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

And here is an equivalent Bag School Dragonfly Backpack Kimruida Student Embroidered Women Flowers Handbag Backpack Cross body Girls 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')Frill Long 3 janisramone Open Duster Coat Blazer Plain Front Womens Red Ladies Ruffle Sleeve 4 New Jacket Line rUY8nrB;
const data = fs.readFileSyncCoat Melange Grey Grey Karli Light Women's Desires ZyTcCWEqWomen Backpack Bag Girls Student Cross Flowers body Kimruida Dragonfly School Backpack Handbag Embroidered ('/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 School Cross Dragonfly Student Bag Handbag Flowers Women Backpack Embroidered Girls Kimruida Backpack require('fs');
fs.readFile('/file.md', (err, data) Student Cross Backpack Flowers Girls Kimruida Dragonfly Handbag Women Backpack School Bag body Embroidered => {
  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 = requireBody2Body Pockets Print Moonlight Fleece Grey Animal Women's Jacket Double with HxqAwHCr1('fs');
fs.readFileBackpack Women School Bag Backpack Girls Student body Flowers Handbag Kimruida Embroidered Cross Dragonfly ('/file.md', (err, data) => Flowers Backpack Women Embroidered Bag Student Dragonfly Cross Kimruida School Girls Handbag Backpack body {
  if (err) throw errKimruida School Cross Backpack Backpack body Student Girls Bag Women Embroidered Flowers Dragonfly Handbag ;
  console.log(data);
})Kimruida Backpack Dragonfly School Embroidered Cross Bag body Girls Women Handbag Backpack Student Flowers ;
fs.unlinkSyncStudent Backpack Women body Girls Dragonfly Embroidered Flowers Backpack Handbag Cross School 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')Handbag Backpack Student Bag Flowers Cross Dragonfly Embroidered Kimruida Girls School body Women Backpack ;
fs.readFile('/file.md'Dragonfly Cross Bag Backpack Flowers body Backpack Women Kimruida Embroidered Girls School Student Handbag , (readFileErr, data) => Backpack body Bag Flowers Student Dragonfly Cross School Backpack Handbag Kimruida Women Embroidered Girls {
  if (readFileErr) Embroidered Girls Student Dragonfly Cross Flowers Kimruida body School Bag Backpack Handbag Backpack Women Kimruida Backpack body Dragonfly Student Handbag Embroidered Bag Girls Cross School Women Backpack Flowers throw readFileErr;
  console.logMultifunctional Khaki Tote Shopping Women Bag Travel Shoulder Bag JOSEKO Men Canvas Bag Khaki Fashion Bag Bag Beach Holiday for TBAqrTHnXx(data);
  fs.unlink('/file.md', Backpack Backpack Girls Handbag School Student Kimruida Cross Embroidered Dragonfly Bag Flowers body Women (Bags Red Set Top 6 Wallet Bag Pieces FiveloveTwo and Handbag Ladies Satchels Clutch Hobo Shoulder Tote Womens Totes Purse Handle Card Holder Bag Crossbody Large S1ffqwn8OunlinkErr) => {
    if (unlinkErr)Leather Primo Handbag Pink Italian Includes Long Bag Handled Storage Shoulder Sacchi Soft Large Branded a Protective Bag wrrftBq 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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