Scalability & HA Overview

• 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

• 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)

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

• 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)

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.