今天分享的题目比较简单，主要考察应试者是否熟悉不同种类EC2实例的所需花费，以及如何根据实际需求，利用这些实例种类，构建最优性价比的解决方案。
A Solutions Architect is working with a company that operates a standard three-tier web application in AWS. The web and application tiers run on Amazon EC2 and the database tier runs on Amazon RDS. The company is redesigning the web and application tiers to use Amazon API Gateway and AWS Lambda, and the company intends to deploy the new application within 6 months. The IT Manager has asked the Solutions Architect to reduce costs in the interim. Which solution will be MOST cost effective while maintaining reliability?
A. Use Spot Instances for the web tier, On-Demand Instances for the application tier, and Reserved Instances for the database tier.
B. Use On-Demand Instances for the web and application tiers, and Reserved Instances for the database tier.
C. Use Spot Instances for the web and application tiers, and Reserved Instances for the database tier.
D. Use Reserved Instances for the web, application, and database tiers.
答案：B
题目难度：低
题目解析：题目描述的场景非常简单，标准的三层网络应用架构，EC2实例上跑Web页面和应用服务，数据层由RDS来支撑。至于以后要使用API Gateway，Lambda等，其实都是干扰项。重点是要在满足可靠性的前提下，确保花费最少。
首选排除包括Spot实例的选项。因为Spot实例虽然价格低廉，但是没法确保系统的可靠性。
剩下的就看，如果只使用Reserved实例（RI），能不能在系统满足可靠性的同时，确保费用最低。
理论上，提前预定6个月内需要用到的实例，费用支出肯定会节省一些。不过，RI的数量怎么样来确定呢？按照性能需求低谷或者平均值来预估，肯定无法满足峰值要求；但如果按照峰值数据预定RI，性价比明显会降低。
因此选择On-Demand与Reserved实例混合的方案最符合题目要求。
Tips：跟其它的认证考试类似，类似于选项D这种一刀切的方案，除非有非常充足的理由，否则都应该避开。
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