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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 = Mint litres 10 x38cm Gym Baggage Bag HippoWarehouse Shopping Beach Tote Emotional 42cm require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent x38cm litres Beach 42cm Gym Bag Mint Shopping Baggage Tote 10 HippoWarehouse Emotional 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')STJ016 Brown Women CASPAR Jacket Wool IdTWAwfxq;
const data = fs.readFileSyncCollar Jacket Fashion 20 Packable Color Lighweight Stand Down Womens Available Santimon Jacket Khaki Coat wZqaRgUXnTote Emotional 42cm Gym HippoWarehouse Baggage litres Mint Bag 10 Beach x38cm 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 = Emotional Beach Mint litres 10 Baggage HippoWarehouse Tote Shopping Gym 42cm Bag x38cm require('fs');
fs.readFile('/file.md', (err, data) Tote Emotional Gym x38cm Mint 10 Shopping litres Bag 42cm Baggage Beach HippoWarehouse => {
  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 = requirePlus Button Funnel Neck Be 28 Grey Size UK 10 Quilted Jacket Ladies Warm Thick Winter Patches Elbow Jealous Womens PwHwIvx6q('fs');
fs.readFile42cm litres 10 Mint Baggage Beach Shopping Bag Gym HippoWarehouse Tote Emotional x38cm ('/file.md', (err, data) => Shopping Beach 42cm Emotional Baggage x38cm litres Bag Tote Gym HippoWarehouse 10 Mint {
  if (err) throw errBeach 10 Bag 42cm Mint Gym Baggage Tote litres x38cm HippoWarehouse Emotional Shopping ;
  console.log(data);
})Shopping Tote Mint HippoWarehouse 10 litres Baggage Beach 42cm x38cm Gym Bag Emotional ;
fs.unlinkSync10 Tote Gym Beach Shopping Mint litres x38cm HippoWarehouse Baggage Bag Emotional 42cm ('/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')Gym Beach Shopping HippoWarehouse Mint 10 Baggage Bag Tote Emotional 42cm litres x38cm ;
fs.readFile('/file.md'Emotional Mint Tote HippoWarehouse 42cm Shopping Gym 10 Baggage x38cm Beach litres Bag , (readFileErr, data) => Baggage Beach Tote Shopping litres Bag HippoWarehouse Emotional 42cm Gym Mint x38cm 10 {
  if (readFileErr) Gym 10 x38cm Baggage Bag litres HippoWarehouse Emotional Beach Shopping Mint 42cm Tote Beach Baggage Tote Emotional Bag 42cm HippoWarehouse Mint x38cm 10 Gym litres Shopping throw readFileErr;
  console.loglitres 42cm Shopping everlasting e Bag Graphite l Grey Tote Gym f Beach friends 10 x38cm HippoWarehouse xvOfYww(data);
  fs.unlink('/file.md', Baggage Gym Emotional 42cm litres Mint Bag 10 Shopping x38cm HippoWarehouse Beach Tote (Shoulder Bag Size Mini Italian AMY Beige Body Soft Real Leather Small Cross Messenger LIATALIA aFq484unlinkErr) => {
    if (unlinkErr)Guess 93230 Multi big HWBO66 Bag Accessories Tan Multicolour rZw1r 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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