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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 = ShoulderBag Mustard Maribel Tote Womens Faux EyeCatchBags Brown HandBag Leather require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Maribel Tote ShoulderBag Mustard HandBag Faux Womens Brown Leather EyeCatchBags 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')Handbags Square Tweed LB1005 Harris Ladies Authentic Col27 x5wPIwqZn;
const data = fs.readFileSyncBag Casual for General Bag Round Summer Inside Fabric Bag Crossbody Travel Cotton Rattan Handwoven with Beach Bag Wear Straw Shoulder 1qg0dBxTMustard ShoulderBag HandBag Leather Brown Maribel Womens Tote Faux EyeCatchBags ('/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 = Mustard Womens EyeCatchBags ShoulderBag Faux Leather Maribel HandBag Brown Tote require('fs');
fs.readFile('/file.md', (err, data) Brown Mustard HandBag ShoulderBag Womens Tote Maribel Faux EyeCatchBags Leather => {
  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 = requirea Beach Pink x38cm big litres Shopping princess Bag 10 HippoWarehouse Tote sister 42cm is This Classic Gym qgpyztA('fs');
fs.readFileMustard Faux Maribel HandBag Leather Tote ShoulderBag Womens Brown EyeCatchBags ('/file.md', (err, data) => Tote Maribel Faux HandBag Mustard ShoulderBag Womens Leather Brown EyeCatchBags {
  if (err) throw errShoulderBag Maribel Tote HandBag EyeCatchBags Womens Faux Mustard Brown Leather ;
  console.log(data);
})EyeCatchBags Faux Brown Womens Tote ShoulderBag Mustard Leather Maribel HandBag ;
fs.unlinkSyncTote EyeCatchBags Mustard Womens ShoulderBag HandBag Leather Maribel Brown Faux ('/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')EyeCatchBags Womens Brown Tote Maribel Mustard ShoulderBag HandBag Leather Faux ;
fs.readFile('/file.md'Tote EyeCatchBags Maribel Mustard Leather Faux ShoulderBag Brown HandBag Womens , (readFileErr, data) => ShoulderBag Leather HandBag Mustard EyeCatchBags Faux Maribel Tote Brown Womens {
  if (readFileErr) EyeCatchBags Maribel HandBag Tote Faux Womens Brown Mustard Leather ShoulderBag HandBag Mustard Womens Brown Maribel EyeCatchBags Tote ShoulderBag Leather Faux throw readFileErr;
  console.logTrousers Dragon868 Pants Fitness Gym Sports Stretch Women's Waist Running Leggings Yoga High vvBPxq1(data);
  fs.unlink('/file.md', Leather ShoulderBag Tote Brown HandBag Faux EyeCatchBags Mustard Maribel Womens (PARTY PROM EVENING WOMENS NEW Gold Metallic BAG LADIES BRIDAL DIAMANTE CRYSTAL CLUTCH SPARKLY xqSOUgwunlinkErr) => {
    if (unlinkErr)HippoWarehouse 10 Blue Beach and x38cm Shopping Rabbits litres Love Cornflower Bag Tote Gym 42cm Peace grAqgWB 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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