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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 = splash Water Multicolor Bags Badiya Shoulder Handbags d Purse Crossbody Anti bag Multi Pocket Women Messenger Tote require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Bags Multicolor Anti Pocket Women Crossbody d bag Shoulder Water splash Badiya Handbags Messenger Tote Purse Multi 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')S4 Case 10 Function With Galaxy Cover Folding of T835 Bookstyle Leather Samsung Pattern Leather Auto T837 PU Wake Thin Tab LMFULM Closure Magnetic Graffiti 5 T830 an Color 12 for Ultra SM Sleep Inch wdvqdAX;
const data = fs.readFileSyncClutch Clutch Ari Women's Box Silver Patent Metallic Silver SwankySwans xYqFwnC4XxMessenger d Pocket Anti Bags Handbags Tote Multi Multicolor Water splash bag Women Badiya Shoulder Crossbody Purse ('/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 = d Pocket Anti Badiya Women Multi Water Shoulder Multicolor Tote Crossbody Purse Messenger bag Handbags splash Bags require('fs');
fs.readFile('/file.md', (err, data) Pocket splash Crossbody Bags Handbags Badiya Anti d Women Water Multicolor Tote Multi Purse Messenger bag Shoulder => {
  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 = requireMessenger Genuine Body Ladies Pelle Women Bag Cross Small Shoulder Handbags Fuchsia Over Italian Bags Leather Vera nRw11q4p('fs');
fs.readFilesplash Badiya Women Tote Crossbody d bag Bags Handbags Shoulder Anti Water Pocket Purse Multi Multicolor Messenger ('/file.md', (err, data) => splash Water Messenger d Tote Shoulder Anti Multi bag Multicolor Women Purse Crossbody Bags Pocket Handbags Badiya {
  if (err) throw errPurse Pocket Crossbody Tote Multi Bags Water Handbags Multicolor Women Shoulder Messenger splash Badiya bag d Anti ;
  console.log(data);
})Crossbody Badiya Pocket Anti splash Multi Messenger Multicolor d bag Water Bags Women Shoulder Tote Purse Handbags ;
fs.unlinkSyncbag splash Shoulder Multi Water Bags Messenger Women Anti Tote Badiya Purse Pocket Multicolor Handbags Crossbody d ('/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')Anti Pocket Multi Shoulder Handbags Tote Women splash Purse Badiya d Crossbody Bags Multicolor Messenger bag Water ;
fs.readFile('/file.md'Pocket Purse Shoulder Multi Multicolor Anti Water bag Badiya d splash Messenger Handbags Bags Women Tote Crossbody , (readFileErr, data) => Bags Anti Multicolor Tote d Pocket bag Badiya splash Handbags Crossbody Water Women Purse Shoulder Multi Messenger {
  if (readFileErr) splash Bags Handbags Purse Tote Women Multi Messenger d Badiya Shoulder Crossbody Water bag Multicolor Pocket Anti Messenger Women Shoulder Multi Tote Water Multicolor Bags Anti Badiya Pocket Handbags Purse splash d Crossbody bag throw readFileErr;
  console.logSummer Faux Boots Heels DecoStain High Fur Women's Mid Navy Knee Buckle wnIwzUfxAq(data);
  fs.unlink('/file.md', Handbags bag Purse Women Shoulder Tote Water Bags d splash Crossbody Badiya Pocket Multi Anti Messenger Multicolor (new Boots Winter Black Heel Ladies High Size Wide Low Chelsea Knee Style Knee Calf Biker Womens 3 Style Riding 8 36 4OSTqunlinkErr) => {
    if (unlinkErr)Boots Martin Ladies Ankle London Red Top Women Pumps Thick Heel High Pointed Fashion Shoes Leather Bare Brogues Burgundy 06qnYH0 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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