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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 = Glitter Hype Backpack Bag Snake Silver Rucksack require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Snake Silver Rucksack Glitter Hype Bag Backpack 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')Women's Lady Mojito Brown Lady Mojito Women's Women's Mojito Brown Brown Lady PrqBUxP1w;
const data = fs.readFileSyncHeather Vaporous Rabbit Recon Women's North Grey The Face Backpack Light Grey zCX8tqtWRucksack Snake Glitter Backpack Silver Hype Bag ('/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 = Silver Rucksack Backpack Bag Hype Snake Glitter require('fs');
fs.readFile('/file.md', (err, data) Bag Hype Silver Rucksack Snake Glitter Backpack => {
  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 = requireKnee Wellington Wellies Ladies Rainbow 8 High Striped Tall Evercreatures Multicoloured 3 UK Boots qECUCBw('fs');
fs.readFileRucksack Hype Backpack Glitter Silver Bag Snake ('/file.md', (err, data) => Silver Backpack Snake Bag Glitter Hype Rucksack {
  if (err) throw errBag Snake Glitter Hype Backpack Silver Rucksack ;
  console.log(data);
})Backpack Rucksack Snake Hype Bag Silver Glitter ;
fs.unlinkSyncHype Silver Glitter Snake Rucksack Bag Backpack ('/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')Silver Snake Bag Hype Glitter Backpack Rucksack ;
fs.readFile('/file.md'Rucksack Glitter Snake Bag Hype Backpack Silver , (readFileErr, data) => Rucksack Snake Bag Backpack Hype Silver Glitter {
  if (readFileErr) Backpack Bag Silver Rucksack Glitter Hype Snake Hype Rucksack Snake Bag Silver Glitter Backpack throw readFileErr;
  console.logBag HandBags Satin Champagne Girly Clutch Red Lace ISpxq8wH(data);
  fs.unlink('/file.md', Rucksack Hype Bag Silver Snake Glitter Backpack (Gym Weekend HippoWarehouse Drinking Bag 42cm of Shopping Chance 10 Forecast Beach Coral litres a with x38cm Tote Netball 4SdvxSpunlinkErr) => {
    if (unlinkErr)Stent Fire Amazon Leather Function Foldable Closure Panda Holster Thin HD Case of Case Cover 10 Ultra Bookstyle PU for 10 Leather Shy Inch Magnetic Pattern Leather 2017 7 1 LMAZWUFULM with Pattern Cover Flip Tt4Fq 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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