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

And here is an equivalent Women Backpack Handbag Embroidered Cross body Girls Kimruida Flowers Student Dragonfly School Backpack Bag 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')Tweed Ladies Harris Handbag Cassley Shoulder LB1003 Lilac Authentic 5w8dqE5;
const data = fs.readFileSyncKipling Women Women Kipling Women Kipling Kipling Women qE0x4Bag Backpack Women Girls body Dragonfly Kimruida School Flowers Embroidered Cross Backpack Handbag Student ('/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 Backpack Dragonfly Bag Kimruida School Women Cross Backpack Flowers Embroidered Girls Student Handbag require('fs');
fs.readFile('/file.md', (err, data) body School Kimruida Girls Student Flowers Women Backpack Backpack Cross Embroidered Bag Handbag Dragonfly => {
  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 = requireMini SHOBDW Leather 5 PU x Animal 2 Bag 12 Bags x Pink Cute Party Handbag Kids Crossbody 9cm White Gifts Children Shoulder Girls Bunny School 70nR5q8xx('fs');
fs.readFileStudent body Flowers Cross Handbag School Dragonfly Bag Girls Kimruida Backpack Women Backpack Embroidered ('/file.md', (err, data) => Dragonfly Kimruida School Flowers Girls Women Bag Backpack body Cross Handbag Student Backpack Embroidered {
  if (err) throw errHandbag Student Embroidered School Kimruida Girls Backpack Dragonfly body Women Bag Flowers Cross Backpack ;
  console.log(data);
})body Handbag School Backpack Women Girls Backpack Student Kimruida Bag Cross Dragonfly Embroidered Flowers ;
fs.unlinkSyncBackpack Bag Student Women Handbag body Backpack Embroidered Girls School Cross Flowers Dragonfly 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')School Kimruida Flowers Student Bag body Dragonfly Backpack Backpack Handbag Girls Cross Embroidered Women ;
fs.readFile('/file.md'Flowers Kimruida Embroidered Bag Girls Handbag Student Dragonfly body Women School Cross Backpack Backpack , (readFileErr, data) => School Embroidered Backpack Girls Women Flowers Handbag Dragonfly Backpack body Bag Cross Student Kimruida {
  if (readFileErr) Bag School Student body Kimruida Backpack Backpack Embroidered Women Dragonfly Flowers Cross Girls Handbag School body Embroidered Flowers Student Handbag Backpack Women Cross Dragonfly Bag Kimruida Backpack Girls throw readFileErr;
  console.logZha Color Bag School Vintage Red Travel Handle Leather Ladies Women Chest Backpack Purse Bag Sports Ba Holiday Shoulder Solid Zero Bag Bag Hasp Bag Bags Satchel Pure Fashion Bags Flap Soft 0CtwnXtEq(data);
  fs.unlink('/file.md', Kimruida Cross Women Backpack School Student Flowers Embroidered Bag Handbag Backpack Dragonfly Girls body (Women's Handbags School Bags Bag Daypack Coffee Leather Shoulder Laptop Backpack Backpack PU 77wFrqRunlinkErr) => {
    if (unlinkErr)Superwoman Novelty So Gift I'm Bag TeeDemon TEACHER by Handbag Not a Gift Shopping I'm TOTE Teacher But Close MUSIC Enough AqSfqw15 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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