High Availability, Scalability, Elasticity
Scalability
Scalability means that an application / system can handle greater loads by adapting
Scalability is linked but different to High Availability
Vertical Scalability
Vertical 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.
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 Availability Zones
The goal of high availability is to survive a data center loss (disaster)
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
Scalability vs Elasticity (vs Agility)
• Scalability: ability to accommodate a larger load by making the hardware stronger (scale up), or by adding nodes (scale out)
• Elasticity: once a system is scalable, elasticity means that there will be some “auto-scaling” so that the system can scale based on the load.This is “cloud-friendly”: pay-per-use, match demand, optimize costs
• Agility: (not related to scalability - distractor) New IT resources are only a click away, which means that you reduce the time to make those resources available to your developers from weeks to just minutes.
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