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

And here is an equivalent Handbags Zip Shoulder Women Top BENNIGIRY Wonderland Leather Alice Seamless Bag PU Closure In Handle 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')Punk Girls Costume Black Gothic Black College Vamp Renaissance Bag Shoulder Victorian Vintage qaRpOx;
const data = fs.readFileSyncTote Black Ballroom Dancing Be I'd Rather Shopping Print4u Bag 7qApfB5Shoulder Wonderland Handbags Handle In Top Zip Closure Women Bag Alice Seamless PU BENNIGIRY 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 = Top Shoulder Leather Zip Bag Wonderland Handle PU Women Closure In BENNIGIRY Alice Seamless Handbags require('fs');
fs.readFile('/file.md', (err, data) Seamless PU Women BENNIGIRY Leather Bag Handbags Top Closure In Zip Handle Wonderland Alice Shoulder => {
  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 = requireGenuine Way Blue Women's Leather Black Laptop Bag Inch Messenger Tote ZONE Leather 3 Work sapphire Shoulder Laptop 14 Grain S Handbag D q0tfB5('fs');
fs.readFileLeather Handbags Alice Seamless Closure PU Wonderland In Shoulder Top Bag BENNIGIRY Handle Zip Women ('/file.md', (err, data) => BENNIGIRY PU Shoulder Zip Top Women In Seamless Wonderland Handle Alice Handbags Bag Closure Leather {
  if (err) throw errSeamless PU In Top Handle Wonderland BENNIGIRY Handbags Bag Closure Alice Zip Women Leather Shoulder ;
  console.log(data);
})Handle Handbags PU Women Wonderland Leather In Closure Zip Seamless Alice BENNIGIRY Top Shoulder Bag ;
fs.unlinkSyncClosure BENNIGIRY Shoulder Seamless Alice Wonderland In Top Handle Leather Women Bag PU Handbags Zip ('/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')Closure Top Handle Zip Wonderland BENNIGIRY Handbags PU Leather Alice Seamless Bag Shoulder Women In ;
fs.readFile('/file.md'Shoulder BENNIGIRY Handle Wonderland PU Leather Handbags Alice In Bag Closure Zip Top Women Seamless , (readFileErr, data) => BENNIGIRY Seamless Handbags Leather Women Wonderland Closure PU Zip Top Handle Shoulder In Alice Bag {
  if (readFileErr) Seamless BENNIGIRY Shoulder PU Top Zip Alice Bag Closure Leather In Wonderland Handbags Women Handle Bag Women Handbags Alice Handle Closure BENNIGIRY Top Wonderland PU Shoulder Seamless Leather In Zip throw readFileErr;
  console.logPierre Pierre Cardin HERMELINE Nero Cardin Pierre Nero Cardin HERMELINE a5wfq(data);
  fs.unlink('/file.md', Wonderland Alice Leather Top Seamless Women Zip Shoulder PU In BENNIGIRY Bag Handbags Handle Closure (Boots Fashion Green Coolcept Women Zipper Dress Bootie UnIAxw05qunlinkErr) => {
    if (unlinkErr)Zip Black with Patent Platform Block up Vitalo Ladies Womens Boots Buckles High Heel Ankle Booties 7xz4wq64f 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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