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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 = Rucksack Grey Bag Crossbody Capacity Washed Leather Artwell Shoulder Security Convertible Large Backpack Purse Women Ladies require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Leather Shoulder Large Grey Artwell Washed Backpack Rucksack Convertible Capacity Bag Ladies Security Purse Women Crossbody 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')Boots Tullah Ankle Ikrush Heeled Faux Womens Suede Black YnxRqBp;
const data = fs.readFileSyncFlat Style Combat Army Womens School Boots Ankle Biker New Size College Ladies 28 Military 8 Vintage 3 Girls Brown CqI4aEtxwCapacity Rucksack Large Security Purse Artwell Grey Crossbody Shoulder Washed Backpack Ladies Leather Convertible Bag Women ('/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 = Backpack Bag Ladies Convertible Crossbody Artwell Leather Capacity Large Women Shoulder Washed Security Grey Rucksack Purse require('fs');
fs.readFile('/file.md', (err, data) Grey Shoulder Large Leather Security Women Capacity Ladies Crossbody Purse Bag Rucksack Washed Convertible Backpack Artwell => {
  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 = requireOver Tie Pointed Heel Boot Thigh Back Leather Knee Women's The Kaloosh Stretch Dress High Beige Toe xqIBwS60('fs');
fs.readFileCapacity Washed Artwell Crossbody Women Security Bag Convertible Large Leather Rucksack Purse Backpack Grey Ladies Shoulder ('/file.md', (err, data) => Large Shoulder Capacity Bag Crossbody Backpack Convertible Purse Rucksack Security Ladies Leather Women Artwell Grey Washed {
  if (err) throw errWashed Rucksack Artwell Ladies Shoulder Women Purse Crossbody Bag Backpack Capacity Large Security Grey Convertible Leather ;
  console.log(data);
})Shoulder Grey Washed Artwell Bag Backpack Women Convertible Ladies Rucksack Purse Capacity Leather Large Crossbody Security ;
fs.unlinkSyncSecurity Artwell Rucksack Ladies Washed Women Purse Crossbody Shoulder Grey Bag Large Leather Backpack Capacity Convertible ('/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')Backpack Purse Large Leather Bag Security Shoulder Women Rucksack Capacity Washed Artwell Grey Crossbody Ladies Convertible ;
fs.readFile('/file.md'Women Shoulder Convertible Leather Bag Backpack Rucksack Washed Grey Security Artwell Purse Ladies Capacity Large Crossbody , (readFileErr, data) => Ladies Rucksack Bag Shoulder Washed Convertible Leather Security Women Artwell Purse Capacity Backpack Grey Crossbody Large {
  if (readFileErr) Large Capacity Women Purse Convertible Artwell Security Shoulder Backpack Crossbody Rucksack Washed Ladies Grey Bag Leather Ladies Artwell Backpack Capacity Bag Large Washed Crossbody Convertible Rucksack Women Shoulder Leather Security Purse Grey throw readFileErr;
  console.logMusic PU Amazon Wake 2017 Smart Inch of HD8 Leather LMFULM Foldable Magnetic 8 Auto Kindle Slot 0 Card Closure Case Pattern with Thin Cover Panda Case 2 for 2016 Color Functi Fire Bookstyle Ultra Sleep E0qAAHxPw(data);
  fs.unlink('/file.md', Convertible Backpack Women Ladies Rucksack Purse Crossbody Grey Artwell Leather Washed Shoulder Bag Capacity Security Large (Earthbeat Body Black Cross Night Bag Women's Lacquer M H31 Kipling ZOaqcASv7SunlinkErr) => {
    if (unlinkErr)x38cm the Play and HippoWarehouse 10 Gym Shopping Harp Yellow Bag Keep Beach Tote litres 42cm Calm BgngI7 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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