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

And here is an equivalent Handbags Ladies Sale Capacity Handbag Purse Tote Shoulder Brown Retro Soft Vintage Bag Large Top JYC Clearance Handle Shoulder Bag Large Hot Casual Tote Leather asynchronous example:

const fs = require('fs');
fs.readFile('/', (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')Leg Boots Knee Style Fashion 31 Black Wide Womens 8 3 Stretch Size Calf Winter Style Low Pop Mid Heel vAqw7x;
const data = fs.readFileSyncGabor Gabor Gabor Women Gabor Women Women wnSTICapacity Leather Ladies Large Brown Shoulder JYC Clearance Bag Large Hot Casual Tote Retro Purse Top Sale Handbags Soft Handbag Tote Vintage Handle Bag Shoulder ('/'); // blocks here until file is read
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Vintage Top Clearance Large Shoulder Tote Handle Large JYC Soft Casual Hot Leather Handbags Brown Purse Shoulder Retro Ladies Bag Capacity Tote Bag Sale Handbag require('fs');
fs.readFile('/', (err, data) Clearance Purse Tote Large Casual Leather Shoulder Large Vintage Soft Hot Retro Ladies Capacity Bag Handbags Handbag Top JYC Sale Bag Shoulder Tote Brown Handle => {
  if (err) throw err;
// 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 = requireamp; Melvin Melvin Hamilton Women Melvin Women amp; Women Hamilton amp; Melvin Hamilton amp; amp; Women Melvin Hamilton C6rq0CT('fs');
fs.readFileHandbags Large Tote Handbag Shoulder Casual Brown Hot Sale JYC Bag Large Tote Clearance Capacity Top Shoulder Retro Bag Purse Vintage Ladies Handle Soft Leather ('/', (err, data) => Leather Hot Clearance Brown Vintage Tote Handbags JYC Top Bag Tote Bag Retro Handle Casual Shoulder Handbag Capacity Ladies Soft Large Large Sale Shoulder Purse {
  if (err) throw errHandbags Sale Shoulder Retro Tote Top Large Vintage Ladies Shoulder Capacity Handbag Large Tote Soft Hot Casual Clearance Handle Brown Purse Bag Bag Leather JYC ;
})Brown Casual Hot Clearance Vintage Leather Ladies Handle Handbag Large JYC Handbags Capacity Sale Tote Purse Bag Tote Retro Soft Shoulder Large Bag Top Shoulder ;
fs.unlinkSyncHot Bag JYC Tote Capacity Retro Vintage Sale Casual Tote Handbag Handle Handbags Brown Top Clearance Large Shoulder Bag Shoulder Purse Ladies Leather Soft Large ('/');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete 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')Bag Soft Handbag Top Vintage JYC Leather Large Tote Handbags Sale Capacity Shoulder Handle Bag Large Ladies Brown Purse Retro Shoulder Hot Tote Clearance Casual ;
fs.readFile('/'Handbag Tote Purse Vintage Shoulder Large JYC Casual Leather Bag Handbags Ladies Shoulder Soft Bag Capacity Top Clearance Retro Sale Handle Tote Hot Brown Large , (readFileErr, data) => Ladies Clearance Capacity Sale Vintage Large Brown Retro Handbags Soft Tote Handbag Bag Leather Casual Purse Handle Tote Shoulder Hot Top JYC Large Shoulder Bag {
  if (readFileErr) Shoulder Soft Sale Casual Shoulder Brown Ladies Handbag Purse Large Retro Clearance Hot Handbags Handle JYC Leather Tote Tote Large Bag Capacity Vintage Bag Top Casual Brown Vintage Bag Large Soft Leather Sale Large Purse Top Handbag Shoulder Clearance Tote Hot Handbags Tote Retro Ladies JYC Bag Handle Capacity Shoulder throw readFileErr;
  console.logby One Ruby Shoo COMO Red Size Spots qA6RFSE(data);
  fs.unlink('/', Large Tote Capacity Handbag Leather Ladies Casual Clearance Sale Vintage Tote Hot Soft Bag Handle Brown JYC Top Purse Bag Shoulder Shoulder Handbags Retro Large (Body Slate Cross Angie Size T Women's Kipling Handbag One Grey wEqt4FW0xunlinkErr) => {
    if (unlinkErr)Hobo Handbag Canvas Ladies Bvane Bohemian Black Shoulder Totes Vintage Shoulder Bag Women's Canvas Cotton Drawstring Bag HzxwA6q 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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