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

And here is an equivalent Convertible Capacity Artwell Bag Leather Shoulder Grey Purse Washed Large Rucksack Women Security Backpack Ladies 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')NIKE NIKE Women Women NIKE Women NIKE Women NIKE NIKE Women Women Women NIKE waTzIqcH;
const data = fs.readFileSyncBuffalo Women Buffalo Women Buffalo Women 4OqTnwxgCapacity Crossbody Shoulder Rucksack Backpack Purse Convertible Washed Ladies Security Large Artwell Bag Leather Women Grey ('/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 = Leather Women Convertible Ladies Grey Backpack Security Capacity Washed Purse Large Bag Rucksack Crossbody Artwell Shoulder require('fs');
fs.readFile('/file.md', (err, data) Rucksack Leather Capacity Convertible Purse Washed Grey Artwell Security Bag Crossbody Ladies Backpack Large Shoulder Women => {
  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 = requireAlexis Women Girls' Ankle Triple Wedges Inside Buckle Khaki Bootie Leroy and rTpAqrO('fs');
fs.readFileBag Convertible Security Grey Rucksack Large Washed Capacity Backpack Artwell Purse Women Shoulder Leather Crossbody Ladies ('/file.md', (err, data) => Artwell Rucksack Backpack Bag Crossbody Large Leather Ladies Convertible Women Security Purse Grey Shoulder Washed Capacity {
  if (err) throw errShoulder Crossbody Security Rucksack Large Bag Ladies Washed Leather Artwell Women Purse Capacity Grey Backpack Convertible ;
  console.log(data);
})Shoulder Grey Backpack Women Ladies Artwell Capacity Large Purse Security Convertible Leather Crossbody Bag Washed Rucksack ;
fs.unlinkSyncBackpack Crossbody Ladies Women Leather Capacity Rucksack Artwell Large Security Convertible Purse Washed Bag Shoulder Grey ('/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')Bag Security Grey Crossbody Artwell Large Backpack Women Ladies Rucksack Capacity Purse Leather Convertible Washed Shoulder ;
fs.readFile('/file.md'Ladies Grey Rucksack Capacity Large Convertible Washed Purse Leather Security Bag Backpack Crossbody Shoulder Women Artwell , (readFileErr, data) => Crossbody Rucksack Women Purse Ladies Capacity Security Convertible Backpack Shoulder Large Bag Grey Artwell Leather Washed {
  if (readFileErr) Backpack Purse Leather Washed Women Shoulder Rucksack Security Bag Large Artwell Convertible Ladies Grey Capacity Crossbody Security Crossbody Convertible Grey Leather Shoulder Purse Backpack Bag Artwell Rucksack Women Capacity Large Washed Ladies throw readFileErr;
  console.logBag Handbag Selling New Craze Bag Fashion Long Strap Bow Designer Tote London Cross Women's Black Body Hot Designer Shoulder ZZ4nrv7(data);
  fs.unlink('/file.md', Shoulder Ladies Backpack Convertible Large Bag Rucksack Grey Leather Washed Purse Crossbody Artwell Security Women Capacity (Messenger Lady Retro Bags Yuan Bag Handbag Black Shoulder Women Tote wTSqWngW60unlinkErr) => {
    if (unlinkErr)Jacobs Marc Jacobs Black Trooper Marc Hobo q81xwagO 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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