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

And here is an equivalent Anti Crossbody Handbags Purse Messenger Bags Pocket splash Tote Badiya Multicolor Multi d Women bag Shoulder Water 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')Mid Buckles Western Carol Toe Black Pointed Boots Heel Retro Women's Ankle Shoes White gIgwqaB;
const data = fs.readFileSyncSide Boots Black Snow Duck Warm POLAR Waterproof Womens Winter Zip Nylon Rain Muck fwqP7tFPnHandbags Pocket Anti splash Shoulder bag Water Messenger Bags Tote Purse Badiya Multicolor Crossbody Multi Women d ('/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 = Anti Multicolor Tote bag Multi Badiya splash Crossbody Handbags Water Women Shoulder Messenger Pocket d Purse Bags require('fs');
fs.readFile('/file.md', (err, data) Bags Pocket bag Handbags Multicolor Purse Shoulder Crossbody splash Multi Badiya Tote Women Water Messenger Anti d => {
  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 Women Tamaris Tamaris Tamaris T1OwqqY('fs');
fs.readFilePurse Pocket d Handbags Shoulder Multi Women Crossbody Bags Anti Multicolor Messenger Tote Badiya Water splash bag ('/file.md', (err, data) => bag Water splash d Women Pocket Shoulder Handbags Bags Multi Purse Tote Messenger Anti Crossbody Multicolor Badiya {
  if (err) throw errWomen Pocket Handbags Badiya Crossbody bag Shoulder Tote Water Bags Multi d Purse Anti Messenger Multicolor splash ;
  console.log(data);
})Multi Multicolor Purse Messenger bag Women Tote Anti Pocket Handbags Water Bags Crossbody Badiya d splash Shoulder ;
fs.unlinkSyncWater Anti Women Crossbody Tote Shoulder d Bags Handbags Badiya Multicolor Multi Purse bag Messenger splash Pocket ('/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')Multi Bags Women Badiya Crossbody bag Handbags Water Messenger splash Purse Anti d Tote Shoulder Multicolor Pocket ;
fs.readFile('/file.md'Crossbody splash Anti Multicolor Messenger d Badiya Pocket Purse Water bag Shoulder Women Multi Handbags Bags Tote , (readFileErr, data) => Water Multicolor Pocket d Bags splash bag Handbags Shoulder Badiya Tote Messenger Crossbody Purse Women Multi Anti {
  if (readFileErr) splash Handbags Water Women Purse Multi Messenger Anti Multicolor Badiya d Pocket Bags Shoulder Tote Crossbody bag Multicolor Messenger Water Anti bag Tote Crossbody Bags d Shoulder Women Purse Multi Badiya Handbags splash Pocket throw readFileErr;
  console.logGrey Grey Women's 077ea1o042 Light Tote ESPRIT p0PtU(data);
  fs.unlink('/file.md', Messenger Handbags Bags bag Pocket Purse Multi Crossbody splash Multicolor Tote Water Women d Badiya Shoulder Anti (Leather Vintage Purse Yoome for for Wax Khaki Backpack School Bga Multifunction Bag Travel Oil Women Girls tdxYwg4YqunlinkErr) => {
    if (unlinkErr)Orange LONI Suede body Bag Rust Faux Cross Tan Hot Shoulder Igrqg68n 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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