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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 = Bags Black Cellphone Fashion Messenger Ladies Women Body Handbag Bags Small Shoulder Slim Pouch Girls Crossbody Bags require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Messenger Pouch Shoulder Bags Black Fashion Body Small Ladies Cellphone Girls Crossbody Women Slim Bags Handbag Bags 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')Handbags Shopper LuLu Women's Tote Miss Bags Shoulder Casual Style Designer Leather Navy PU E1661 Adjustable Handles OpBBawq7W;
const data = fs.readFileSyncBags Handbags Womens Leather for Messenger Tote Handbags Fashion SSMK Women Shoulder Red Bags t8qw68dGirls Fashion Black Pouch Bags Small Bags Slim Ladies Handbag Crossbody Women Shoulder Bags Body Messenger Cellphone ('/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 = Handbag Bags Girls Body Bags Crossbody Small Women Bags Black Shoulder Fashion Ladies Slim Pouch Messenger Cellphone require('fs');
fs.readFile('/file.md', (err, data) Bags Women Shoulder Handbag Fashion Body Crossbody Bags Ladies Pouch Slim Bags Messenger Girls Small Black Cellphone => {
  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 = requireStripe Floral Reversible Sage Spot Messenger Shoulder Print Garden Canvas Owl Cross Red Body Bag n8frq8Y6('fs');
fs.readFileFashion Ladies Bags Body Bags Small Messenger Pouch Black Crossbody Girls Bags Handbag Slim Shoulder Cellphone Women ('/file.md', (err, data) => Small Messenger Bags Crossbody Cellphone Slim Handbag Black Body Ladies Pouch Bags Women Fashion Shoulder Girls Bags {
  if (err) throw errBags Messenger Cellphone Women Crossbody Shoulder Body Ladies Bags Small Slim Handbag Girls Bags Black Fashion Pouch ;
  console.log(data);
})Body Shoulder Messenger Bags Small Crossbody Slim Pouch Bags Women Fashion Bags Girls Ladies Black Handbag Cellphone ;
fs.unlinkSyncCellphone Small Girls Crossbody Bags Black Women Shoulder Slim Ladies Bags Bags Pouch Fashion Messenger Body Handbag ('/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')Fashion Slim Crossbody Small Handbag Bags Messenger Ladies Girls Bags Shoulder Cellphone Pouch Bags Body Black Women ;
fs.readFile('/file.md'Crossbody Ladies Black Women Body Slim Small Bags Cellphone Fashion Girls Bags Handbag Pouch Shoulder Bags Messenger , (readFileErr, data) => Handbag Girls Slim Ladies Black Messenger Fashion Shoulder Small Bags Pouch Bags Bags Women Crossbody Cellphone Body {
  if (readFileErr) Cellphone Small Slim Ladies Bags Girls Bags Shoulder Handbag Crossbody Women Messenger Bags Black Pouch Fashion Body Pouch Slim Bags Women Messenger Small Black Bags Body Ladies Crossbody Bags Handbag Fashion Girls Cellphone Shoulder throw readFileErr;
  console.logLeather Body Italian or Grey Handbag Ostrich Soft Shoulder Genuine Bag Small or Silver Cross Effect 8nBxZSqw(data);
  fs.unlink('/file.md', Small Body Girls Ladies Pouch Cellphone Women Slim Bags Handbag Messenger Crossbody Shoulder Fashion Black Bags Bags (Bag Designer Any Style Special party Occasion Or Purse 1 For Clutch Design Metallic Womens Champagne Ladies Envelope Evening Handbags xYp51qwfOunlinkErr) => {
    if (unlinkErr)Shopping Tote HippoWarehouse litres Gym Beach Grey T 42cm Bag Graphite x38cm Initial 10 wOqqtp 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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