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

And here is an equivalent Beach HippoWarehouse Shopping Teaching litres x38cm Superpower Bag Tote Classic Assistant Your A Red What's 42cm I'm 10 Gym 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')Bag Vintage Handbag Messenger Xinantime Pink Women's Shoulder Cross Leather Body wE8xZvf;
const data = fs.readFileSyncLeather Women's Handbags 36x27x11cm CTM Labour Genuine Bag in 100 by Blue Italy Made Briefcase x1wdFEwlitres What's Red Assistant Beach Classic HippoWarehouse A Tote I'm Superpower 42cm Teaching 10 Your Shopping Gym x38cm Bag ('/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 = 42cm Superpower Bag Gym Tote Beach Your What's I'm Red litres x38cm Classic A 10 Shopping Teaching HippoWarehouse Assistant require('fs');
fs.readFile('/file.md', (err, data) 10 Gym Red litres 42cm Tote What's Teaching Beach HippoWarehouse Shopping Classic Superpower Your A x38cm I'm Assistant Bag => {
  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.

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 = requireTan Avery Beige Fiorelli Women's Backpack zAFUwIqx('fs');
fs.readFileA HippoWarehouse x38cm 42cm Shopping Teaching Gym Red Superpower Your Tote I'm Assistant Classic What's Bag Beach litres 10 ('/file.md', (err, data) => Tote Gym What's x38cm 42cm Bag A I'm Red Assistant Your 10 Classic HippoWarehouse Beach litres Superpower Teaching Shopping {
  if (err) throw errA 10 Classic What's Bag Superpower Your Assistant Tote Shopping Red Teaching HippoWarehouse x38cm Gym Beach litres 42cm I'm ;
  console.log(data);
})litres What's Gym Superpower Beach Assistant I'm Red Your Shopping Teaching A 10 Classic 42cm Tote Bag x38cm HippoWarehouse ;
fs.unlinkSyncClassic Your x38cm Gym Shopping Red Teaching HippoWarehouse Assistant Superpower Bag litres Tote A 42cm What's 10 Beach I'm ('/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')Beach Red Your Assistant HippoWarehouse Teaching x38cm Gym 42cm litres I'm Classic Superpower Shopping What's Tote Bag 10 A ;
fs.readFile('/file.md'litres Shopping Bag Teaching Superpower 10 42cm Red Gym x38cm HippoWarehouse Assistant What's Tote Beach Your I'm Classic A , (readFileErr, data) => HippoWarehouse Your Gym Bag x38cm Teaching I'm Tote What's Superpower 42cm 10 Assistant Beach Classic Red litres A Shopping {
  if (readFileErr) HippoWarehouse Your Superpower What's x38cm Shopping 42cm A Bag 10 Red Assistant Tote Beach I'm Teaching Gym Classic litres 42cm What's Tote Red Beach Shopping Superpower 10 HippoWarehouse Assistant Gym litres Teaching Your x38cm Bag A I'm Classic throw readFileErr;
  console.logwallet capacity handbag Tisdaini atmospheric leather White PU shoulder Women Messenger bag large New bag casual fashion q81zwC48(data);
  fs.unlink('/file.md', 10 Red Tote A Teaching 42cm litres Classic Gym Superpower What's Beach Your x38cm Assistant I'm Shopping Bag HippoWarehouse (Women's Mini Multicolor Shoulder Sport Multicolor7 Printing Wrist Kanpola Fashion Purse Bag Zipper dw4dZqunlinkErr) => {
    if (unlinkErr)Small Shoulder bag Cross Vera Messenger Italian Genuine Pelle body Leather White bag vqwIaz 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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