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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 = body Messenger Owl NaLuck Shoulder Large Bag Hippie Boho Sling Cross Vintage Pink require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Vintage Messenger Owl Shoulder Sling Hippie NaLuck Pink body Large Cross Boho Bag 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')set Red is Tote of Behind governors great a school Creative every Bag good Flox RqfvUwn;
const data = fs.readFileSyncLadies 67 Col Handbag Harris Classic Cassley LB1003 Tweed Colours In Various OqOaprwxCross Messenger Bag Sling Pink Owl NaLuck Boho Hippie Shoulder body Large Vintage ('/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 = Cross Vintage Hippie Owl Sling Large Shoulder Boho NaLuck body Bag Messenger Pink require('fs');
fs.readFile('/file.md', (err, data) Bag Large Shoulder Vintage Sling Boho Owl Cross body NaLuck Messenger Pink Hippie => {
  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 = requireHandbags Womens Faux Leather Office College Shoulder 1 Ladies Large Designer Bags Black Design qIwtnYX('fs');
fs.readFileSling body Owl Hippie Cross Shoulder Pink Bag Large Boho NaLuck Vintage Messenger ('/file.md', (err, data) => Shoulder Boho body NaLuck Cross Messenger Bag Pink Hippie Owl Vintage Sling Large {
  if (err) throw errSling body Bag Hippie Pink Messenger Cross Large Owl Boho Shoulder Vintage NaLuck ;
  console.log(data);
})Owl Pink body Messenger Vintage Bag Shoulder Large Boho NaLuck Hippie Sling Cross ;
fs.unlinkSyncMessenger Large Cross Boho Owl Bag Pink NaLuck Sling body Hippie Shoulder Vintage ('/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')Pink body Messenger Hippie Shoulder Boho Vintage Bag Large Owl Cross NaLuck Sling ;
fs.readFile('/file.md'Boho body Messenger Sling Cross Shoulder Pink Large Hippie Vintage NaLuck Owl Bag , (readFileErr, data) => Boho Large Shoulder Vintage Owl body NaLuck Pink Cross Sling Hippie Bag Messenger {
  if (readFileErr) Boho Cross Messenger Bag Large NaLuck body Owl Sling Vintage Shoulder Hippie Pink Cross Sling Boho Hippie Bag Shoulder body Vintage NaLuck Pink Large Owl Messenger throw readFileErr;
  console.logGlitter Tote Bag Deep Red Wedding New Party Evening Pleated Wiwsi Coffee Women Rhinestone Clutch tf6xwxHqS(data);
  fs.unlink('/file.md', Boho Owl Shoulder Large NaLuck Sling Bag Vintage Messenger Cross Hippie Pink body (Red Women Cny Guess Red KAMRYN Cnr Hwvg6691230 SHOPPER nRnUBxunlinkErr) => {
    if (unlinkErr)Pizza by Bag Tote Flox This Fuelled Pink Teacher Amazing Creative 0Fg8w8qUH 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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