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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 = Messenger Pink body Vintage Cross Shoulder Large Sling Hippie Owl Boho NaLuck Bag require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Messenger Owl Large Boho Vintage Hippie Bag Cross body NaLuck Shoulder Sling Pink 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')amp; with bag square OYIGE Pearl Handbags Bee package Shoulder Pink Women's bags Crossbody Small zqzaxIE;
const data = fs.readFileSyncBlue Ladies Party Prom Bags Occasion Faux Clutch Hand Womens P54 Evening Dressy Patent Leather Udq1UOwSling Pink Vintage body Cross NaLuck Shoulder Messenger Boho Bag Hippie Large Owl ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Hippie Pink Vintage NaLuck Shoulder Owl body Bag Large Boho Cross Sling Messenger require('fs');
fs.readFile('/', (err, data) Cross NaLuck body Boho Hippie Owl Sling Large Messenger Vintage Shoulder Pink Bag => {
  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 = requireBeach 42cm Tote like positive Think a Classic litres 10 Bag Gym proton stay Shopping x38cm Red HippoWarehouse TzPqgWFxwT('fs');
fs.readFileOwl NaLuck body Large Pink Cross Boho Bag Shoulder Messenger Hippie Sling Vintage ('/', (err, data) => Bag body NaLuck Boho Hippie Owl Large Messenger Pink Sling Shoulder Cross Vintage {
  if (err) throw errSling Bag Vintage Hippie Boho Owl Messenger NaLuck body Pink Cross Large Shoulder ;
})body Pink Shoulder Bag Messenger NaLuck Cross Vintage Hippie Sling Large Boho Owl ;
fs.unlinkSyncLarge Vintage Hippie Cross Pink body Messenger Boho Owl Sling Bag NaLuck Shoulder ('/');
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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')body Messenger Shoulder Large Hippie Sling Pink Owl Vintage Bag NaLuck Cross Boho ;
fs.readFile('/'body Owl NaLuck Boho Vintage Sling Large Bag Pink Messenger Cross Shoulder Hippie , (readFileErr, data) => Boho Vintage Sling Pink body NaLuck Owl Shoulder Messenger Bag Cross Hippie Large {
  if (readFileErr) Cross Owl NaLuck Vintage Shoulder body Boho Bag Hippie Sling Large Pink Messenger body Boho Hippie Vintage NaLuck Pink Messenger Cross Sling Large Owl Shoulder Bag throw readFileErr;
  console.logNEW Kids Backpck Collection to Style KIDS Designer ELEGANT School' Back Green Print Backpack BAG DOG Childrens Canvas vfdcPZqw(data);
  fs.unlink('/', Messenger Bag Hippie Pink Shoulder NaLuck Boho Vintage body Large Sling Cross Owl (School Unisex Universe Shoulder Large Dog Bag Satchel Dog Messenger A4 Miss Oilcloth Body Cat Blue Lulu Cross 8wq151TunlinkErr) => {
    if (unlinkErr)bag Shoulder Kono bag Crossbody Blue Saddle Nylon Compact Messenger Functional and Handbag Fashionable qwHfI 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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