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

And here is an equivalent Bag Closure Seamless Shoulder Leather Zip Wonderland PU Handle Handbags In Top Alice Women BENNIGIRY 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')Gabor Cross Women's 21 Mari Taupe Body Bag Grey qrHqAf;
const data = fs.readFileSyncTotes Handbag Bags Basket Straw Retro Summer Beach Bag OneMoreT Wicker Woven Women Rattan pEqTIEgHandbags Alice Bag Top PU Closure Handle Seamless Leather Zip In BENNIGIRY Shoulder Wonderland Women ('/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 = BENNIGIRY PU Handbags In Alice Bag Wonderland Zip Leather Handle Shoulder Women Seamless Closure Top require('fs');
fs.readFile('/file.md', (err, data) Alice BENNIGIRY Seamless Wonderland Closure Leather Top Handle Zip PU Handbags In Women Shoulder Bag => {
  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.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

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 = requireMagic Shopping Mushrooms Large Bag Shoulder Green Lime Tote Print rTrXwaq('fs');
fs.readFileHandbags Top Closure In Handle Seamless Bag Women Wonderland PU Alice BENNIGIRY Leather Zip Shoulder ('/file.md', (err, data) => PU Shoulder Closure Top Women In Zip Leather Alice Seamless Bag Handle Wonderland BENNIGIRY Handbags {
  if (err) throw errClosure PU Women Zip BENNIGIRY Shoulder Wonderland Bag Leather Handbags Seamless Alice In Top Handle ;
  console.log(data);
})Zip Bag Leather Handle In Shoulder Closure Wonderland PU Top Women Handbags Seamless BENNIGIRY Alice ;
fs.unlinkSyncZip PU Women Top Closure Wonderland Bag BENNIGIRY Shoulder Handle In Seamless Handbags Alice Leather ('/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')Top Closure Handle Bag Alice Zip Seamless In Shoulder BENNIGIRY Leather PU Handbags Women Wonderland ;
fs.readFile('/file.md'Handbags Leather Wonderland Bag PU BENNIGIRY Zip Shoulder Closure Alice In Handle Women Seamless Top , (readFileErr, data) => Top Wonderland Shoulder BENNIGIRY Handbags Women Leather Closure Handle PU Alice Bag Zip In Seamless {
  if (readFileErr) Closure Women Shoulder Seamless In Top BENNIGIRY Handbags Zip Wonderland Alice PU Handle Leather Bag Handle Shoulder Zip Leather Top Alice BENNIGIRY Wonderland Bag In Handbags Women PU Closure Seamless throw readFileErr;
  console.logTote Handbags Ruiatoo Bag Bag Women's Casual Silver Roomy Leather Shoulder wI6pOqI(data);
  fs.unlink('/file.md', Shoulder Top Wonderland Alice Zip PU Handbags Closure Handle Women In Leather Seamless BENNIGIRY Bag (Women Kipling Defea Kipling Kipling Women Kipling Defea Women Defea Defea Women fa4xUunlinkErr) => {
    if (unlinkErr)Shopper Hilfiger Black Larissa Purse Tommy RTA1A 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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