# Elastic Load Balancing & Auto Scaling Groups

- [Elastic Load Balancing \& Auto Scaling Groups](#elastic-load-balancing--auto-scaling-groups)
  - [Scalability \& High Availability](#scalability--high-availability)
  - [Vertical Scalability](#vertical-scalability)
  - [Horizontal Scalability](#horizontal-scalability)
  - [High Availability](#high-availability)
  - [High Availability \& Scalability for EC2](#high-availability--scalability-for-ec2)
  - [Scalability vs Elasticity (vs Agility)](#scalability-vs-elasticity-vs-agility)
  - [What is Load Balancing?](#what-is-load-balancing)
    - [Why Use a Load Balancer?](#why-use-a-load-balancer)
    - [Why Use an Elastic Load Balancer?](#why-use-an-elastic-load-balancer)
      - [Types of ELB](#types-of-elb)
  - [What’s an Auto Scaling Group?](#whats-an-auto-scaling-group)
    - [Auto Scaling Group Scaling Strategies](#auto-scaling-group-scaling-strategies)
  - [ELB \& ASG Summary](#elb--asg-summary)

## Scalability & High Availability

- **Scalability**: Ability of a system to handle an increase in load by adapting to the demand.
- **High Availability**: Ensures a system is operational and accessible for a high percentage of time, often achieved by reducing the impact of failures.
- There are two kinds of scalability:
  - Vertical Scalability
  - Horizontal Scalability (= elasticity)
- Scalability is linked but different to High Availability

## Vertical Scalability

- Increasing the capacity of a single instance (e.g., moving from t3.medium to t3.large).
- Suitable for databases or applications where upgrading a single resource is more efficient.
- Limited by hardware constraints (can only scale up to a certain point).

## Horizontal Scalability

- Adding more instances (servers) to distribute the load across multiple resources.
- Achieved through technologies like **Auto Scaling Groups (ASG)** and **Elastic Load Balancing (ELB)**.
- Preferred for applications needing resilience and distributed workloads.
- Horizontal scaling implies distributed systems.

## High Availability

- Implemented by deploying resources across multiple **Availability Zones** (AZs).
- Ensures failover and redundancy in case of failures in one AZ.
- High Availability usually goes hand in hand with horizontal scaling

## 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)

| **Term**        | **Definition**                                                                                                    |
| --------------- | ----------------------------------------------------------------------------------------------------------------- |
| **Scalability** | Ability to increase or decrease the capacity to handle varying levels of traffic or load.                         |
| **Elasticity**  | Automatically adjusts resources up or down based on the load in real-time, preventing under or over-provisioning. |
| **Agility**     | The ability to deploy and manage resources quickly and efficiently in response to changing demands.               |

## What is Load Balancing?

- Distributes incoming traffic across multiple targets (EC2 instances, containers, IP addresses) to ensure that no single resource is overwhelmed.

### Why Use a Load Balancer?

- Ensures application fault tolerance and high availability by spreading the load across multiple servers.
- Protects against failures in a single resource by rerouting traffic automatically.
- Do regular health checks to your instances
- Provide SSL termination (HTTPS) for your websites

### Why Use an Elastic Load Balancer?

- **Elastic Load Balancer (ELB)** is a fully managed service that automatically distributes incoming application traffic across multiple targets in one or more Availability Zones.
- It improves fault tolerance, enhances performance, and scales according to demand.
- AWS guarantees that it will be working
- AWS takes care of upgrades, maintenance, high availability
- AWS provides only a few configuration knobs

#### Types of ELB

1. **Application Load Balancer (ALB)**: For HTTP and HTTPS traffic, operates at Layer 7 (application level).
2. **Network Load Balancer (NLB)**: Handles high-performance traffic at Layer 4 (transport level).
3. **Classic Load Balancer**: (slowly retiring) – Layer 4 & 7

## What’s an Auto Scaling Group?

- An **Auto Scaling Group (ASG)** ensures the right number of EC2 instances are running to handle the load.
- Automatically adjusts the number of instances based on metrics such as CPU utilization or custom-defined thresholds.
- Can span across multiple AZs to ensure high availability.
- 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 Group Scaling Strategies

- **Manual Scaling**: Adjusting the number of instances manually based on load prediction.
- **Dynamic Scaling**: Automatically adjusts the number of instances based on demand (e.g., CPU usage).
  - 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 patterns and scales proactively.

## 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