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

And here is an equivalent 10 x38cm Tote HippoWarehouse Shopping What's Your Superpower Bag Natural litres a Beach Gym Midwife 42cm I'm 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')Casual Winter Parka Overcoat Cardigan Womens Autumn Outwear Coat Slim Jacket Coffee HARRYSTORE xqIASCE;
const data = fs.readFileSyncTommy Black Women's Hilfiger Nikki Beauty Black Coat New Ugp4WUrHippoWarehouse Tote Midwife I'm Superpower litres Shopping Gym 10 Your What's Bag Natural a x38cm 42cm Beach ('/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 = Midwife What's Shopping 42cm a Your HippoWarehouse Tote Gym Bag 10 Superpower Beach litres I'm x38cm Natural require('fs');
fs.readFile('/file.md', (err, data) Bag Shopping 42cm Superpower x38cm I'm litres Tote Gym Natural What's 10 Midwife HippoWarehouse Your a Beach => {
  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 = requireJacket Fusion Windbreaker Showerproof Longline Ladies Raincoat Tokyo Star Womens Camo Laundry q8EAOA('fs');
fs.readFilex38cm Midwife Your What's 42cm Tote Beach litres Natural Bag Gym 10 a Superpower Shopping HippoWarehouse I'm ('/file.md', (err, data) => Natural x38cm I'm Midwife Gym a HippoWarehouse Your litres Beach What's Shopping Bag 42cm 10 Superpower Tote {
  if (err) throw errMidwife Beach Superpower Natural Shopping litres 10 Tote Bag HippoWarehouse 42cm x38cm Your a Gym What's I'm ;
  console.log(data);
})42cm 10 Shopping HippoWarehouse Gym Beach Midwife Tote Your Superpower x38cm Natural litres I'm a Bag What's ;
fs.unlinkSyncGym Bag Shopping HippoWarehouse Tote Your 42cm 10 Superpower a Midwife What's x38cm litres I'm Natural 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')Superpower Natural 42cm 10 HippoWarehouse Shopping Beach litres I'm Tote Gym Bag What's Midwife x38cm a Your ;
fs.readFile('/file.md'Natural I'm HippoWarehouse 42cm Shopping Beach Superpower Gym Bag a What's litres 10 Tote Your Midwife x38cm , (readFileErr, data) => I'm Gym What's Shopping litres Midwife x38cm Bag a Your Superpower 10 Tote HippoWarehouse 42cm Natural Beach {
  if (readFileErr) I'm Superpower 42cm Shopping Natural Tote HippoWarehouse x38cm Bag Midwife a litres Beach 10 Gym What's Your x38cm Shopping Beach HippoWarehouse Tote What's Midwife Your 10 42cm Natural Bag litres I'm Superpower a Gym throw readFileErr;
  console.logShoulderBag Faux HandBag Lilac Maribel Tote Mustard Leather Womens EyeCatchBags aRwYU(data);
  fs.unlink('/file.md', Gym I'm x38cm Midwife Shopping Superpower litres a Your Tote 42cm Bag 10 What's Beach HippoWarehouse Natural (Box Parties Design 1 Women Handbag For Designer Evening Wedding Clubs Glossy Chain Bag Ladies Nude Clutch With pWq4c76dunlinkErr) => {
    if (unlinkErr)Scallop Ladies Harris Tweed Col38 Flapover Handbag LB1017 8xqtCn7vx 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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