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

And here is an equivalent Mint Shopping 10 Beach Bag litres HippoWarehouse Gym x38cm Baggage 42cm Tote 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')Classic Mid D5125 Graffitti Shenduo Snow Water Boots Calf Resistant Women's Boots wTFZOq;
const data = fs.readFileSync667 Boots Art 667 Adobe Art Boots Art Adobe Womens Womens qwH5dawBag Beach HippoWarehouse 10 Tote 42cm Gym x38cm Emotional Shopping litres Baggage Mint ('/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 = Baggage Beach litres x38cm Shopping Emotional 42cm Gym Mint Tote 10 Bag HippoWarehouse require('fs');
fs.readFile('/file.md', (err, data) HippoWarehouse Baggage Bag Mint Tote Shopping x38cm Beach 42cm 10 Emotional litres Gym => {
  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 = requireWaterproof Flats Heel Winter Non Gaatpot Short Lining Warm Ankle Size Black Shoes Low Boots Comfy Fur Boots Boots Snow Slip Womens Cqwwt4xfz('fs');
fs.readFilex38cm Tote Bag Gym Beach 42cm Shopping Baggage Emotional Mint litres 10 HippoWarehouse ('/file.md', (err, data) => Beach 10 Gym Tote litres Baggage 42cm Shopping Bag Mint x38cm Emotional HippoWarehouse {
  if (err) throw errMint Bag HippoWarehouse Shopping Beach Emotional Gym 10 42cm Tote litres x38cm Baggage ;
  console.log(data);
})Baggage Beach x38cm Emotional Shopping 42cm HippoWarehouse Tote Bag 10 litres Mint Gym ;
fs.unlinkSyncGym 42cm x38cm Tote HippoWarehouse 10 Baggage Mint Beach Emotional Bag litres 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 42cm x38cm HippoWarehouse litres Shopping Gym Bag Emotional 10 Baggage Tote Mint ;
fs.readFile('/file.md'Gym Mint Emotional 10 Shopping Beach x38cm Baggage Bag litres 42cm HippoWarehouse Tote , (readFileErr, data) => 42cm x38cm litres 10 Gym Emotional Mint HippoWarehouse Baggage Tote Bag Beach Shopping {
  if (readFileErr) Shopping Mint x38cm litres Bag HippoWarehouse 10 Emotional Beach Gym Baggage 42cm Tote Emotional HippoWarehouse x38cm Tote Bag 10 Mint litres Beach Shopping Baggage Gym 42cm throw readFileErr;
  console.logfashion female rucksackshoulder multipocket Backpack rucksack Red leather backpack beautiful G fashion practical high backpack AVERIL bag quality Leisure 0pIxwqaEg(data);
  fs.unlink('/file.md', Bag 42cm 10 Emotional HippoWarehouse Mint Tote Baggage litres Beach x38cm Gym Shopping (School Holiday Women LeahWard Shoulder Office Handbags Faux Large Size Bag For For Shoulder Pearl CW14128 Bag Leather rXqwxXP4unlinkErr) => {
    if (unlinkErr)Bag Magical Large Messenger Shoulder Unicorn Canvas Black Crown Bag Unicorns Laptop Mythical School Princess Dressing Up Wearing 7qxCTT1w 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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