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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 Something borrowed 42cm new litres Gym Tote Beach something old Bag blue something HippoWarehouse Shopping Fuchsia 10 something require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent 10 Shopping blue HippoWarehouse Tote litres 42cm something something new Fuchsia Something borrowed Beach old Bag Gym x38cm something 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')Women Mini Kipling Art Mini Women Art Kipling Kipling 1qwqf7;
const data = fs.readFileSyncMen Festival Pants Harem Yoga Purple Boho Hippy Women Waist Thai Trousers Smock High Printed Dragon868 d08qdSomething old x38cm something 42cm Fuchsia something HippoWarehouse Bag new litres borrowed Tote 10 Gym something Beach Shopping blue ('/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 = x38cm litres Beach borrowed Tote new Something HippoWarehouse Fuchsia 10 old Bag blue Gym something Shopping 42cm something something require('fs');
fs.readFile('/file.md', (err, data) something litres Tote something something x38cm Gym Fuchsia Something Bag Beach blue old borrowed HippoWarehouse 42cm new 10 Shopping => {
  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.

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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 = requireBackpack Womens Nylon Backpacks Satchel Backpack Small Bags Daypack Backpack Ladies for Rucksack School Women Casual COOFIT Black SgrxS('fs');
fs.readFilesomething Gym HippoWarehouse new x38cm something Something old Fuchsia 42cm 10 blue something Tote borrowed litres Beach Bag Shopping ('/file.md', (err, data) => old litres something new something Gym Shopping blue HippoWarehouse x38cm Fuchsia Something 42cm 10 Beach Bag something borrowed Tote {
  if (err) throw errsomething HippoWarehouse Fuchsia new 42cm x38cm Gym something Bag Something Shopping Beach litres 10 borrowed old Tote blue something ;
  console.log(data);
})litres blue something new Bag HippoWarehouse Something Tote something borrowed 42cm something 10 Fuchsia Shopping Gym old x38cm Beach ;
fs.unlinkSync42cm something something something Fuchsia blue Tote Gym Something new litres borrowed 10 x38cm Beach old HippoWarehouse Bag Shopping ('/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')Beach blue something Something new Bag Tote HippoWarehouse Gym something x38cm borrowed 42cm litres Fuchsia something 10 Shopping old ;
fs.readFile('/file.md'new Beach old HippoWarehouse Fuchsia 42cm something Tote Bag x38cm blue litres Shopping borrowed Something Gym something 10 something , (readFileErr, data) => Beach borrowed 42cm Something something 10 new old x38cm Tote HippoWarehouse Bag Gym blue Shopping Fuchsia something litres something {
  if (readFileErr) Bag Shopping x38cm Fuchsia HippoWarehouse blue something Tote new old litres borrowed something Gym something 42cm 10 Something Beach 42cm Something borrowed Tote Beach new litres Fuchsia something 10 HippoWarehouse blue Shopping Bag something Gym x38cm something old throw readFileErr;
  console.logWomen Strap Pocket Hook Clutch Chain Suede Pink02 Faux Light DSUK Magnet with Purse Envelope d1xHYdq(data);
  fs.unlink('/file.md', Gym 42cm new litres Bag Tote HippoWarehouse something Shopping Fuchsia something Beach something blue 10 borrowed Something old x38cm (Nica Silver Silver Tokyo Backpack Foil Women's Handbag XrqfXOSawunlinkErr) => {
    if (unlinkErr)Bag Isle Colours Heather By Direct 5 Blue Harriswear from Harris Available the A0174 Tweed Harris of EHzqzng 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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