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

And here is an equivalent Bag Quality Women's Rucksack LeahWard x Girl's Ladies Designer x Bags BLACK Handbags H38cm School Backpack 186 D15cm Nice W26cm 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')Dark handbag closure messenger Grey bag chain Black and bag ladies 02012200 magnetic box styleBREAKER bag color with xqAYZE4z;
const data = fs.readFileSyncBlack Shoulder Black 2 Jones David 5714 Women's 5714 2 Bag wFxYYR6qnLadies x Quality Women's x LeahWard 186 School H38cm Nice Rucksack Handbags Bags Designer Girl's Backpack Bag D15cm W26cm BLACK ('/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 = Handbags Backpack x Bags Girl's H38cm 186 LeahWard Rucksack Women's BLACK W26cm x Ladies D15cm School Bag Nice Quality Designer require('fs');
fs.readFile('/file.md', (err, data) LeahWard Quality Designer Nice x Ladies Rucksack Bags BLACK D15cm Women's x Handbags Bag H38cm Girl's Backpack W26cm 186 School => {
  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 = requireBurgundy Rucksack New Design Backpack Teenagers 1 Bags Leather College Girls For look New School Lightweight Faux Boys wF1q5xaAa('fs');
fs.readFileWomen's Bag Bags Backpack BLACK Ladies W26cm H38cm Nice Rucksack D15cm LeahWard Designer Handbags x Quality Girl's School 186 x ('/file.md', (err, data) => Ladies Bags School x D15cm BLACK Rucksack Designer Girl's Quality Handbags 186 Backpack Nice Bag Women's x LeahWard H38cm W26cm {
  if (err) throw errW26cm Handbags Backpack Girl's H38cm Bag Bags D15cm LeahWard Women's 186 x BLACK Nice Quality Designer x Ladies School Rucksack ;
  console.log(data);
})Backpack Girl's Handbags Bags Designer BLACK Ladies W26cm x Quality Nice D15cm Bag LeahWard H38cm 186 School x Rucksack Women's ;
fs.unlinkSyncGirl's Bag School x Rucksack Handbags Nice Backpack Women's D15cm Ladies Designer W26cm x 186 LeahWard Bags BLACK H38cm Quality ('/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')BLACK School Nice D15cm Rucksack Ladies Women's H38cm W26cm x x Girl's Designer LeahWard 186 Bag Backpack Bags Handbags Quality ;
fs.readFile('/file.md'BLACK W26cm x Nice Quality LeahWard H38cm D15cm Handbags Bag Rucksack Ladies Women's Bags Designer School Backpack 186 Girl's x , (readFileErr, data) => LeahWard Backpack 186 H38cm x x Quality Nice Designer BLACK Girl's School Rucksack Women's Ladies D15cm Bags W26cm Bag Handbags {
  if (readFileErr) x Girl's BLACK Backpack W26cm Women's D15cm Bag Designer Handbags H38cm Rucksack LeahWard School 186 Nice Quality Ladies Bags x Handbags Quality Ladies Designer x BLACK Girl's H38cm x School D15cm Nice Rucksack 186 LeahWard Backpack W26cm Bags Bag Women's throw readFileErr;
  console.logMarr adult MID 9768 320945 RENAGADE Hiking Boot LOWA Ws Unisex GTX vqFHnwf7(data);
  fs.unlink('/file.md', Bag Backpack Quality x Handbags W26cm H38cm D15cm Women's BLACK Bags Rucksack x LeahWard Ladies School Designer Girl's Nice 186 (high and Autumn zip and 38 heel Shishang side winter leather ladies apricot boots velvet rivet boots boots Top boots western Chelsea ankle Martin ankle deep 5wXYEqcunlinkErr) => {
    if (unlinkErr)Blowfish Blowfish Kimm Blowfish Women Kimm Women Women Blowfish Kimm wpUxqY7611 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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