Your web application needs four instances to support steady traffic all of the time. On the last day of the month, the traffic triples. What is the most cost-effective way to handle this pattern?

Running four Reserved Instances constantly, then adding eight On-Demand Instances is the most cost-effective way to manage the variable traffic pattern effectively.

Reserved Instances provide a significant discount compared to On-Demand pricing for instances that are needed consistently, which is beneficial for the normal, steady traffic that your application experiences. By having four instances reserved, you ensure that you have the necessary capacity at a lower cost during regular usage.

When the traffic significantly increases, as noted on the last day of the month when it triples, the additional On-Demand Instances can be quickly deployed to scale based on the immediate demand. On-Demand pricing allows you to adjust capacity without long-term commitments, which is ideal for handling sudden spikes in traffic.

This approach balances both the cost and flexibility needs of your web application—the Reserved Instances provide predictable costs and stability during steady traffic, and the On-Demand Instances allow you to scale up as needed without pre-purchasing capacity that could lead to wasted resources during low-traffic periods.

Upgrading to larger Reserved Instances would increase the costs unnecessarily since the current reserved capacity is sufficient for regular traffic. Similarly, relying only on Spot Instances poses a risk, as their availability fluctuates and they can be terminated at any time, which