Scalability & High Availability

Scalability means that an application / system can handle greater loads by adapting

There are two kinds of scalability:

a) Vertical Scalability

b) Horizontal Scalability (= elasticity)

Scalability is linked but different to High Availability

 

Vertical Scalability

 

It 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 system, 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

 

it 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 offering such as Amazon EC2

 

High Availability

 

it usually goes hand in hand with horizontal scaling

it means running your application / system in a least 2 data center (== Availablity 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 Availablity & Scalability For EC2

 

Vertical Scaling: Increase instance size (=scale up / down)

 a) From:t2.nano - 0.5G of RAM, 1 vCPU

 b) To: u-l2tbl.metal -12.3TB of RAM, 448 vCPUs

 

Horizontal Scaling: Increase number of instances (=scale out / in)

 a) Auto Scaling Group

 b) Load Balancer

 

High Availability: Run instances for the same application across multi AZ

 a) Auto Scaling Group multi AZ

 b) Load Balancer multi AZ