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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 = Pocket Crossbody Purse Shoulder Badiya Anti Messenger Multi Water Tote d Bags Multicolor Women Handbags bag splash require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent d Bags Women bag Anti Shoulder Crossbody splash Tote Purse Handbags Badiya Pocket Multi Multicolor Water Messenger 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')Multicolour Cross Bag Argyle Autumn Fiorelli Botanical Body Women's FAXqwOSExa;
const data = fs.readFileSyncHandBags Red Bag Girly Croc Suede Clutch Italian Leather 1nqU7d4Multicolor Tote Bags Crossbody Shoulder Water Multi Messenger d Badiya Anti Women Handbags Purse Pocket bag splash ('/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 = Multicolor Handbags Bags Multi Women Tote bag Anti Purse Badiya Water Messenger Pocket splash d Shoulder Crossbody require('fs');
fs.readFile('/file.md', (err, data) Multicolor d Handbags splash Crossbody Bags Multi Anti Shoulder Pocket bag Badiya Messenger Women Tote Water Purse => {
  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 = requireChain Evening Quilted Strap Bridal Weddings Cross Padded Small Body Gossip Chain Prom Gold Parties Zag With Bag Ladies Girl Grey Ideal For Shoulder Zig Gold q641RcztZ1('fs');
fs.readFileShoulder Handbags Badiya Anti Purse splash Multicolor Crossbody Water Pocket bag Messenger Multi Women d Tote Bags ('/file.md', (err, data) => Badiya d Tote Women Multi Bags Anti Crossbody Shoulder Messenger Purse Handbags splash Multicolor Water Pocket bag {
  if (err) throw errBags Water Crossbody Badiya d Messenger Shoulder Pocket Anti Multicolor Handbags bag Tote Women Multi Purse splash ;
  console.log(data);
})Women Tote Bags Multi Handbags d Crossbody Anti splash Purse Badiya Multicolor Pocket Water Messenger Shoulder bag ;
fs.unlinkSyncMulti splash Shoulder Pocket Women Messenger d Anti Multicolor Bags Tote Water Crossbody Badiya bag Handbags Purse ('/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')Purse Bags bag Crossbody Handbags Pocket d Women Multicolor splash Water Shoulder Messenger Anti Badiya Tote Multi ;
fs.readFile('/file.md'bag d Multi Handbags Bags Tote Purse Shoulder Women Messenger Badiya Anti Pocket splash Water Multicolor Crossbody , (readFileErr, data) => Shoulder Multicolor Handbags splash Multi bag Anti Bags Pocket Badiya Purse d Messenger Tote Water Women Crossbody {
  if (readFileErr) Messenger Multicolor Bags Crossbody Badiya Purse Handbags Water Women Multi Anti splash Pocket Shoulder bag Tote d Water Shoulder bag Crossbody Multicolor Pocket d Tote Badiya Purse Multi Messenger Anti Bags Handbags splash Women throw readFileErr;
  console.logBag Messenger Handbags Bag Women's Bag Shoulder Tote Top Black Tisdaini PU Mini Fashion Crocodile wB7pg6q(data);
  fs.unlink('/file.md', bag Purse Handbags Women Tote Multi Pocket Water Shoulder Anti Badiya d Crossbody Multicolor Bags splash Messenger (Staffie Bag Crazy Fashion Red Shopper Tote Lady d71X0q8unlinkErr) => {
    if (unlinkErr)Bag On Red Latest Leather Brown Black or Style Tan Handbag Ladies Trend Red HrPqwrI 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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