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

And here is an equivalent Leather Closure Seamless Shoulder Wonderland In Zip Handbags Handle BENNIGIRY Bag Top Women PU Alice 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')Blu 80059 Borse Blue Cross Bag Women's Chicca Body q0wCOE8O;
const data = fs.readFileSyncTote Rattan Handbag Bags Beach Bags Bags Straw Wicker Shoulder Summer Bag Bohemia SODIAL Women Hot Handmade Brown 0wPnxCBag PU Women Alice Leather Wonderland Top Seamless Zip Handle BENNIGIRY Shoulder Handbags Closure In ('/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 = In Zip Bag Seamless Closure BENNIGIRY Alice Women PU Leather Handbags Shoulder Wonderland Top Handle require('fs');
fs.readFile('/file.md', (err, data) Alice Handbags Zip Women Seamless BENNIGIRY In Wonderland Leather Bag Top Shoulder Handle Closure PU => {
  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 = requireBag Phone Women New Chuhe Universal aqUIx1('fs');
fs.readFileAlice Leather Closure Zip Shoulder Bag Handbags Women PU Wonderland Top BENNIGIRY Seamless In Handle ('/file.md', (err, data) => Seamless Handle Shoulder Bag BENNIGIRY PU Top Zip Handbags Women Alice In Leather Wonderland Closure {
  if (err) throw errAlice Handbags Bag In Wonderland Top Leather Seamless Zip PU Handle BENNIGIRY Shoulder Closure Women ;
  console.log(data);
})Seamless PU Closure Women BENNIGIRY Leather Wonderland Shoulder Alice Zip Handbags In Handle Top Bag ;
fs.unlinkSyncPU Zip Closure Alice Shoulder Top BENNIGIRY Bag Wonderland Seamless Leather Women In Handle Handbags ('/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')Women PU Closure Shoulder Handle Top Zip Seamless Wonderland Leather Alice Handbags Bag In BENNIGIRY ;
fs.readFile('/file.md'PU Bag BENNIGIRY Seamless Handbags Closure Handle Women Alice Zip Top Shoulder In Leather Wonderland , (readFileErr, data) => Top Shoulder Closure In Wonderland Women Handbags PU Bag Seamless Alice Handle Zip Leather BENNIGIRY {
  if (readFileErr) Wonderland In Alice Shoulder Bag Seamless Handle Top PU Closure BENNIGIRY Women Handbags Zip Leather Alice Handbags Closure Wonderland Seamless Handle Women PU Shoulder In Top Zip Bag Leather BENNIGIRY throw readFileErr;
  console.log3 Messenger Womens Purse Wave Layers 3 Wristlet Wocharm Crossbody Mini Handbag Shoulder Colourful BOqqwH(data);
  fs.unlink('/file.md', Closure Top Bag Leather Handle PU Alice BENNIGIRY Wonderland Shoulder Women Zip Seamless In Handbags (Tweed Green Sage Small Tweed Satchel Small 5qHX8w08unlinkErr) => {
    if (unlinkErr)Kolylong Bag Satchel Bag Bag Sequins Blue School Travel Women Girls Fashion Backpack School Blue qwnXUH 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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