Final answer:
The most cost-effective way to handle the traffic pattern is to run four on-demand instances constantly and add eight more on-demand instances on the last day of each month.
Step-by-step explanation:
The student's question is about finding the most cost-effective way to handle a web application's traffic that triples on the last day of the month. Considering the patterns of usage, the most cost-effective solution would be to run four on-demand instances constantly and add eight additional on-demand instances only on the last day of each month. This approach allows the system to scale up resources when necessary without incurring the continuous cost of running extra capacity that is not used most of the time.
If your web application needs four instances to support steady traffic all the time, and on the last day of the month, the traffic triples, the most cost-effective way to handle this pattern would be to run four on-demand instances constantly, then add eight more on-demand instances on the last day of each month. This approach allows you to have the required number of instances during regular traffic and scale up to meet the increased demand on the last day of the month without paying for reserved instances that you won't fully utilize. This way, you optimize cost by running on-demand instances for most of the month and only pay for the additional instances when they are needed.