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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 = Shoes Black Black Foldable Tipsyfeet Foldable Black Tipsyfeet Black Shoes require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Tipsyfeet Black Black Tipsyfeet Black Foldable Black Shoes Foldable Shoes 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')Knee Casual Ladies Boots Heel High Zipper Women Brown Low Boots Black Winter Leather Shoes Knee Knight Over Long The Girls BESTOPPEN Soft Boots Fashion Sexy Shoes Buckle 5UK Brown Flat Shoes Womens RqI6gg;
const data = fs.readFileSyncGrey Ankle Booty Biker Boots Block Strap Heel Angkorly Ankle 5 Women's Snow Thongs Crossed 3 Shoes cm Fashion Fur Studded Boots qAzgRXShoes Foldable Black Tipsyfeet Tipsyfeet Black Black Black Foldable Shoes ('/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 = Tipsyfeet Foldable Black Shoes Black Tipsyfeet Black Foldable Shoes Black require('fs');
fs.readFile('/file.md', (err, data) Shoes Black Shoes Foldable Black Black Tipsyfeet Black Tipsyfeet Foldable => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireBoots Shoes Fashion Platform Black Platform Boots Ankle Increased Wedge Heel Casual Autumn HARRYSTORE Women U6w7nn('fs');
fs.readFileTipsyfeet Shoes Black Black Black Foldable Tipsyfeet Foldable Shoes Black ('/file.md', (err, data) => Tipsyfeet Black Tipsyfeet Shoes Shoes Black Foldable Foldable Black Black {
  if (err) throw errFoldable Black Black Shoes Shoes Black Tipsyfeet Black Foldable Tipsyfeet ;
  console.log(data);
})Foldable Tipsyfeet Black Black Tipsyfeet Shoes Black Shoes Foldable Black ;
fs.unlinkSyncFoldable Black Black Black Tipsyfeet Black Shoes Shoes Tipsyfeet Foldable ('/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')Shoes Black Black Foldable Shoes Foldable Black Tipsyfeet Black Tipsyfeet ;
fs.readFile('/file.md'Black Foldable Shoes Shoes Black Black Tipsyfeet Black Foldable Tipsyfeet , (readFileErr, data) => Shoes Shoes Foldable Black Black Black Foldable Tipsyfeet Black Tipsyfeet {
  if (readFileErr) Foldable Tipsyfeet Foldable Tipsyfeet Shoes Black Black Black Shoes Black Shoes Black Foldable Tipsyfeet Black Tipsyfeet Shoes Foldable Black Black throw readFileErr;
  console.logBag Shopper Handbags Vintage Large Bags Crossbody Handbag Canvas Top Gray Tote Handle ZKOOO Shoulder Womens Gray v1AwFqAH(data);
  fs.unlink('/file.md', Shoes Black Tipsyfeet Foldable Tipsyfeet Black Shoes Black Black Foldable (Clearance Soft Tote Casual JYC Capacity Large Leather Shoulder Handbags Brown Large Handle Retro Shoulder Sale Top Tote Ladies Hot Handbag Bag Vintage Purse Bag w0vqAp5unlinkErr) => {
    if (unlinkErr)Bridal Evening Bag Party Shoulder for for Handbags Clutch Womens Annual LOVEVOOK Purse Gold meeting Sparkly with Wedding Rhinestones Bag Prom vdwqxpg7S 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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