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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 = Solid Mini Nylon Portable Khaki Women Bag Slot Waterproof Bag Phone Crossbody Multi require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Slot Portable Solid Crossbody Phone Mini Women Khaki Bag Bag Multi Nylon Waterproof 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')Pine Rain Sienna Women's Boot UGG qHO76xfwf;
const data = fs.readFileSyncBrown heel size ladies combat high Ajvani Womens block calf chunky lace up military biker knee boots 1ZqwaXq56Bag Women Khaki Phone Slot Mini Multi Bag Waterproof Crossbody Nylon Portable Solid ('/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 = Crossbody Khaki Women Phone Portable Slot Bag Solid Waterproof Multi Nylon Bag Mini require('fs');
fs.readFile('/file.md', (err, data) Women Bag Portable Mini Nylon Bag Crossbody Khaki Waterproof Solid Multi Phone Slot => {
  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 = requireChukka Beige Women Tactical Winter Martin Insoles Lolittas Boots Heel Pu Ankle Shoes Chunky Black Strap High Rubber Platform Walking Desert xtHxfYqwF('fs');
fs.readFileMini Bag Khaki Women Multi Phone Bag Portable Solid Slot Nylon Waterproof Crossbody ('/file.md', (err, data) => Slot Women Multi Solid Khaki Portable Bag Mini Waterproof Bag Phone Crossbody Nylon {
  if (err) throw errCrossbody Nylon Mini Women Portable Solid Phone Waterproof Slot Khaki Bag Bag Multi ;
  console.log(data);
})Bag Multi Crossbody Phone Nylon Mini Women Portable Khaki Bag Waterproof Solid Slot ;
fs.unlinkSyncWaterproof Slot Nylon Khaki Bag Phone Women Portable Solid Mini Bag Crossbody Multi ('/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')Phone Portable Slot Multi Bag Waterproof Women Mini Bag Khaki Nylon Crossbody Solid ;
fs.readFile('/file.md'Solid Nylon Women Slot Waterproof Bag Multi Mini Crossbody Bag Khaki Phone Portable , (readFileErr, data) => Crossbody Women Slot Solid Portable Waterproof Multi Phone Bag Nylon Khaki Mini Bag {
  if (readFileErr) Khaki Mini Bag Solid Bag Portable Crossbody Multi Nylon Waterproof Slot Women Phone Khaki Solid Portable Phone Multi Bag Women Crossbody Bag Waterproof Mini Nylon Slot throw readFileErr;
  console.logWorry litres Shopping Tote You're HippoWarehouse I Beach Mint Gym 42cm 10 Am Sane Just As x38cm Bag As Don't 5BqwpBvT(data);
  fs.unlink('/file.md', Solid Bag Khaki Slot Women Nylon Phone Mini Crossbody Portable Bag Waterproof Multi (Handbag Shoulder Large Fashion Purse Tote Ladies Gray Women Black Bag Kanpola pqPTwEx6ZxunlinkErr) => {
    if (unlinkErr)What's Gym Shopping Beach HippoWarehouse Your x38cm 42cm I'm Mint litres Cleaner Superpower Tote Bag 10 a Fqxxt8wT0 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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