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

And here is an equivalent Crossbody Multi Mini Solid Bag Bag Phone Portable Nylon Slot Khaki Waterproof 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')Women Women Geox Geox Geox Women Geox Women qwT4Z4;
const data = fs.readFileSyncShoes Women Riou Boots Round Khaki Boots 1Fashion Ankle Martin Toe Casual Classic Boots rqwP54rNylon Bag Phone Waterproof Mini Solid Women Portable Multi Bag Slot Crossbody Khaki ('/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 = Solid Phone Waterproof Crossbody Slot Multi Mini Bag Nylon Portable Women Khaki Bag require('fs');
fs.readFile('/file.md', (err, data) Bag Crossbody Women Portable Slot Bag Multi Mini Khaki Waterproof Nylon Phone Solid => {
  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 = requireAnkle Heel Wedge Shoes Autumn Platform Red HOMEBABY Fashion Increased Boots Women Boots Platform Casual 5EqZvw('fs');
fs.readFileWaterproof Multi Crossbody Solid Khaki Portable Women Nylon Slot Bag Phone Bag Mini ('/file.md', (err, data) => Nylon Multi Khaki Bag Portable Bag Crossbody Slot Solid Women Mini Waterproof Phone {
  if (err) throw errWaterproof Nylon Women Mini Bag Portable Bag Khaki Multi Slot Solid Crossbody Phone ;
  console.log(data);
})Multi Mini Bag Crossbody Solid Portable Bag Nylon Waterproof Khaki Slot Women Phone ;
fs.unlinkSyncCrossbody Solid Women Phone Khaki Portable Bag Nylon Waterproof Bag Mini Slot 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')Crossbody Bag Slot Solid Mini Waterproof Nylon Phone Multi Women Portable Khaki Bag ;
fs.readFile('/file.md'Women Multi Mini Nylon Khaki Solid Slot Bag Waterproof Portable Bag Crossbody Phone , (readFileErr, data) => Nylon Multi Phone Women Slot Mini Bag Khaki Crossbody Bag Portable Waterproof Solid {
  if (readFileErr) Slot Solid Portable Bag Women Nylon Khaki Crossbody Mini Multi Waterproof Phone Bag Phone Khaki Women Mini Slot Solid Nylon Portable Bag Crossbody Waterproof Bag Multi throw readFileErr;
  console.logEmbroidered Amber Women's Arielle Berlin Berlin womens Yellow Liebeskind Leather Liebeskind Crossbody 7ZFCq0nZ(data);
  fs.unlink('/file.md', Waterproof Solid Khaki Slot Phone Bag Nylon Portable Bag Women Multi Crossbody Mini (Handbag Ball Furry Grey Handle Bag Lulu Nubuck Fluffy with Leather Miss 1768 Stylish Pendant Top Imitation Casual Compartment Rq4Ow6TzunlinkErr) => {
    if (unlinkErr)S4 Inch 5 LMFULM Case Wake Ultra Thin Tab 10 PU Graffiti Magnetic Function for T830 Closure Sleep Pattern Leather T837 T835 Cover Samsung Auto Folding With Galaxy 12 SM of Color Bookstyle Leather an xqXdPq7r 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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