You need a persistent and durable storage to trace call activity of an IVR (Interactive Voice Response) system. Call duration is mostly in the 2-3 minutes timeframe. Each traced call can be either active or terminated. An external application needs to know each minute the list of currently active calls. Usually there are a few calls/second, but once per month there is a periodic peak up to 1000 calls/second for a few hours. The system is open 24/7 and any downtime should be avoided. Historical data is periodically archived to files. Cost saving is a priority for this project.

What database implementation would better fit this scenario, keeping costs as low as possible?

A. Use DynamoDB with a -Calls- table and a Global Secondary Index on a -State- attribute that can equal to -active- or -terminated-. In this way the Global Secondary Index can be used for all items in the table.

B. Use RDS Multi-AZ with a -CALLS- table and an indexed -STATE- field that can be equal to -ACTIVE- or TERMINATED-. In this way the SQL query is optimized by the use of the Index.

C. Use RDS Multi-AZ with two tables, one for -ACTIVE_CALLS- and one for -TERMINATED_CALLS-. In this way the -ACTIVE_CALLS- table is always small and effective to access.

D. Use DynamoDB with a -Calls- table and a Global Secondary Index on a -IsActive- attribute that is present for active calls only. In this way the Global Secondary Index is sparse and more effective.