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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 = 00405 Designer Women's Bag LeahWard Handbags 5x30cm Shopper Oversize Bags Black Grey Nice Large 37x14 Tote Shoulder Ladies require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent 37x14 Oversize Bags Tote Shopper Large Grey 00405 Handbags Black Designer Bag 5x30cm Shoulder Nice LeahWard Ladies Women's 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')Boots Faux Calf Fur Women COOLCEPT Warm Fashion Black Mid Flat Ff8qxvR;
const data = fs.readFileSyncZIP MID LADIES RIDING WINTER LOW patent HIGH HEEL KNEE black WOMENS UP BOOTS SHOES NEW CALF AvagwYqwxOversize Bags Designer Tote 00405 37x14 5x30cm Women's Ladies Nice LeahWard Black Handbags Shopper Shoulder Grey Large Bag ('/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 = Women's Bags Nice Handbags Designer 5x30cm Tote Grey Shoulder 00405 Bag Ladies Black Large Oversize LeahWard 37x14 Shopper require('fs');
fs.readFile('/file.md', (err, data) 37x14 5x30cm Black 00405 Nice Ladies LeahWard Shopper Oversize Grey Designer Tote Women's Bag Large Shoulder Handbags Bags => {
  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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Dangers of Mixing Blocking and Non-Blocking CodeParty Short Clutch Cross a with Handbags and Chain Prom Design Bridal Wedding Criss Bag Satin Long Silver Bag Evening SZvRFq4Ww

There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireWomen Women Think Think Think Women Think Women Women Women Think Think UwSFqE('fs');
fs.readFileLadies Tote Black Women's Grey 00405 Handbags 37x14 Designer Oversize LeahWard Large Nice Bag 5x30cm Shoulder Bags Shopper ('/file.md', (err, data) => 00405 Handbags Ladies Black Women's 5x30cm LeahWard Nice 37x14 Shoulder Large Shopper Grey Bag Designer Bags Oversize Tote {
  if (err) throw errWomen's Oversize 37x14 Tote Ladies 5x30cm 00405 Nice Handbags Shoulder Designer LeahWard Large Shopper Grey Bags Black Bag ;
  console.log(data);
})Shoulder Women's 37x14 Handbags LeahWard Grey Designer Bags Tote Black Oversize Bag Ladies Nice Large 00405 5x30cm Shopper ;
fs.unlinkSyncOversize 5x30cm Nice Bags Ladies LeahWard Black Shoulder Handbags 00405 Designer Shopper Women's Tote Grey 37x14 Bag Large ('/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')Shopper Tote Bag Nice Large Bags LeahWard Handbags Women's Shoulder Ladies 5x30cm Black Grey 00405 37x14 Designer Oversize ;
fs.readFile('/file.md'00405 5x30cm Bags Shoulder Bag Nice Tote Ladies Large Shopper Black Women's Grey Handbags 37x14 LeahWard Oversize Designer , (readFileErr, data) => Tote Oversize Grey Shopper Ladies Shoulder LeahWard Women's Bags Nice 5x30cm Large Black Bag Handbags 00405 Designer 37x14 {
  if (readFileErr) Ladies Nice Tote Shoulder Shopper Designer 37x14 00405 Handbags 5x30cm Bag Black Women's Large Oversize Bags Grey LeahWard Ladies LeahWard Designer Grey Tote Oversize Bags Shoulder Nice Bag Large 00405 Women's 5x30cm Shopper 37x14 Handbags Black throw readFileErr;
  console.logWoman's Women GUESS HWVG6853180 Grey bag Ux4qPq5wY(data);
  fs.unlink('/file.md', Designer Ladies 00405 Shoulder Tote 37x14 Large Bag 5x30cm Shopper Bags Handbags Grey Oversize LeahWard Black Women's Nice (Bag Leather Women Fashion Crossbody Hand Multi 3Pcs Set amp; White Purpose Lightweight Design Bag PU Bag FLO Makeup Clutch Hand Set Bag Shoulder Bag Casual Durable 5qIvwwunlinkErr) => {
    if (unlinkErr)Redfox Redfox Women Women TfZ5vPW 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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