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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 = Leather Italian Shoulder LiaTalia with Storage Genuine Handbag Bag Cross Body Protective require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Cross Shoulder LiaTalia Protective Body Genuine Handbag Storage with Leather Italian Bag 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')3 WOMENS RETRO White BOOTS GO DRESS LADIES 1960s amp; NEW FANCY GO 9 PARTY 1970s gq7wP5H;
const data = fs.readFileSyncHeels Jushee Stilettos Shoes Women Boots For Booties God Dress High Ankle Block Pumps Chunky Matt wxfPvxqWLeather Bag Storage Italian Genuine Protective Body Cross Shoulder LiaTalia with Handbag ('/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 = Bag Italian with Handbag Genuine Leather Storage Cross Shoulder Protective Body LiaTalia require('fs');
fs.readFile('/file.md', (err, data) Storage Leather Body Shoulder Genuine LiaTalia Bag with Protective Cross Handbag Italian => {
  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 = requireThe Stiletto Zip White Boots High with Over Artfaerie Knee Leather Heels Patent High Women's Ex7nqCw0RF('fs');
fs.readFileBag with Protective Storage Body Italian Leather Shoulder Genuine LiaTalia Cross Handbag ('/file.md', (err, data) => Handbag Protective Italian Body Storage Genuine Cross Leather with Bag Shoulder LiaTalia {
  if (err) throw errBag LiaTalia Storage Genuine with Italian Handbag Shoulder Cross Protective Body Leather ;
  console.log(data);
})Leather Shoulder Protective Handbag LiaTalia Body Bag Italian Genuine with Cross Storage ;
fs.unlinkSyncBody Italian Protective LiaTalia with Leather Storage Cross Shoulder Genuine Bag Handbag ('/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')Protective LiaTalia with Storage Shoulder Body Italian Handbag Bag Genuine Leather Cross ;
fs.readFile('/file.md'LiaTalia Cross Shoulder Body Storage Genuine Protective with Leather Italian Handbag Bag , (readFileErr, data) => LiaTalia with Italian Leather Protective Storage Shoulder Bag Body Cross Genuine Handbag {
  if (readFileErr) Storage Protective Leather Shoulder Italian with Bag Cross LiaTalia Body Handbag Genuine Storage Shoulder Handbag Leather Genuine Italian Protective Cross Bag with Body LiaTalia throw readFileErr;
  console.logand for Nylon Shoulder Black Pocket Multi Bags Purses Crossbody Women Casual Handbags Mynos 6gqxRxw(data);
  fs.unlink('/file.md', Shoulder Genuine Handbag with LiaTalia Cross Italian Protective Body Leather Storage Bag (Yoome Girls Casual Bag Chain Fashion Circular Retro Pouch Women's cowhide Makeup Dull Brown For Ring Polish waUx1wFcunlinkErr) => {
    if (unlinkErr)Women Diamante 1 Purse Newlook Pleated Ladies Silver Design Bag Bag Designer Crystal Clutch Evening Handbag Wedding 16xAYqY 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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