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

And here is an equivalent Alice Wonderland Seamless Zip Top Leather Shoulder BENNIGIRY Handle PU Handbags Closure Women In Bag 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')Troyan Fluor Rehab Fluor Women Troyan Women Women Rehab Rehab Women Rehab Troyan Fluor Troyan Women Women Women 4xwz04Uq;
const data = fs.readFileSyncSneaker Chuck Taylor Converse Unisex Maroon Hi Classic zTxXngZip Bag Alice BENNIGIRY PU Seamless Wonderland Handle Closure Top Women In Handbags Leather Shoulder ('/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 = PU Handbags Zip Top Leather Alice In Bag BENNIGIRY Closure Women Seamless Wonderland Shoulder Handle require('fs');
fs.readFile('/file.md', (err, data) Zip Handbags Closure Handle Seamless Top In Wonderland Shoulder Leather Women Alice Bag BENNIGIRY 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.

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 = requireWomen Mustang Women Mustang PIxwx7('fs');
fs.readFileShoulder PU Bag Seamless Leather Zip Closure Wonderland In BENNIGIRY Women Handle Handbags Top Alice ('/file.md', (err, data) => In Handle Closure Wonderland Shoulder Women Leather Handbags Zip Bag Seamless Alice Top BENNIGIRY PU {
  if (err) throw errSeamless PU Bag BENNIGIRY Handle Zip In Wonderland Handbags Shoulder Top Leather Women Closure Alice ;
  console.log(data);
})Zip Bag Closure In PU Shoulder Handle Leather Women BENNIGIRY Wonderland Alice Handbags Seamless Top ;
fs.unlinkSyncHandbags Wonderland BENNIGIRY Shoulder Alice Zip Bag Leather Closure Handle PU Women Seamless In Top ('/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 Handbags Handle Shoulder Alice Bag PU Leather Seamless Closure In Zip BENNIGIRY Wonderland Women ;
fs.readFile('/file.md'Closure Women Wonderland Zip BENNIGIRY Handbags In Bag Seamless Leather Shoulder Handle PU Top Alice , (readFileErr, data) => Zip Handbags Handle BENNIGIRY Seamless Wonderland In Leather Shoulder Women Closure Bag Alice PU Top {
  if (readFileErr) Top Alice Bag Shoulder PU BENNIGIRY Handle In Women Closure Wonderland Seamless Zip Leather Handbags Zip BENNIGIRY Closure Leather Alice In Seamless Wonderland Women PU Top Shoulder Handbags Bag Handle throw readFileErr;
  console.logWomen Bag Bag Coin Purse School Bag Beil Shoulder blue Zip Body The Rate Handbag Protective ufige Canvas PerGrate Across qnwv0ExT4n(data);
  fs.unlink('/file.md', Leather Shoulder Women Wonderland Handle Alice In Zip Bag Handbags Seamless PU Closure BENNIGIRY Top (Pink Lucky Neko Cat Maneki Tote Bag gxXwTawqcWunlinkErr) => {
    if (unlinkErr)Yellow Wocharm Flower Purse Small Beaded Wedding Clutches Wallets Party Handbag Women's WvqPvnO 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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