Note_Tech

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AWS - High Availability & Scalability

Scalability means that an application / system can handle greater loads by adapting.
There are two kinds of scalability:
- Vertical Scalability
- Horizontal Scalability (= elasticity)
Vertical Scalability
- Vertically 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.
  - RDS, ElastiCache are services that can scale vertically.
- There’s usually a limit to how much you can vertically scale (hardware limit)

Vertical Scalability

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
Scalability is linked but different to High Availability

Horizontal Scalability

High Availability usually goes hand in hand with horizontal scaling
High availability means running your application / system in at least 2 data centers (== Availability Zones)
The goal of high availability is to survive a data center loss
The high availability can be passive (for RDS Multi AZ for example)
The high availability can be active (for horizontal scaling)

Availability

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