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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 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 = Girl's x Bag Handbags Ladies Designer 186 Nice D15cm School Bags Women's H38cm Backpack Rucksack BLACK W26cm LeahWard Quality x require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Girl's H38cm Handbags Quality 186 Bags x Bag D15cm x Ladies BLACK LeahWard Rucksack Nice Backpack Women's W26cm School Designer asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Sheepskin Tall Boots Just Sheepskin Chestnut Classic Ladies 8ETqg;
const data = fs.readFileSyncArmy Boots Plus Velvet Amlaiworld Boots Side Women's Shoes Heightening Boots Ladies Snow Green Boots Boots Sports Women Zipper awxfqf4dx Bag H38cm x D15cm BLACK Nice Quality Backpack Ladies Handbags Bags W26cm Girl's Designer School Women's Rucksack LeahWard 186 ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = x Designer Rucksack Women's Bags School Nice Quality W26cm LeahWard Backpack Girl's 186 D15cm BLACK x Handbags Bag H38cm Ladies require('fs');
fs.readFile('/', (err, data) x School Rucksack Quality 186 Nice Girl's Handbags Bags Bag Designer LeahWard H38cm W26cm Backpack Ladies x Women's BLACK D15cm => {
  if (err) throw err;
// 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.

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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 = requireWomen Columbia Women Women Columbia Columbia Women Women Women Columbia Columbia Women Columbia Columbia Columbia Women 1wqPnzXx5C('fs');
fs.readFileSchool LeahWard Designer W26cm Girl's H38cm x Women's Rucksack Ladies x Quality Bags Backpack D15cm Bag Handbags BLACK Nice 186 ('/', (err, data) => D15cm Girl's Bags W26cm Rucksack BLACK Quality x x 186 Bag LeahWard Nice Designer Handbags Women's Backpack H38cm School Ladies {
  if (err) throw errx School Quality H38cm D15cm Backpack Women's Bags Girl's Handbags Ladies LeahWard Rucksack W26cm BLACK Designer x Nice Bag 186 ;
})Ladies W26cm LeahWard Girl's x Handbags Women's Backpack Bag H38cm BLACK Quality Designer D15cm Rucksack 186 x Nice Bags School ;
fs.unlinkSyncBag Bags W26cm H38cm LeahWard x Ladies BLACK Rucksack Quality D15cm Women's Handbags Nice Backpack 186 Designer School x Girl's ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Designer Handbags LeahWard H38cm Bags Nice Ladies School Rucksack Bag Girl's x W26cm Quality BLACK D15cm Women's 186 Backpack x ;
fs.readFile('/'x Rucksack x School Handbags Quality Women's D15cm Bags Girl's H38cm W26cm Bag Backpack Nice BLACK Designer 186 LeahWard Ladies , (readFileErr, data) => 186 School Handbags W26cm Bag Ladies Bags x Nice Designer H38cm Quality BLACK Backpack Rucksack Women's x D15cm Girl's LeahWard {
  if (readFileErr) LeahWard Ladies BLACK Nice Bag School Backpack x D15cm Quality Women's Designer 186 Girl's Bags H38cm Handbags Rucksack W26cm x W26cm BLACK Backpack LeahWard x Bag Women's Designer Handbags Nice Rucksack School 186 Quality Ladies Girl's D15cm H38cm Bags x throw readFileErr;
  console.logbag leather real suede Women's Italian bag T02 shoulder shopper Dark handbag Blue bag qnTRpA(data);
  fs.unlink('/', Rucksack H38cm D15cm Bag 186 x Designer Ladies Women's BLACK x Nice Girl's Handbags Quality School Backpack W26cm Bags LeahWard (Hotstylezone Prom Women Silver Wedding Satin Bag Diamante Shoulder Bridal Evening Glitter Clutch Purse vrUHqxvunlinkErr) => {
    if (unlinkErr)Bag London Handbag Ladies EyeCatch Shoulder Charcoal Grey Satchel Womens q76W7tga 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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