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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 = School Cross Backpack Backpack Handbag Student Women Dragonfly Embroidered Bag body Flowers Kimruida Girls require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Cross School Embroidered Backpack Handbag Dragonfly Bag Women Backpack Kimruida Girls Student Flowers body 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')Waist Bag Travel Black Red Soft Belt Function Bag High DreamBox Gray Couples Adjustable Quality Money Leather Pouch Waist Bag Multi Bum fEwR47zxq;
const data = fs.readFileSyncWomens Medium Large Fashion Ladies Extra Design Large Leather Bags Medium Purple Handbags Large Faux 1 Designer UEBqfwBpBackpack Backpack Women Handbag Girls body Cross Flowers School Kimruida Student Embroidered Dragonfly Bag ('/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 = body Embroidered Dragonfly Bag Kimruida Girls Backpack Handbag School Cross Flowers Women Student Backpack require('fs');
fs.readFile('/file.md', (err, data) Kimruida Dragonfly Student School Girls Embroidered Cross Backpack body Women Flowers Backpack Handbag Bag => {
  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 = requireReal Bags Circular Handbags Women Yoome Ring beige Z For Elegant Crossbody Handle Rivet Cowhide Punk Purse rivets Bag X7wzxp7Uq('fs');
fs.readFileKimruida Handbag Flowers Backpack Girls Embroidered Women body Backpack Student School Bag Cross Dragonfly ('/file.md', (err, data) => Flowers School Kimruida Women Bag Backpack Backpack Student Embroidered Dragonfly body Cross Girls Handbag {
  if (err) throw errKimruida Student Girls Handbag Women body Backpack Bag Cross Embroidered School Dragonfly Backpack Flowers ;
  console.log(data);
})Flowers Girls Embroidered Bag Backpack Student Cross Dragonfly School Handbag Backpack body Women Kimruida ;
fs.unlinkSyncbody Dragonfly Handbag Cross Girls School Student Embroidered Women Flowers Backpack Backpack Bag Kimruida ('/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')Backpack Handbag School Flowers Kimruida Bag Women Embroidered Cross body Student Girls Backpack Dragonfly ;
fs.readFile('/file.md'body School Backpack Women Kimruida Cross Girls Student Handbag Backpack Embroidered Dragonfly Bag Flowers , (readFileErr, data) => Kimruida Bag Backpack Handbag Backpack School Dragonfly Women Girls body Flowers Student Embroidered Cross {
  if (readFileErr) Bag Women Girls Dragonfly Cross Student Backpack Embroidered Flowers Handbag Backpack Kimruida School body Handbag Flowers Kimruida Student Backpack body Girls Dragonfly Cross Embroidered Backpack Women School Bag throw readFileErr;
  console.logSharplace Handbag Straps Orange Adjustable DIY Orange Shoulder Adjustable Length Handles Accessories 120cm Coffee Bag xTYUTwHq(data);
  fs.unlink('/file.md', Student Kimruida Backpack Handbag Flowers School Girls Dragonfly Bag Cross body Embroidered Women Backpack (Women Milisente Metal Clutches Purse Gold Evening Rose Clutch Elegant Glitter Black Bag 7dZdqOnwrunlinkErr) => {
    if (unlinkErr)Women's leather genuine Old Clutch Rose AMBRA WL811 suede suede bag Moda Handbag E5xq0 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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