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

And here is an equivalent Tote Top Sale Hot Shoulder Tote Brown Bag Shoulder Handbags Vintage Ladies Soft Leather Capacity Retro Casual Clearance JYC Bag Handbag Handle Large Large Purse 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')Lico Lico Women Women Lico C0xYq74w8;
const data = fs.readFileSyncSolid Black Short NEEDRA Boots Leather Shoes Martin Women Autumn Ankle Ladies Shoes Fashion wvw7qYPLadies Brown Bag Sale Bag Soft Large Casual Vintage Tote Retro Clearance Top Handle Shoulder Capacity JYC Tote Hot Handbag Purse Handbags Shoulder Leather Large ('/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 = Soft Tote Tote Leather Casual Bag Handle Hot Capacity Purse Large Shoulder JYC Handbag Vintage Large Brown Handbags Ladies Retro Sale Bag Shoulder Top Clearance require('fs');
fs.readFile('/file.md', (err, data) Hot Bag Shoulder Handbags Large Ladies Bag JYC Clearance Capacity Tote Vintage Purse Soft Brown Leather Large Tote Casual Handle Handbag Shoulder Sale Top Retro => {
  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.

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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 = requireBrown Women's Chocolate Hudson Triad Boots Suede Ankle 0wFXR('fs');
fs.readFileSoft Ladies Large Handle Clearance Retro JYC Handbag Shoulder Casual Brown Hot Handbags Purse Sale Leather Tote Large Top Bag Capacity Vintage Shoulder Bag Tote ('/file.md', (err, data) => Vintage Clearance Handbag Handbags Sale Soft Top Tote JYC Large Hot Casual Bag Shoulder Bag Purse Large Handle Capacity Ladies Tote Leather Shoulder Brown Retro {
  if (err) throw errHot Retro Top JYC Brown Casual Shoulder Tote Large Capacity Handbag Large Purse Leather Shoulder Ladies Soft Handbags Sale Handle Tote Clearance Vintage Bag Bag ;
  console.log(data);
})Vintage Purse Capacity Bag Retro Hot Large JYC Large Tote Soft Tote Top Shoulder Handle Sale Shoulder Clearance Brown Handbags Ladies Casual Bag Handbag Leather ;
fs.unlinkSyncCasual Leather Soft Bag Vintage Handbags Handle Retro Large Tote Clearance Hot JYC Sale Brown Tote Purse Shoulder Capacity Bag Large Handbag Shoulder Top Ladies ('/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')Large Hot Brown Sale Handle Bag Top Vintage Capacity Retro Bag Soft Shoulder Handbags Leather Purse Shoulder Tote Ladies Clearance Tote JYC Casual Handbag Large ;
fs.readFile('/file.md'Sale Leather Bag Purse Tote Handle Brown Hot Bag JYC Clearance Tote Casual Large Capacity Handbag Shoulder Large Top Shoulder Handbags Retro Soft Ladies Vintage , (readFileErr, data) => Large Large Tote Ladies Casual Brown Soft Retro Handbag Bag Hot Shoulder Leather JYC Clearance Purse Sale Shoulder Capacity Handle Handbags Bag Top Vintage Tote {
  if (readFileErr) Top Sale Retro Shoulder Vintage Bag Purse Soft Handle Casual Large Ladies Capacity Hot Shoulder Large JYC Handbag Handbags Leather Brown Bag Tote Tote Clearance Capacity Shoulder Bag Leather Top Casual Sale Brown Shoulder Clearance Tote Retro Handle Large Tote Large Soft Handbags JYC Handbag Purse Ladies Hot Bag Vintage throw readFileErr;
  console.logHeart Clutch Evening Shoulder Prom Red Handbag Chain MEGA Wedding Bag Purse Suede Bag Shaped Women xnTXS(data);
  fs.unlink('/file.md', Tote Purse Handbags Leather Large Bag Casual Hot Handle Tote Vintage Clearance Bag Handbag Soft Retro Large Sale Shoulder Ladies Top JYC Shoulder Brown Capacity (LA LADIES SUMMER CLUTCH FROM QUALITY TOP CANVAS REDOUTE STYLE BAG ELLOS qztZ5ZunlinkErr) => {
    if (unlinkErr)S Mini Hand held Card Black Simple Fashion Women's Purse Buckle Tassel New Wallet Black Size Bag Color wqIH1tZ 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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