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The purpose of design is to allow you to do design later, and it's primary goal is to reduce the cost of change.

Single Responsibility Principle: a class should have only a single responsibility
Open-Closed Principle: Software entities should be open for extension, but closed for modification (inherit instead of modifying existing classes).
Liskov Substitution: Objects in a program should be replaceable with instances of their subtypes without altering the correctness of that program.
Interface Segregation: Many client-specific interfaces are better than one general-purpose interface.

System Design Cheatsheet

Picking the right architecture = Picking the right battles + Managing trade-offs

User cases (description of sequences of events that, taken together, lead to a system doing something useful)
- Who is going to use it?
- How are they going to use it?

sudo mkdir /etc/nginx/sites-available
sudo mkdir /etc/nginx/sites-enabled

include /etc/nginx/sites-enabled/*;

	# frozen_string_literal: true

	# Controller concern for basic crud.
	# Can be added to any controller like so
	# include Crud
	# Assumptions:
	# 1. Model class defines a constant called WHITELIST_FIELDS
	# as an array of symbols for attributed that are allowed for creating a record.
	# 2. Model is serialized elsewhwere. This can be done either by overwriting as_json
	# method in the model class, or by using separate serializers. By default as_json

	class CreditCardPayment
	def pay(amount)
	puts "Processing credit card payment of #{amount}"
	end
	end

	class PayPalPayment
	def pay(amount)
	puts "Processing PayPal payment of #{amount}"
	end

	var mediaJSON = { "categories" : [ { "name" : "Movies",
	"videos" : [
	{ "description" : "Big Buck Bunny tells the story of a giant rabbit with a heart bigger than himself. When one sunny day three rodents rudely harass him, something snaps... and the rabbit ain't no bunny anymore! In the typical cartoon tradition he prepares the nasty rodents a comical revenge.\n\nLicensed under the Creative Commons Attribution license\nhttp://www.bigbuckbunny.org",
	"sources" : [ "http://commondatastorage.googleapis.com/gtv-videos-bucket/sample/BigBuckBunny.mp4" ],
	"subtitle" : "By Blender Foundation",
	"thumb" : "images/BigBuckBunny.jpg",
	"title" : "Big Buck Bunny"
	},
	{ "description" : "The first Blender Open Movie from 2006",
	"sources" : [ "http://commondatastorage.googleapis.com/gtv-videos-bucket/sample/ElephantsDream.mp4" ],

	# MySQL. Versions 4.1 and 5.0 are recommended.
	#
	# Install the MySQL driver:
	# gem install mysql2
	#
	# And be sure to use new-style password hashing:
	# http://dev.mysql.com/doc/refman/5.0/en/old-client.html
	development:
	adapter: mysql2
	encoding: utf8

	# frozen_string_literal: true

	# Creates a new blob on the server side in anticipation of a direct-to-service upload from the client side.
	# When the client-side upload is completed, the signed_blob_id can be submitted as part of the form to reference
	# the blob that was created up front.
	class ActiveStorage::DirectUploadsController < ActiveStorage::BaseController
	skip_before_action :verify_authenticity_token, :only => [:create]

	def create
	blob = ActiveStorage::Blob.create_before_direct_upload!(blob_args)

	function PrivateRoute ({component: Component, authenticated, ...rest}) {
	return (
	<Route
	{...rest}
	render={(props) => authenticated === true
	? <Component {...props} />
	: <Redirect to={{pathname: '/login', state: {from: props.location}}} />}
	/>
	)
	}