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)

Vertical Scalability

• Vertical Scalability means increasing the size of the instance. (scale up / down)
• 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, ElasticCache are services that can scale vertically.
• 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. (scale out / in)
• Horizontal scaling implies distributed systems.
• This is very common for web applications / modern applications.
• Auto Scaling Group (ASG) & Elastic Load Balancer (ELB).

High Availability

• 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(AZ))