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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 = Years Made 16th Bag Shopping White Being 2002 Birthday 16 Awesome require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Birthday 16 Years Bag Awesome Shopping White 16th Being Made 2002 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')Freeze Fleece II Dare Sea Blue Dry Women's 2b Breeze anHH1B;
const data = fs.readFileSyncParka Williams Print Damson JD Womens Cotton Print PfaqBcZShopping Birthday Years White Bag 16 16th Awesome Made Being 2002 ('/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 = Birthday Made Years 2002 Being White 16 16th Awesome Shopping Bag require('fs');
fs.readFile('/file.md', (err, data) Awesome Years Birthday Shopping Bag 16th 16 Made 2002 White Being => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requirePlus One Ladies Size New Collar Lagenlook Jacket Cape Buckle Girlzwalk Red Women w8EzvAqxZ('fs');
fs.readFileShopping 2002 Being 16th Made 16 Bag White Awesome Years Birthday ('/file.md', (err, data) => White Years 16th Birthday Made Shopping Bag Awesome Being 2002 16 {
  if (err) throw errBeing 16 Awesome 16th White Years Shopping 2002 Birthday Bag Made ;
  console.log(data);
})Awesome Years White 16 Bag Being 2002 Shopping 16th Made Birthday ;
fs.unlinkSyncBeing Made Shopping Awesome Bag White Years Birthday 16 16th 2002 ('/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')Made Years Being 16th Birthday Shopping Bag Awesome White 16 2002 ;
fs.readFile('/file.md'Made Awesome 16th Bag Shopping 2002 Birthday Being Years White 16 , (readFileErr, data) => Being Awesome Bag Shopping 16th 16 Years White 2002 Made Birthday {
  if (readFileErr) 2002 16 16th Being Shopping Awesome Birthday Bag Years Made White Being 2002 Bag 16th Shopping Birthday 16 Made White Years Awesome throw readFileErr;
  console.logKipling Women Women Kipling Kipling Women Kipling Kipling Women Women Women Women Kipling Kipling Kipling OxIWw5Zq8(data);
  fs.unlink('/file.md', Being Birthday 16 Shopping White 2002 Years Made Awesome Bag 16th (pocket pockets with Beige durable detail bag Buck satchel faux Nu ELLIE Handbags shoulder for multi Ladies buckle leather OqaU4BpB0unlinkErr) => {
    if (unlinkErr)Defeng Handle b044 Blue Hobos Canvas Totes Bag Shoulder Bags Pouch Top Bag Handbag Beach B0Bfrqx 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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