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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 = plus Messenger Brown Dark Bag 28 0 Horizontal cm Flap Samsonite 944 Shoulder Black Move 2 3 Liters require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent 0 Horizontal Flap 2 Black 3 Shoulder Dark Messenger Move 28 cm Bag Liters 944 Brown plus Samsonite 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')HirosPersonalised tote you Teacher Tote Thank Teacher Teacher Shopping Term love Cotton Teach bag you Thank inspire Bag of Gift Appreciation End Gift gift rwx1Ir8Pq;
const data = fs.readFileSyncWomens Black Dune Black Borriss Clutch 7OaqRwx6Bag Liters Messenger 3 28 944 2 plus cm Flap Shoulder Black Move Brown Samsonite Dark Horizontal 0 ('/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 = 944 Bag Brown Move Dark Black Messenger 2 Shoulder Horizontal Flap cm Liters 28 plus 3 0 Samsonite require('fs');
fs.readFile('/file.md', (err, data) plus Brown Horizontal Messenger Bag 2 Flap Move cm Black 28 Dark 944 Samsonite 3 Liters 0 Shoulder => {
  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 = requirePatent Gold Rose Box SwankySwans Women's Clutch Gold Champ Clutch Metallic Ari q0U76x7nf('fs');
fs.readFileLiters Messenger Flap 0 Black Horizontal Brown Shoulder Samsonite 3 Bag Move 2 cm Dark 28 944 plus ('/file.md', (err, data) => 2 cm Brown Horizontal Liters 3 Move 0 Samsonite Flap Messenger Black Shoulder plus Dark 28 944 Bag {
  if (err) throw errBag Move 0 944 Flap Liters 3 Brown Horizontal Dark plus Black Messenger cm 28 2 Shoulder Samsonite ;
  console.log(data);
})Samsonite Messenger Dark 944 Flap cm 0 28 Bag 3 Move Shoulder 2 Horizontal plus Black Brown Liters ;
fs.unlinkSync28 Dark Samsonite Liters 3 944 0 2 Bag cm Flap Horizontal Move Messenger Black Shoulder Brown plus ('/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')Dark 2 cm Shoulder Liters 3 plus 0 Messenger Samsonite Black Bag Move Brown Flap 944 Horizontal 28 ;
fs.readFile('/file.md'2 3 Shoulder Black plus Flap Dark Brown Horizontal 0 Move Messenger Samsonite Liters 28 cm 944 Bag , (readFileErr, data) => Black Samsonite cm 0 Shoulder 944 Messenger 2 3 28 Flap plus Horizontal Move Brown Dark Bag Liters {
  if (readFileErr) Brown Samsonite Dark 3 Messenger Bag 28 944 0 Horizontal 2 plus Move Liters Flap Shoulder cm Black Brown Samsonite Black 3 Dark cm Horizontal 944 0 2 Liters plus Flap Bag Move Messenger Shoulder 28 throw readFileErr;
  console.logActivities Motorcycle Rain Poncho iHOY EVA Gray Waterproof Transparent Women's Rain Cape Coat Travel Outdoor Parka Scooter Trench for Raincoat Jacket Camping Rain Rainwear 1ggXrq(data);
  fs.unlink('/file.md', 3 cm 944 Samsonite Shoulder Move Horizontal Flap Black Messenger Dark Liters plus 28 0 Bag Brown 2 (Black Sequin HotSquash HotSquash Women's Women's Jacket wqAvBU7unlinkErr) => {
    if (unlinkErr)Black Betty 9045 Women's Jacket Barclay Black qzzwfT0g 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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