# Elastic Load Balancing & Auto Scaling Groups

## Scalability & High Availability

* Scalability means that an application / system can handle greater loads by adapting.
* There are two kinds of scalability:
  * Vertical Scalability
  * Horizontal Scalability (= elasticity)
* Scalability is linked but different to High Availability
* Let’s deep dive into the distinction, using a call center as an example

## Vertical Scalability

* Vertical Scalability means increasing the size of the instance
* For example, your application runs on a t2.micro
* Scaling that application vertically means running it on a t2.large
* Vertical scalability is very common for non distributed systems, such as a database.
* There’s usually a limit to how much you can vertically scale (hardware limit)

## Horizontal Scalability

* Horizontal Scalability means increasing the number of instances / systems for your application
* Horizontal scaling implies distributed systems.
* This is very common for web applications / modern applications
* It’s easy to horizontally scale thanks the cloud offerings such as Amazon EC2

## High Availability first building in New York

* High Availability usually goes hand in hand with horizontal scaling
* High availability means running your application / system in at least 2 Availability Zones
* The goal of high availability is to survive a data center loss (disaster)

## High Availability & Scalability For EC2

* Vertical Scaling: Increase instance size (= scale up / down)
  * From: t2.nano - 0.5G of RAM, 1 vCPU
  * To: u-12tb1.metal – 12.3 TB of RAM, 448 vCPUs
* Horizontal Scaling: Increase number of instances (= scale out / in)
  * Auto Scaling Group
  * Load Balancer
* High Availability: Run instances for the same application across multi AZ
  * Auto Scaling Group multi AZ
  * Load Balancer multi AZ

## Scalability vs Elasticity (vs Agility)

* Scalability: ability to accommodate a larger load by making the hardware stronger (scale up), or by adding nodes (scale out)
* Elasticity: once a system is scalable, elasticity means that there will be some “auto-scaling” so that the system can scale based on the load. This is “cloud-friendly”: pay-per-use, match demand, optimize costs
* Agility: (not related to scalability - distractor) new IT resources are only a click away, which means that you reduce the time to make those resources available to your developers from weeks to just minutes.

## What is load balancing?

* Load balancers are servers that forward internet traffic to multiple servers (EC2 Instances) downstream.

## Why use a load balancer?

* Spread load across multiple downstream instances
* Expose a single point of access (DNS) to your application
* Seamlessly handle failures of downstream instances
* Do regular health checks to your instances
* Provide SSL termination (HTTPS) for your websites
* High availability across zones

## Why use an Elastic Load Balancer?

* An ELB (Elastic Load Balancer) is a managed load balancer
  * AWS guarantees that it will be working
  * AWS takes care of upgrades, maintenance, high availability
  * AWS provides only a few configuration knobs
* It costs less to setup your own load balancer but it will be a lot more effort on your end (maintenance, integrations)
* 3 kinds of load balancers offered by AWS:
  * Application Load Balancer (HTTP / HTTPS only) – Layer 7
  * Network Load Balancer (ultra-high performance, allows for TCP) – Layer 4
  * Classic Load Balancer (slowly retiring) – Layer 4 & 7

## What’s an Auto Scaling Group?

* In real-life, the load on your websites and application can change
* In the cloud, you can create and get rid of servers very quickly
* The goal of an Auto Scaling Group (ASG) is to:
  * Scale out (add EC2 instances) to match an increased load
  * Scale in (remove EC2 instances) to match a decreased load
  * Ensure we have a minimum and a maximum number of machines running
  * Automatically register new instances to a load balancer
  * Replace unhealthy instances
* Cost Savings: only run at an optimal capacity (principle of the cloud)

## Auto Scaling Groups – Scaling Strategies

* Manual Scaling: Update the size of an ASG manually
* Dynamic Scaling: Respond to changing demand
  * Simple / Step Scaling
    * When a CloudWatch alarm is triggered (example CPU > 70%), then add 2 units
    * When a CloudWatch alarm is triggered (example CPU < 30%), then remove 1
  * Target Tracking Scaling
    * Example: I want the average ASG CPU to stay at around 40%
  * Scheduled Scaling
    * Anticipate a scaling based on known usage patterns
    * Example: increase the min. capacity to 10 at 5 pm on Fridays
* Predictive Scaling
  * Uses Machine Learning to predict future traffic ahead of time
  * Automatically provisions the right number of EC2 instances in advance
* Useful when your load has predictable time - based patterns

## ELB & ASG – Summary

* High Availability vs Scalability (vertical and horizontal) vs Elasticity vs Agility in the Cloud
* Elastic Load Balancers (ELB)
  * Distribute traffic across backend EC2 instances, can be Multi-AZ
  * Supports health checks
  * 3 types: Application LB (HTTP – L7), Network LB (TCP – L4), Classic LB (old)
* Auto Scaling Groups (ASG)
  * Implement Elasticity for your application, across multiple AZ
  * Scale EC2 instances based on the demand on your system, replace unhealthy
  * Integrated with the ELB