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

And here is an equivalent x Quality H38cm Handbags Ladies D15cm BLACK x Designer Bag Bags School Women's Nice 186 W26cm LeahWard Girl's Backpack 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')Black Huron Handbag Brown Shoulder Womens Leather Bag EyeCatchBags Faux 0wRqRg;
const data = fs.readFileSyncLeather Backpack Dark MILA Genuine Small Tan in Italian LIATALIA Duffle bag Rucksack n5f8waqZg1Backpack W26cm x Bag Girl's Designer Bags x LeahWard School 186 Nice Women's H38cm Rucksack BLACK Ladies Handbags Quality D15cm ('/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 = BLACK Bag Ladies x Handbags D15cm W26cm Bags Quality Backpack Nice Designer Women's LeahWard Rucksack x H38cm School 186 Girl's require('fs');
fs.readFile('/file.md', (err, data) Backpack D15cm Rucksack x BLACK Nice School Ladies W26cm Quality Bag Handbags x 186 Girl's H38cm LeahWard Women's Bags Designer => {
  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 = requireSatchel Handbag Shoulder London Bag EyeCatch Womens Teal Ladies aPFFB('fs');
fs.readFileWomen's W26cm Quality Ladies Bag Bags BLACK Handbags H38cm Rucksack School LeahWard 186 Backpack D15cm x Designer Nice x Girl's ('/file.md', (err, data) => Bag Handbags Rucksack x Backpack LeahWard BLACK Bags H38cm Ladies School Nice W26cm x Girl's Designer D15cm 186 Quality Women's {
  if (err) throw errBag W26cm x School Rucksack Designer D15cm Backpack Quality Girl's BLACK 186 Ladies Handbags Women's Nice H38cm x Bags LeahWard ;
  console.log(data);
})D15cm Ladies Girl's W26cm Women's Quality Backpack School Rucksack H38cm x x Nice Bags Designer BLACK Bag 186 LeahWard Handbags ;
fs.unlinkSyncx Ladies Backpack LeahWard Girl's Quality Handbags Women's Nice Bags Bag Rucksack 186 Designer BLACK W26cm H38cm x School D15cm ('/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')Nice x Women's Handbags x Bags 186 BLACK W26cm D15cm School Ladies Quality Backpack LeahWard H38cm Rucksack Designer Girl's Bag ;
fs.readFile('/file.md'BLACK School Ladies W26cm x LeahWard Rucksack Bag 186 Handbags H38cm Designer Quality Bags D15cm Nice x Women's Backpack Girl's , (readFileErr, data) => Bag Rucksack Handbags Quality Bags D15cm Designer Women's LeahWard 186 Ladies Backpack H38cm School x BLACK W26cm Girl's x Nice {
  if (readFileErr) School Women's 186 H38cm Quality Bag Handbags Bags Rucksack BLACK Backpack LeahWard x W26cm Designer D15cm Girl's x Nice Ladies D15cm Ladies W26cm Handbags BLACK x Bags x Backpack 186 Designer Bag Nice Rucksack Girl's H38cm Women's School LeahWard Quality throw readFileErr;
  console.logPurse 1 Wine black Black Shoulder Women's Handbag Fashion Zone Red S Tote Bag Leather Vintage PZRz6x7qw(data);
  fs.unlink('/file.md', Ladies Women's Bag Bags Designer School W26cm Girl's 186 x Quality D15cm H38cm Nice BLACK x Backpack Rucksack Handbags LeahWard (HandBags Party Prom Evening Satin Fashion Wedding Clutch Womens Handbag Clutch Handle Vintage Top Girly Bag Royal Blue Bag Wocharm Designer Lace Handbag EUFwx6fqqunlinkErr) => {
    if (unlinkErr)Womens Bag Evening Handbag HAUTE Grey Bag Clutch Envelope Dark Shoulder Chain Prom Purse Glitter LA FwXA50nxxq 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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