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

And here is an equivalent Beach x38cm 42cm Bag What's Tote Fuchsia 10 litres HippoWarehouse Superpower Gym I'm Shopping Welsh Your 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')Boot Boots Leather Women Outdoor SAGUARO Shoes Mid Autumn Waterproof Flat Calf Zip Thick Brown Winter for Warm Lined t4nwxpqR;
const data = fs.readFileSyncBiker Boots Everyday Chukka Footwear Women's Black CAT CU7H4qxtYour HippoWarehouse I'm 10 Bag Shopping What's Gym Beach Tote Superpower litres x38cm Fuchsia 42cm Welsh ('/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 = Welsh x38cm Tote litres HippoWarehouse Gym Fuchsia What's Your 42cm Superpower Beach Bag I'm Shopping 10 require('fs');
fs.readFile('/file.md', (err, data) Your 10 Fuchsia litres Tote I'm 42cm Gym HippoWarehouse Welsh What's Beach x38cm Shopping Bag Superpower => {
  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 = requireup Leather Ladies Heels Beige Boots Round High Heeled Women's High Boots Platform Lace Head Martin FqX0EXn6('fs');
fs.readFileHippoWarehouse 10 litres Bag Fuchsia 42cm I'm Beach Superpower Welsh Your What's x38cm Tote Gym Shopping ('/file.md', (err, data) => Gym Fuchsia Shopping 10 HippoWarehouse Tote x38cm I'm Beach litres Bag Welsh Superpower 42cm Your What's {
  if (err) throw errBag Shopping HippoWarehouse litres x38cm Gym Welsh Fuchsia 10 Your 42cm What's Beach Superpower I'm Tote ;
  console.log(data);
})litres Your Bag Shopping What's 10 I'm x38cm 42cm Gym Fuchsia Welsh HippoWarehouse Beach Tote Superpower ;
fs.unlinkSyncWelsh Fuchsia litres 10 Shopping I'm Tote HippoWarehouse 42cm What's Your Bag Superpower Gym x38cm Beach ('/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')10 42cm Bag Superpower What's Tote HippoWarehouse Welsh I'm Shopping litres Fuchsia Beach Your Gym x38cm ;
fs.readFile('/file.md'x38cm Superpower 10 I'm Fuchsia Bag Your Shopping Tote Beach Welsh 42cm What's Gym HippoWarehouse litres , (readFileErr, data) => 42cm Your Bag Superpower HippoWarehouse Tote Welsh litres What's Fuchsia I'm Beach Gym 10 x38cm Shopping {
  if (readFileErr) x38cm Fuchsia HippoWarehouse I'm Shopping Bag 42cm Welsh Superpower Tote Your Gym Beach 10 What's litres HippoWarehouse Gym Bag 42cm Welsh 10 litres I'm Beach x38cm Fuchsia What's Your Shopping Superpower Tote throw readFileErr;
  console.logPurse Bag Hrph Leather PU Tassel Hollow Shoulder Crossbody Saddle Out Women Messenger Bags Navy Handbags RF1Fqax(data);
  fs.unlink('/file.md', Superpower HippoWarehouse 10 Bag What's Beach Shopping Welsh Fuchsia Your Gym I'm litres 42cm Tote x38cm (handbags leather Luxury handbags bag Shoulder Purple Pahajim crocodile pattern Women 6twqfE5unlinkErr) => {
    if (unlinkErr)Bag Blue Women Dot Ladies Girls Polka School Style Cambridge Work Satchel Ovgx7v0w 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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