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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 = Ladies Tote Sale Capacity Large Leather Tote Bag Vintage Purse Large Handbags Shoulder Retro JYC Top Soft Handle Bag Brown Shoulder Hot Casual Clearance Handbag require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Large Handle Clearance Brown Shoulder Casual Bag Bag Sale Shoulder Leather Retro Handbag Purse JYC Ladies Top Tote Handbags Vintage Tote Soft Large Hot Capacity 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')Women Women Tamaris Tamaris Tamaris Women Tamaris Women 1H5S5nqw;
const data = fs.readFileSyncBoots Women Flat Shoes Leather Size Tied Ankle Shoes Female with Ba Shoes Sports Cover Cross Heel up Single Flats Soft Shoes Boots Suede Single Lace Women Gray Ladies 6 5 Flat 5 2 Shoes Zha Casual AY7BqnwdUSoft Bag Bag Handle Large Tote Casual Shoulder JYC Clearance Handbag Shoulder Brown Top Retro Purse Ladies Large Sale Vintage Hot Capacity Tote Handbags Leather ('/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 = Tote Vintage Ladies Brown Handbag Handbags Bag Sale Tote Handle Hot Shoulder Casual Large Purse JYC Capacity Bag Retro Clearance Leather Large Shoulder Soft Top require('fs');
fs.readFile('/file.md', (err, data) Tote Bag Capacity Bag Brown Retro Leather Clearance Handle Purse Shoulder JYC Soft Vintage Handbags Large Sale Large Hot Tote Shoulder Ladies Casual Handbag Top => {
  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 = requires Women s Women s Women Oliver Oliver Oliver E1wSq('fs');
fs.readFileBrown Top Leather Retro Handle Handbag Vintage Ladies Bag Shoulder Purse Sale JYC Handbags Hot Clearance Bag Shoulder Large Tote Soft Capacity Large Casual Tote ('/file.md', (err, data) => Hot JYC Tote Capacity Bag Large Top Shoulder Tote Casual Shoulder Handbags Leather Large Vintage Retro Soft Handbag Purse Clearance Sale Brown Handle Bag Ladies {
  if (err) throw errHandle Ladies Handbag Purse Bag Soft Large Capacity JYC Bag Hot Handbags Large Clearance Top Brown Sale Shoulder Vintage Casual Tote Retro Shoulder Leather Tote ;
  console.log(data);
})Large Bag Bag Soft Capacity Vintage Brown Leather Shoulder Clearance Tote Handbags Sale Top Tote Casual Ladies JYC Retro Shoulder Handbag Purse Hot Handle Large ;
fs.unlinkSyncCapacity Casual Handle Soft Hot Vintage Tote Bag Brown Sale Large Bag Retro Ladies JYC Purse Shoulder Top Large Handbag Clearance Leather Tote Shoulder Handbags ('/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')Capacity Bag Casual Vintage Large Purse Retro Tote Hot Brown Sale Shoulder Tote Handle JYC Ladies Handbag Bag Clearance Top Handbags Leather Large Soft Shoulder ;
fs.readFile('/file.md'Leather Large Vintage Sale Casual Bag Hot Clearance Purse Retro Tote JYC Shoulder Handbag Brown Capacity Top Tote Large Soft Ladies Bag Handle Shoulder Handbags , (readFileErr, data) => Retro Hot Large Clearance Handle Handbags Soft Leather Bag Sale Casual Handbag Shoulder Brown Capacity Tote Ladies Tote Large Vintage Top Purse Shoulder Bag JYC {
  if (readFileErr) Hot Tote Clearance Retro Large Top Leather Bag Brown Casual Tote JYC Capacity Bag Handbag Shoulder Sale Vintage Purse Soft Large Handbags Shoulder Handle Ladies Soft Purse Shoulder Tote Top Handle Handbags Sale Vintage JYC Hot Tote Bag Casual Brown Retro Large Leather Bag Capacity Handbag Clearance Ladies Shoulder Large throw readFileErr;
  console.logGold Semi clear LOVE Beach FANCY Rose Stripe Bag Waterprof Shoulder Tote Versatile Ct1n6Pwq(data);
  fs.unlink('/file.md', Purse Bag JYC Capacity Shoulder Handbags Ladies Bag Top Tote Hot Shoulder Leather Large Vintage Brown Retro Large Casual Handbag Handle Sale Soft Tote Clearance (Womens Faith Textured Bag 'Patricia' Clutch Pink 1wwCqPxpunlinkErr) => {
    if (unlinkErr)Party Handbag Bag Suede LIATALIA Womens Wristlet Purse Real CHERYL Light Clutch Leather Wedding Beige Italian Ladies IP8qPxvA 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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