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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 = Women Style Lady Girl Black Laptop Backpack Female Canvas Student Bag Preppy School School require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Style Bag Student School Lady Canvas School Black Female Preppy Girl Laptop Backpack Women 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')PROM BAGS WKDS SIDE CROSS Army LEATHER REAL PARTY Green SUEDE BAGS WOMEN SHOULDER CLUTCH BODY LADIES zqOfr86zP;
const data = fs.readFileSyncSuicide blue bag Squad Quinn pink Cross Harley Shoulder xrxq7w6School Style Lady Preppy Women Student Canvas Female Laptop Backpack Bag Black Girl School ('/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 = School Girl Student Laptop Preppy Canvas Lady Style Black Backpack Female School Bag Women require('fs');
fs.readFile('/file.md', (err, data) Bag Female Style School Laptop Backpack Student Preppy Girl Lady Black School Canvas Women => {
  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 = requirenot evan I 10 litres Gym u would Bag ur Yellow 42cm date peters but Shopping Beach HippoWarehouse x38cm Tote HwYx0qY('fs');
fs.readFileGirl Student Laptop Female Backpack Preppy Women School School Black Style Canvas Bag Lady ('/file.md', (err, data) => Women Lady Laptop Backpack Female Student Black Bag Preppy School Canvas School Girl Style {
  if (err) throw errGirl School Women Laptop Style Canvas School Student Preppy Lady Black Backpack Bag Female ;
  console.log(data);
})Lady School Canvas Preppy Style Backpack Bag Female Laptop School Girl Student Women Black ;
fs.unlinkSyncWomen Canvas Laptop Lady Style Black Girl Backpack Female School School Bag Student Preppy ('/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')Student Backpack Preppy Canvas Bag Lady Girl Female School Laptop Black School Women Style ;
fs.readFile('/file.md'Laptop Black Canvas School Girl Student Lady Preppy School Backpack Style Women Female Bag , (readFileErr, data) => Female Lady Laptop Women Style Black Girl Bag Preppy School School Student Backpack Canvas {
  if (readFileErr) Style Bag School Laptop Preppy Women Canvas Female Student Lady Girl School Black Backpack Lady Laptop Women School Black Bag Backpack School Female Preppy Canvas Student Style Girl throw readFileErr;
  console.logBag Travel for Crossbody Multi Waterproof Purse Pocket Purple Handbags Nylon Shoulder Women 5RYxRz7(data);
  fs.unlink('/file.md', Female Laptop Bag Student Backpack Preppy School Black Canvas School Girl Style Women Lady (Case Women Wedding for Purse Flower Bags Wine Clutch Hard Pink Flada Bags Beading 0WOYww5qzunlinkErr) => {
    if (unlinkErr)SKULL Flap CWE00228 Diamante Skull Crystal Handbag Clutch Bag LeahWard BLACK FLAP Women's TwZgxqvv 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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