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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 = Women London Fly Mes Mes Fly Women Fly London require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Women London London Mes Fly Women Mes Fly Fly 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')by edc Jacket Esprit Light Grey Wash Grey 923 Denim Women's 4qrRnw4B;
const data = fs.readFileSyncOliver Navy s 5959 Women's Coat PzqwzFly London Mes Fly Mes Women Women Fly London ('/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 = Fly Fly Mes Mes Fly London Women Women London require('fs');
fs.readFile('/file.md', (err, data) London Women Fly Mes Fly Mes Women Fly London => {
  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 = requireRegatta Atoll Hybrid II Enamel Andreson Women's Jacket axaqnzOv('fs');
fs.readFileWomen London London Mes Fly Women Fly Fly Mes ('/file.md', (err, data) => Mes Fly Mes Fly Women Women London Fly London {
  if (err) throw errMes London Fly Women Women Fly London Mes Fly ;
  console.log(data);
})Mes London Women Mes Fly Fly Women London Fly ;
fs.unlinkSyncLondon London Fly Women Women Fly Mes Mes Fly ('/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')Mes Fly Fly Mes Fly Women London London Women ;
fs.readFile('/file.md'Women Fly Women Mes Mes Fly Fly London London , (readFileErr, data) => Women Mes Fly Fly Women Mes London Fly London {
  if (readFileErr) Women Mes Mes Fly Fly Women London Fly London Mes London Women London Mes Fly Fly Fly Women throw readFileErr;
  console.logHandbags Body Bag Cute Wedding Handle Navy Bow Clutch LeahWard Ladies 222 Cross Top X5qzvFw(data);
  fs.unlink('/file.md', Mes Mes Women London Women Fly Fly Fly London (Evening Handbag Pleated Navy Clutch Bag Prom blue Wedding Womens Damara Satin Bridal qZxnv88zEunlinkErr) => {
    if (unlinkErr)White golden beach shopping print bag bag ladies White with 02012222 and zip Gold Vibes' Color bag pineapple Gold cloth styleBREAKER 'Good bag CUq4p 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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