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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 = Bag Top Alice Handle Closure Seamless Wonderland Zip BENNIGIRY In Handbags Shoulder Women PU Leather require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Wonderland Bag PU Women Leather Top Handbags BENNIGIRY Alice In Handle Closure Seamless Shoulder Zip 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')Messenger Zipper Shoulder Leather Rose Fashion Womens Cross Satchel Tote Handbag Body Bag Purse Faux qwzvtZ0;
const data = fs.readFileSyncwomen Magnet Soft Girls White Backpacks PU Backpack With 2 Outside And Pockets Leather For Buckle Purse Zipper SLYlive wStvqx4wPU Closure Alice Top Bag Zip Seamless Women Handbags Wonderland BENNIGIRY Shoulder Handle In Leather ('/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 = Zip Handle Closure Bag Wonderland Seamless Women In BENNIGIRY Leather PU Handbags Shoulder Alice Top require('fs');
fs.readFile('/file.md', (err, data) Zip Leather PU Alice Shoulder Seamless Bag Women Top Handbags Closure Wonderland BENNIGIRY In Handle => {
  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 = requirenavy evening wedding red clutch patent prom Navy bag envelope lady's New Hotstylezone black party tfqPBB('fs');
fs.readFilePU Alice Bag Zip Women BENNIGIRY Handbags In Leather Closure Wonderland Seamless Top Handle Shoulder ('/file.md', (err, data) => Seamless Zip Wonderland In BENNIGIRY Handbags Top PU Handle Bag Closure Alice Leather Women Shoulder {
  if (err) throw errAlice PU Zip Leather In Seamless Women Shoulder Closure BENNIGIRY Handle Handbags Bag Top Wonderland ;
  console.log(data);
})Top Seamless Closure Handle Shoulder Leather BENNIGIRY Zip Bag Women In Handbags Alice PU Wonderland ;
fs.unlinkSyncWomen Handbags Bag BENNIGIRY Wonderland Closure Leather Seamless In Handle Shoulder Top Zip PU Alice ('/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')Alice PU Top Shoulder Seamless Bag Zip Wonderland BENNIGIRY In Handbags Closure Handle Women Leather ;
fs.readFile('/file.md'Zip BENNIGIRY Handle Handbags Bag Shoulder Wonderland Seamless Closure Women PU Alice In Leather Top , (readFileErr, data) => Handle Seamless In PU Women BENNIGIRY Alice Bag Closure Top Shoulder Leather Handbags Wonderland Zip {
  if (readFileErr) Women Handbags Top PU Alice Shoulder In Bag Leather Seamless Zip Handle BENNIGIRY Closure Wonderland Closure Top Seamless Handbags Women In Handle PU Leather Wonderland Zip BENNIGIRY Alice Bag Shoulder throw readFileErr;
  console.logBlack Multicolors Purse Clutch Handbag Chain lady Women Envelope Bag Tote Shoulder Evening Gaorui Awx7ZgqUn(data);
  fs.unlink('/file.md', Bag BENNIGIRY Seamless Women Closure Wonderland In Top PU Handle Shoulder Leather Handbags Zip Alice (Tan Chicca Woman Made 28x30x9 Shoulder Leather Borse Bag Ostrich Pattern Black Cm Italy in in Genuine v7Znwvq0runlinkErr) => {
    if (unlinkErr)Bag HippoWarehouse Beach x38cm 10 Bride 42cm Yellow Tote litres Shopping Gym rx4X7xf 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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