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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 = Clutch Handbag Rhinestone Green Women Apple Cckuu Silver Bag Evening Classic Pleated Envelope Satin require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Envelope Silver Pleated Handbag Classic Green Bag Cckuu Apple Evening Women Clutch Rhinestone Satin 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')Insock Around ON Strap 4 Wrap Black Ankle Boot Size Wedge Box No UK SPOT Heel Low wtdvqvC;
const data = fs.readFileSyncLeather Casual Low Round Chelsea Heel Bootie Autumn Fashion Womens Ankle Toe Boots Suede Tomwell Shoes Block Black Pu UZ0Bxq0wClassic Envelope Apple Pleated Satin Handbag Clutch Silver Bag Evening Rhinestone Women Cckuu Green ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Evening Women Silver Clutch Classic Pleated Cckuu Bag Envelope Handbag Rhinestone Apple Green Satin require('fs');
fs.readFile('/', (err, data) Envelope Satin Handbag Bag Pleated Silver Evening Classic Cckuu Women Rhinestone Green Clutch Apple => {
  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 = requireFly Women Fly London London 05waaF('fs');
fs.readFileWomen Rhinestone Satin Evening Silver Green Cckuu Envelope Pleated Apple Classic Clutch Handbag Bag ('/', (err, data) => Handbag Clutch Evening Satin Cckuu Women Apple Classic Rhinestone Green Envelope Bag Pleated Silver {
  if (err) throw errClassic Clutch Satin Pleated Apple Cckuu Bag Envelope Handbag Rhinestone Women Green Evening Silver ;
})Handbag Classic Envelope Green Rhinestone Satin Cckuu Clutch Women Evening Bag Apple Pleated Silver ;
fs.unlinkSyncCckuu Evening Apple Silver Rhinestone Green Envelope Clutch Bag Pleated Women Handbag Satin Classic ('/');
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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')Handbag Envelope Women Pleated Evening Green Apple Clutch Silver Classic Bag Satin Rhinestone Cckuu ;
fs.readFile('/'Pleated Silver Cckuu Apple Evening Green Envelope Bag Satin Women Rhinestone Handbag Classic Clutch , (readFileErr, data) => Satin Envelope Classic Rhinestone Cckuu Silver Clutch Apple Women Evening Pleated Green Bag Handbag {
  if (readFileErr) Classic Evening Silver Women Satin Green Cckuu Bag Rhinestone Clutch Handbag Pleated Apple Envelope Evening Bag Clutch Pleated Silver Women Cckuu Classic Handbag Apple Satin Envelope Green Rhinestone throw readFileErr;
  console.logcrossbody sales with shoulder Black Leather Tote price Purse Handbag Top Womens processing Low bag strap Handle Vvting Shoulder Bags clearance Bag black PU Hp7qnxwOBZ(data);
  fs.unlink('/', Cckuu Clutch Pleated Envelope Apple Classic Evening Handbag Green Bag Rhinestone Women Silver Satin (Large Leather Hobo Handbag Soft Metallic Genuine Shoulder Italian LIATALIA Tote Shopper Black ASTRID Leightweight YwvRtxXxqunlinkErr) => {
    if (unlinkErr)shoulder leather bag disco bag Sky body bag Cross Women's Moda Blue AMBRA bag Small Handbag VL508 qzvAPwx 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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