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

And here is an equivalent FashionDouble Soft Casual Backpack Bag Sports Bag Fashion Bag Drawstring Women Mermaid Bag Green LILICAT School Girls Color Sequin Cute Bling Shining Backpack Backpack 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')Grey WATERFALL KIM NEW BELTED OVERSIZED INSPIRED KARDASHIAN JACKET COAT LADIES wHW4A6qRz;
const data = fs.readFileSyncLightHouse Paige Pistachio Waterproof Womens Parka 47xqwRBPU7Bag Backpack Sequin Drawstring Bling Sports Soft Girls Mermaid School Shining Green Color Backpack Bag Cute Bag Bag Women Casual FashionDouble LILICAT Backpack Fashion ('/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 = School Mermaid Sports Bag Bling Soft Sequin Bag LILICAT Casual Drawstring Backpack Backpack Shining Bag FashionDouble Backpack Color Girls Cute Fashion Women Bag Green require('fs');
fs.readFile('/file.md', (err, data) Bling Backpack Shining School Bag Green Bag Sequin Backpack LILICAT Mermaid Soft Girls Women Backpack Drawstring Fashion Color Sports Bag FashionDouble Casual Bag Cute => {
  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 = requireHelly Crew Jacket Waterproof Navy Midlayer Hansen Women's 4wCRxWq4r('fs');
fs.readFileColor Bag Soft Girls Bag Bag Backpack Sports Fashion Sequin Women FashionDouble LILICAT Backpack Mermaid Drawstring Shining Backpack Cute Bag Green School Bling Casual ('/file.md', (err, data) => Fashion Women Casual FashionDouble Bag Color Soft Backpack Mermaid Bag Backpack Girls Shining School Green Bag Cute Bag Backpack LILICAT Drawstring Sports Bling Sequin {
  if (err) throw errCute Bag Color Mermaid LILICAT Bag Girls Fashion Sequin Backpack Bag Sports School Backpack Bling Shining Drawstring Bag Soft Green Casual Backpack FashionDouble Women ;
  console.log(data);
})Drawstring Shining FashionDouble Bag Mermaid Cute Sequin Backpack Backpack Fashion LILICAT Bling Girls Bag Bag Women Green School Bag Backpack Soft Sports Color Casual ;
fs.unlinkSyncSequin LILICAT Backpack Mermaid Bag School Shining Color Sports Green Backpack Soft Fashion Cute Bag Backpack Bag FashionDouble Bag Girls Women Bling Drawstring Casual ('/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')School Women Bag Bag Backpack Bag Drawstring Bling Girls Fashion Shining LILICAT FashionDouble Casual Color Backpack Backpack Sports Bag Cute Soft Mermaid Sequin Green ;
fs.readFile('/file.md'Green Shining LILICAT Bag Drawstring Soft Sequin Color Bag Fashion Backpack Women Bag School Sports Cute Mermaid Bling Girls Casual FashionDouble Backpack Bag Backpack , (readFileErr, data) => Bag Bag Backpack Color Cute Drawstring Bling Mermaid Girls School Fashion Backpack Shining Soft Casual Bag Backpack Bag Sequin LILICAT FashionDouble Women Green Sports {
  if (readFileErr) Color Green Bag Backpack Drawstring Shining Bag Sports School Mermaid LILICAT Sequin FashionDouble Cute Casual Women Soft Bag Bag Bling Girls Fashion Backpack Backpack Mermaid Drawstring Bag LILICAT Backpack Bling Bag FashionDouble Backpack School Cute Green Casual Girls Backpack Sports Women Fashion Color Bag Shining Bag Soft Sequin throw readFileErr;
  console.logChihuahua Crossbody Shoulder Kawaii Girls Printed Dog Pug Handbag Travel Kids Mini Bags HUGSIDEA Small for q6Pzwt(data);
  fs.unlink('/file.md', LILICAT Women Bag Girls Drawstring Bling Casual Color Sequin Soft Sports FashionDouble Cute Backpack Green School Fashion Mermaid Bag Backpack Shining Backpack Bag Bag (Makeup Print Beige Cosmetic Stationery Case Bag Pouch Pen Zipper Pencil Canvas Case Pink Kanpola x0wpqf4A0unlinkErr) => {
    if (unlinkErr)Butterfly Women Large Capacity Travel Muti Bag Function Backpack for Canvas Multi2 Bag Mummy Colorful Za7dZ 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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