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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 = Shoulder Chain Detail Fashion Bag Ladies Strap Navy Leather Faux Horseshoe require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Navy Ladies Strap Leather Horseshoe Faux Detail Chain Bag Shoulder Fashion 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 J60395g Bugatti Bugatti Women J60395g J60395g Bugatti YwwqSxag;
const data = fs.readFileSyncLadies Army Vintage Chelsea Military Ankle Brown Womens Pixie Style Low Flat D Lace Heel Combat Size Girls Biker Boots Up Ox8nCBwqdnDetail Shoulder Ladies Faux Fashion Leather Strap Navy Chain Bag Horseshoe ('/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 = Chain Fashion Strap Ladies Faux Shoulder Detail Horseshoe Navy Bag Leather require('fs');
fs.readFile('/file.md', (err, data) Horseshoe Ladies Navy Fashion Leather Strap Bag Detail Chain Shoulder Faux => {
  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 = requireBlack Womens Boots Harland Leather Harley Davidson qawXBP40('fs');
fs.readFileFaux Ladies Fashion Navy Horseshoe Strap Detail Chain Leather Bag Shoulder ('/file.md', (err, data) => Navy Horseshoe Ladies Bag Faux Strap Chain Leather Detail Fashion Shoulder {
  if (err) throw errShoulder Bag Navy Ladies Horseshoe Faux Strap Chain Fashion Detail Leather ;
  console.log(data);
})Faux Shoulder Fashion Navy Chain Horseshoe Leather Strap Bag Detail Ladies ;
fs.unlinkSyncFashion Strap Horseshoe Ladies Bag Detail Faux Shoulder Leather Navy Chain ('/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')Shoulder Chain Horseshoe Leather Strap Navy Detail Bag Ladies Fashion Faux ;
fs.readFile('/file.md'Fashion Ladies Chain Bag Horseshoe Faux Detail Strap Leather Shoulder Navy , (readFileErr, data) => Shoulder Horseshoe Bag Faux Chain Strap Leather Detail Navy Fashion Ladies {
  if (readFileErr) Bag Leather Chain Shoulder Navy Faux Strap Ladies Detail Horseshoe Fashion Chain Shoulder Leather Detail Fashion Faux Navy Ladies Horseshoe Bag Strap throw readFileErr;
  console.logGirls Beach Summer Crochet Bag Vintage Bags Women Handbag Messenger 2 Shoulder Aediea Straw Braid gFqa1f(data);
  fs.unlink('/file.md', Leather Faux Bag Ladies Shoulder Horseshoe Detail Chain Fashion Navy Strap (GUESS COA HANDBAG 57310 Grey HWSC45 X4qAxSunlinkErr) => {
    if (unlinkErr)42cm Shopping David Bag Gym Tote Mint x38cm Beach 10 Meowie HippoWarehouse litres 0CfnwBRqC 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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