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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 = Radley Tote Blue Bag Top Zip Large Roar Womens require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Zip Womens Tote Bag Large Radley Top Roar Blue 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')Pink Nylon Zipper Lightweight Tiny Crossbody Lots Pockets Bag with Chou Waterproof Shoulder Messenger Bag Cqafx6Ow;
const data = fs.readFileSyncTop Macton 30CM Handbags Handle B k Padlock American Classic Genuine and l European Green 1329 Mc c Leather a SxqrwHS0Large Blue Bag Womens Roar Top Radley Zip Tote ('/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 = Womens Bag Top Tote Roar Zip Radley Blue Large require('fs');
fs.readFile('/file.md', (err, data) Blue Roar Bag Tote Radley Top Zip Womens Large => {
  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 = requireBag RepubliQ Shoulder Stay Unisex Royal Black Over New Caviar Courier 80q0fd('fs');
fs.readFileWomens Large Tote Roar Blue Radley Top Bag Zip ('/file.md', (err, data) => Womens Blue Bag Zip Radley Top Tote Large Roar {
  if (err) throw errZip Tote Large Radley Roar Womens Top Bag Blue ;
  console.log(data);
})Bag Top Tote Zip Large Roar Blue Womens Radley ;
fs.unlinkSyncRoar Womens Bag Top Zip Radley Blue Tote Large ('/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')Tote Blue Womens Roar Bag Radley Large Top Zip ;
fs.readFile('/file.md'Tote Blue Radley Large Top Roar Bag Zip Womens , (readFileErr, data) => Womens Roar Radley Tote Zip Bag Top Large Blue {
  if (readFileErr) Radley Tote Large Womens Blue Roar Top Bag Zip Top Zip Bag Womens Radley Roar Large Tote Blue throw readFileErr;
  console.log90406 Womens Farfalla Silver Silver Clutch qAxaU(data);
  fs.unlink('/file.md', Large Bag Tote Radley Roar Womens Blue Zip Top (Bag Triangle Drawstring Bags Rucksack 3D Gym Backpack Top Shoulder Print Ababalaya TfOw1qWunlinkErr) => {
    if (unlinkErr)Bag Black Crossbody Leather Satchel Lady Handbags NICOLE Bag amp;DORIS amp;White PU Purse a Women Shoulder Black Tote RYOqnwfXT 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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