Day 10: Elasticity, Scalability & High Availability — AWS's 3 Core Value Props

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Continuing AWS fundamentals. Today: the 3 T's of AWS — Elasticity, Scalability, and High Availability — what they actually mean and how they're achieved.

Elasticity

EC2 (Elastic Compute Cloud) is where applications run — instances behind a Load Balancer that distributes traffic to them.

If the Load Balancer starts seeing more traffic, more EC2 instances are launched to handle the load. The increasing and decreasing of the number of servers based on load is called Elasticity.

Elasticity is short-term
Elasticity is achieved in AWS using Auto Scaling
Auto Scaling works in two directions:
- Scale out — increasing/adding instances
- Scale in — decreasing/removing instances
Elasticity is also called Horizontal Scaling

Scalability

Increasing the capacity of a single server (instead of adding more servers) is called Scalability.

Example: a database server running 100 databases sized 10TB on an 8GB RAM machine needs its RAM increased due to slowness — that's scaling the existing server up
Scalability = Scale Up and Scale Down
To scale, you typically need to stop the server/instance
Instance Type = combination of CPU + Memory
Scalability can be achieved in AWS by changing the instance type
Scalability is long-term
Scalability is also called Vertical Scaling

High Availability (HA)

The period of time a service is available to customers is called High Availability
The period of time a service is NOT available to customers is called Downtime
High Availability is measured in percentage
Zero Downtime = Auto Scaling + Fault Tolerance

The three pillars of High Availability:

Redundancy — keeping the same application running on different servers (duplicate setups)
Monitoring — the Load Balancer continuously checks if the application is working, using health checks
- If the Load Balancer gets a success code (200), it considers the application healthy
- Health checks happen every 30 seconds
Failover — if one server goes down, another server takes over the requests from the Load Balancer

IaaS vs PaaS vs SaaS — Applied to AWS

AWS also offers PaaS and SaaS, not just IaaS:

AWS Elastic Beanstalk uses PaaS — you only upload your application, and it takes care of the rest (infrastructure, scaling, deployment)
With EC2, you have to do all of this manually (that's IaaS)
Elastic Beanstalk enables easy and quick deployment of applications in AWS

The biryani analogy, applied to AWS:

Preparing biryani and eating it = IaaS (EC2 — you do the work)
Going and eating in a restaurant = PaaS (Elastic Beanstalk — mostly handled for you)
Ordering via Swiggy/Zomato = SaaS (fully managed, ready to use)

Recap: The 3 T's of AWS

Elasticity — short-term, horizontal scaling, achieved via Auto Scaling
Scalability — long-term, vertical scaling, achieved by changing instance type
High Availability — achieved via Redundancy + Monitoring + Failover