-
Hi Haridev,
Take note that there are two types of load in the scenario, which are:
– Steady-state Load
– Peak Load
The scenario says:
A web application is composed of an Application Load Balancer and EC2 instances across three Availability Zones. During peak load, the web servers operate at 95% utilization. The system is set up to use Reserved Instances to handle steady-state load and On-Demand Instances to handle the peak load. Your manager instructed you to review the current architecture and do the necessary changes to improve the system.
Which of the following provides the most cost-effective architecture to allow the application to recover quickly in the event that an Availability Zone is unavailable during peak load?
The provided answer is:
Launch a Spot Fleet using a diversified allocation strategy, with Auto Scaling enabled on each AZ to handle the peak load instead of On-Demand instances. Retain the current setup for handling the steady-state load.
Throughout the scenario, the steady-state load of the application is still handled by Reserved EC2 instances which means that the application’s availability is not compromised. The only thing that’s changed here is the use of a Spot Fleet for handling the peak load.
Furthermore, we are also using a Spot Fleet with a diversified allocation strategy. This is an advanced compute setup in AWS that balances out the price and computing capability/availability. We have to fully understand why a Spot instance is getting interrupted so we can implement a cost-effective, yet highly available, solution.
AWS usually interrupts a specific Instance type, based on the demand it’s getting so for example, if there’s a surge of On-Demand t3.large instance type, then your Spot t3.large instance would most probably be interrupted. Hence, a Spot Fleet with a variety of EC2 instance types (t3.large, m6.medium, a4.small, etc), provides more durability. If the demand for t3.large instance type goes up and it affects/terminates the t3.large instances on your end, then you still have m6.medium and a4.small EC2 instance types available.
More information here:
I’ve worked with various clients that also implement the same thing in their architecture. Different applications have different SLAs and compliance requirements. It depends on the various factors but the most important thing to note here is that you shouldn’t use a Spot Instance solely for your steady-state load.
Cheers,
Jon Bonso
