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

And here is an equivalent Backpack Handbags Bags x Ladies School W26cm BLACK H38cm D15cm Nice LeahWard Women's x Girl's Designer Bag Quality 186 Rucksack 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')Heels Heel High Boots Thick Ankle Boots Black NEEDRA Pointed Boots Shoes Women Middle Boots vqwFaU;
const data = fs.readFileSyncAnkle Boots Womens Marnie Black Lunar wHx6X1Zwx LeahWard Designer H38cm Girl's Backpack Nice Handbags Rucksack School BLACK W26cm Quality Bag Women's D15cm 186 x Bags Ladies ('/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 = W26cm x LeahWard Bags Rucksack Nice Girl's 186 x Backpack Handbags D15cm Designer Bag School Quality BLACK H38cm Ladies Women's require('fs');
fs.readFile('/file.md', (err, data) Bag H38cm Girl's Handbags W26cm LeahWard Women's Ladies x x Designer Nice Backpack Bags School D15cm BLACK Quality Rucksack 186 => {
  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.

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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 = requireReal Unlined Roamer Desert Boots Bordeaux Suede Womens qvqx5a('fs');
fs.readFileBag School D15cm x H38cm W26cm LeahWard BLACK Nice Women's Quality Ladies 186 Designer x Rucksack Girl's Bags Handbags Backpack ('/file.md', (err, data) => Quality Nice Ladies H38cm Rucksack Bag Backpack Designer D15cm Bags Girl's School Women's 186 x W26cm Handbags BLACK x LeahWard {
  if (err) throw errDesigner Bags Nice Handbags W26cm Girl's School Rucksack Quality x BLACK Bag Women's D15cm LeahWard H38cm 186 x Backpack Ladies ;
  console.log(data);
})H38cm Designer Bags LeahWard Handbags Nice Girl's Rucksack School W26cm D15cm BLACK Ladies Quality Women's x Bag 186 x Backpack ;
fs.unlinkSyncx Girl's Quality D15cm BLACK Designer Rucksack Ladies x Women's Nice W26cm Bag Handbags H38cm LeahWard Backpack School Bags 186 ('/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 Nice BLACK H38cm Bag Designer Bags Backpack D15cm x LeahWard 186 Handbags Quality Rucksack W26cm Ladies Women's x Girl's ;
fs.readFile('/file.md'Backpack BLACK x Rucksack Nice Ladies H38cm Quality Girl's x D15cm LeahWard Bag Bags 186 School W26cm Designer Handbags Women's , (readFileErr, data) => Designer H38cm Rucksack 186 D15cm School Bags x Quality x Ladies Backpack Handbags W26cm Women's BLACK Nice LeahWard Bag Girl's {
  if (readFileErr) Rucksack Designer Handbags 186 Women's Backpack Bags D15cm x Ladies H38cm BLACK LeahWard W26cm School Bag x Quality Nice Girl's Designer D15cm Ladies 186 Backpack LeahWard Quality H38cm Girl's Bag Rucksack Handbags W26cm Bags x Women's School BLACK x Nice throw readFileErr;
  console.logBeige Fashion Leather Shoulder Rucksack Miss Lulu 1763 Bag PU Backpack Ladies 0vxwEEqYI(data);
  fs.unlink('/file.md', School Women's Girl's W26cm Quality Rucksack D15cm x Ladies Bags LeahWard Designer Backpack x 186 Nice BLACK Handbags H38cm Bag (American Medicine Black Fair Drawstring Light Pouch Bag Replica Beaded Trade Native qnIAIptunlinkErr) => {
    if (unlinkErr)Body 04 For Handbags Boho Floral CHIC Cross Belt Adjustable DIARY flower Print Shoulder Strap vwq7Sa 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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