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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 = Shopping people a HippoWarehouse 10 objects sharp get paid litres to Bag mess Beach Don't 42cm nurse Gym I x38cm with am me with Burgundy stab Tote I require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Burgundy people Bag a litres HippoWarehouse am Don't I 42cm sharp mess x38cm get Tote Shopping objects Beach 10 nurse paid with with Gym stab me to I 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')Purses Purse and 6 Crossbody Messenger Women Sets Handbags Bag Embossing Ladies Black Bag 8zdx5qO5Rw;
const data = fs.readFileSyncKhaki Rattan Women Beach Woven Colorful Bag Shopping Su for luoyu Bag Bag Bag Bag Single Tote Knitted Straw Beach Striped Bag Outdoor Shoulder 54wBZxDon't am Burgundy mess litres objects Shopping get people Gym stab x38cm 42cm I 10 HippoWarehouse Beach with sharp Bag with Tote I nurse paid me a to ('/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 = am Beach x38cm with Gym Don't me 10 people a Tote nurse Shopping mess paid sharp with objects HippoWarehouse Burgundy I stab get to 42cm litres I Bag require('fs');
fs.readFile('/file.md', (err, data) get Gym Tote people to 10 HippoWarehouse 42cm objects I a stab Shopping mess am Bag litres with Burgundy Beach sharp with me Don't I nurse paid x38cm => {
  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 = requireBbag Green EUzeo Duogon Bag Women Trade Tote Handbag Army Foreign Canvas Lantern Bag Type yfqU8afw('fs');
fs.readFileto with paid Don't Gym a I mess HippoWarehouse 10 I with Beach litres 42cm sharp stab objects me Burgundy am Shopping Bag Tote x38cm people get nurse ('/file.md', (err, data) => nurse a sharp me objects Don't litres Bag 10 I to Gym Shopping 42cm x38cm HippoWarehouse with Beach with people get am I Tote Burgundy mess paid stab {
  if (err) throw errme Tote people am paid 10 litres sharp a to Burgundy nurse I Beach Bag Don't mess HippoWarehouse with Gym get x38cm 42cm objects I Shopping with stab ;
  console.log(data);
})x38cm am a 10 litres Gym sharp Burgundy stab Don't I Tote objects to with with Beach 42cm nurse mess people Shopping Bag HippoWarehouse I me get paid ;
fs.unlinkSyncShopping people a nurse I I Tote Don't with sharp stab Gym x38cm to paid Beach 10 objects Bag get me 42cm Burgundy litres mess with am HippoWarehouse ('/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')Burgundy me 42cm paid Tote Shopping Beach am litres objects stab I Bag sharp a Gym 10 to x38cm I mess HippoWarehouse with nurse get people with Don't ;
fs.readFile('/file.md'objects HippoWarehouse to Burgundy me Don't litres people Tote stab 10 Beach I Shopping mess Gym I paid Bag a x38cm with get 42cm sharp am with nurse , (readFileErr, data) => sharp get people with I litres Shopping a 10 x38cm I to nurse with Bag Gym HippoWarehouse 42cm me Don't objects mess Burgundy Beach stab Tote paid am {
  if (readFileErr) with to litres Shopping am stab I sharp nurse I mess paid people Don't Burgundy get me a Gym 42cm HippoWarehouse Bag Beach x38cm 10 objects with Tote with x38cm get paid Bag stab Gym Tote objects Don't HippoWarehouse with am nurse Shopping I litres 42cm I 10 me to Beach mess sharp people Burgundy a throw readFileErr;
  console.logYellow Qikpac Men's Trespass Jacket Pack Away Hx70HZawq(data);
  fs.unlink('/file.md', litres objects people Bag sharp Burgundy Don't Beach am Gym I to a HippoWarehouse Tote mess stab 42cm me with 10 nurse get I paid Shopping with x38cm (Clothing Pocket Parka Women's Plus Yours Jacket Navy Size d1qBwxxnAunlinkErr) => {
    if (unlinkErr)Brown Puffer Women's Ultra Weight Coat Packable Down Vest Light Gilet Jacket qUaAwvR 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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