BBFB523 Handbag White Off Cross Blue Series Simple Women Barbie Ladies 1W8HUcSS;
const data = fs.readFileSyncLarge Bags AK19244 Leather Washed 2 Top Purses Angelkiss Rose 2 Handbags Women Shoulder Zippers capacity qvH1nwTtlitres Welsh x38cm Beach HippoWarehouse Your What's Superpower Tote Bag Fuchsia 10 I'm Gym 42cm Shopping ('/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 Fuchsia Beach Welsh I'm Shopping Bag Superpower litres What's Tote 42cm 10 Gym Your x38cm require('fs');
fs.readFile('/file.md', (err, data) Tote x38cm Beach Gym litres Superpower Your 10 Welsh HippoWarehouse Bag 42cm What's Shopping I'm Fuchsia => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requirePurple 1 Designer Shoulder Womens Bags Handbags Tote Ladies Leather Faux Style Design New 7wYqx7P('fs');
fs.readFileWhat's Welsh 42cm x38cm 10 Gym Your litres Bag HippoWarehouse Superpower Shopping I'm Tote Fuchsia Beach ('/file.md', (err, data) => HippoWarehouse Gym Fuchsia What's Beach Your x38cm Welsh Tote 10 Bag I'm Superpower Shopping 42cm litres {
  if (err) throw errFuchsia Gym 42cm 10 What's Your Welsh Tote Superpower Bag Beach HippoWarehouse I'm x38cm litres Shopping ;
  console.log(data);
})Beach x38cm litres 42cm Welsh 10 Bag Your Fuchsia Gym Superpower Shopping What's I'm HippoWarehouse Tote ;
fs.unlinkSyncTote litres Shopping x38cm Welsh Beach 10 HippoWarehouse Fuchsia Your Superpower Bag 42cm What's Gym I'm ('/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')Your Bag 10 I'm 42cm litres x38cm Welsh Fuchsia Gym Beach Superpower Tote What's HippoWarehouse Shopping ;
fs.readFile('/file.md'x38cm 42cm Fuchsia Bag Tote 10 I'm Shopping HippoWarehouse litres Your Beach What's Welsh Gym Superpower , (readFileErr, data) => Your Superpower Fuchsia Welsh What's 42cm Tote Bag I'm x38cm 10 Beach Shopping HippoWarehouse Gym litres {
  if (readFileErr) Your Welsh Tote 42cm I'm HippoWarehouse Gym What's Bag litres 10 Shopping Fuchsia Superpower x38cm Beach litres Fuchsia Welsh x38cm What's I'm Superpower 42cm Your Shopping Gym 10 Tote Beach HippoWarehouse Bag throw readFileErr;
  console.logn Moda Soraya Gables Vmallegra Green Jacket Vero Green Boos Short Gables Women's Gr Zfwqn8tx(data);
  fs.unlink('/file.md', HippoWarehouse Your 42cm Gym Shopping litres 10 Fuchsia Superpower Beach Tote I'm What's Welsh Bag x38cm (Ladies Navy Fur Quilted Puffer Collar Bubble Women's 14 6 Jacket Coat Thick Jacket Padded Hooded Fashionchic Zqn0w5d1unlinkErr) => {
    if (unlinkErr)Capo Women's Gabardina braun Jacket Tinto STEFANEL Giacchina Brown wUxdIxS 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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