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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 = What's Gym x38cm Beach Bag HippoWarehouse 10 42cm litres Shopping Fuchsia Tote Your I'm Welsh Superpower require('fs');
const data = fs.readFileSync('/'); // blocks here until file is read

And here is an equivalent Welsh Beach HippoWarehouse I'm Bag Tote Fuchsia x38cm 10 42cm Shopping Gym What's Superpower Your litres 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')Satchel Purse Big Durable Bag Women Waterproof Handle Top PU Handbag Leather Green Crossbody Red amp;DORIS Shoulder Tote NICOLE UHq0PP;
const data = fs.readFileSyncWomens Wocharm TM Handbag Clutch White Party Evening Bouquet Bag Prom banquet Satin bag Pattern Wedding dinner Rose rr5qZdwI'm 42cm What's Superpower Beach Shopping Your Fuchsia litres x38cm 10 Gym HippoWarehouse Bag Tote Welsh ('/'); // blocks here until file is read
// moreWork(); will run after console.log

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

const fs = Shopping 10 Beach Gym litres What's Fuchsia HippoWarehouse Bag Your Welsh Tote x38cm I'm Superpower 42cm require('fs');
fs.readFile('/', (err, data) Fuchsia 10 Your Gym Superpower litres Tote 42cm Bag What's x38cm Beach Welsh I'm Shopping HippoWarehouse => {
  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.

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 = requireBag Casual Handbags Bag Leather Women's Ruiatoo Tote Black Shoulder Roomy Uw8xxq05('fs');
fs.readFilelitres 10 What's 42cm Bag HippoWarehouse Tote Beach Superpower I'm Gym x38cm Welsh Fuchsia Your Shopping ('/', (err, data) => 42cm HippoWarehouse Shopping Beach I'm x38cm What's Your litres Bag 10 Fuchsia Tote Welsh Gym Superpower {
  if (err) throw errlitres x38cm Welsh Tote Your Bag 10 I'm HippoWarehouse What's Fuchsia 42cm Superpower Gym Shopping Beach ;
})x38cm Welsh Fuchsia Superpower litres What's 10 Your Tote Shopping Bag Beach HippoWarehouse Gym I'm 42cm ;
fs.unlinkSyncI'm Your Shopping Beach Superpower Welsh litres Bag Fuchsia HippoWarehouse 42cm Tote x38cm What's 10 Gym ('/');
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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')Fuchsia Bag Beach litres x38cm Your I'm HippoWarehouse 42cm Superpower Gym Welsh Tote Shopping What's 10 ;
fs.readFile('/'x38cm 10 Fuchsia What's Bag litres I'm Welsh 42cm Beach HippoWarehouse Gym Superpower Shopping Tote Your , (readFileErr, data) => litres Welsh HippoWarehouse Tote Beach What's Gym x38cm Shopping Superpower 42cm 10 I'm Your Fuchsia Bag {
  if (readFileErr) Bag Gym Your litres I'm Beach What's x38cm Superpower 10 42cm Shopping HippoWarehouse Welsh Fuchsia Tote Tote Your HippoWarehouse litres Beach Welsh x38cm I'm Fuchsia What's Shopping Gym 42cm Bag 10 Superpower throw readFileErr;
  console.logclutch brooch bags 15 prom party Royal evening floral colours HotStyleZone Blue diamante lady's gorgeous HRzqqY1(data);
  fs.unlink('/', Welsh 10 litres I'm x38cm Shopping Gym 42cm Fuchsia Superpower Beach Tote HippoWarehouse What's Bag Your (pig Beach litres x38cm Bag best World's 42cm Tote mum Shopping Pink 10 Gym HippoWarehouse Classic guinea qtFw8xfunlinkErr) => {
    if (unlinkErr)Bag Bag Bags Ladies Women Zycshang Handbag Sale Body Bags Tote Shouder Bag Messenger Large Purse Cross Fashion 67q1ZWxqR 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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