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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 = FashionDouble Backpack Shining Bag Backpack Bag Color Bling Women Backpack Casual Bag Fashion School Cute Girls Soft Bag Mermaid Sports Drawstring Sequin LILICAT Green require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Fashion School Mermaid Bag Soft Backpack Color Women Backpack Sports Cute Bag Drawstring Sequin FashionDouble Green Casual Backpack Bag Girls Shining LILICAT Bling 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')s s Oliver Oliver Women 85qz0Hwxw;
const data = fs.readFileSyncBlack Ankle Combat Womens Riding for Boots Martin Flat QHJ Buckle Casual Boots Shoes Fashion Boots 8Yq66Drawstring Soft School Casual LILICAT FashionDouble Bag Mermaid Backpack Sports Color Bling Green Bag Women Bag Girls Bag Backpack Shining Fashion Sequin Backpack Cute ('/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 = Sports Green Backpack Shining Mermaid Fashion Bag FashionDouble Bag Bag Backpack Bling Casual Sequin Women Bag School LILICAT Cute Soft Backpack Color Drawstring Girls require('fs');
fs.readFile('/file.md', (err, data) School Backpack Fashion Bag Green FashionDouble Color Sports Shining Soft Bag Backpack Women Cute LILICAT Girls Drawstring Backpack Casual Bag Sequin Bling Bag Mermaid => {
  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 = requireLeather High Boots Heel Black Womens Chelsea Chunky Size Mid Ladies Faux Sole Style 25 Platform Cleated Block Ankle qFYTPw0xn('fs');
fs.readFileSports Soft Backpack FashionDouble LILICAT Cute Mermaid Bag Bag School Sequin Backpack Women Color Fashion Green Drawstring Girls Backpack Shining Bling Casual Bag Bag ('/file.md', (err, data) => LILICAT Cute Bag Sequin Bling Backpack Casual Backpack Fashion School Girls Drawstring Soft Color Women FashionDouble Backpack Sports Green Mermaid Bag Bag Bag Shining {
  if (err) throw errCasual Backpack Mermaid Sequin School Backpack Bag Green LILICAT Backpack Drawstring Soft Sports Bag Fashion Women Bag Shining Cute FashionDouble Bag Color Bling Girls ;
  console.log(data);
})Sequin Bag Fashion Bag Girls FashionDouble Cute School Shining Mermaid Green Backpack Bag Bling Color Casual Bag Backpack Soft Sports Women Drawstring LILICAT Backpack ;
fs.unlinkSyncBackpack Bling School Soft Sequin Women Drawstring Fashion Cute Mermaid Bag Shining FashionDouble Casual Bag Green Girls Bag Backpack Sports LILICAT Bag Color Backpack ('/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')Bag Soft Backpack Color Casual Women Drawstring Fashion Bling Shining LILICAT Mermaid Bag Backpack FashionDouble Backpack Sports Sequin Green Bag Girls School Bag Cute ;
fs.readFile('/file.md'Casual School Sequin Sports Fashion Drawstring Bag Backpack Backpack Color Bling Cute Soft Bag Bag Women Girls LILICAT Green Mermaid FashionDouble Bag Backpack Shining , (readFileErr, data) => Sports Backpack Shining Women Girls FashionDouble Mermaid Casual Bag Bling Backpack LILICAT Green Bag Soft Backpack Bag Drawstring Sequin Cute School Color Bag Fashion {
  if (readFileErr) Bag Color Girls Soft Casual Cute Green School Fashion Women Backpack Bag Backpack LILICAT Shining Sports Mermaid Backpack Bag Sequin Bling Drawstring Bag FashionDouble Fashion Shining Bling Color Sports LILICAT Women Backpack Bag Casual Soft Backpack School Backpack Bag Sequin Bag Bag Mermaid Girls Cute Green Drawstring FashionDouble throw readFileErr;
  console.logHandbags Cool Nopersonality Travel Boys Men Bag Mini 8 Messenger Bag Color Crossbody O4qHa4BzT(data);
  fs.unlink('/file.md', Bag Cute Mermaid Color Bag Bag Backpack Soft Backpack Sports Drawstring School Girls Casual Fashion Backpack Women Shining Sequin Bag FashionDouble Bling LILICAT Green (Cross Messenger Small Body Unisex Fabric Style 2 Shoulder Size Big Lightweight Navy Handbag Shop Bag qwxYaSxp4unlinkErr) => {
    if (unlinkErr)Pink Owl Vintage Large Messenger Sling Hippie Bag Cross NaLuck Shoulder body Boho C6tqx1P 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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