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, which are usually a few calls/
second. Put 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 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.

B.
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.

C.
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.

D.
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 SOL query Is optimized by the use of the Index.

Explanation:
https://aws.amazon.com/dynamodb/faqs/
Q: Can a global secondary index key be defined on non-unique attributes? Yes. Unlike the primary key on a
table, a GSI index does not require the indexed attributes to be unique.
Q: Are GSI key attributes required in all items of a DynamoDB table? No. GSIs are sparse indexes. Unlike the
requirement of having a primary key, an item in a DynamoDB table does not have to contain any of the GSI
keys. If a GSI key has both hash and range elements, and a table item omits either of them, then that item will
not be indexed by the corresponding GSI. In such cases, a GSI can be very useful in efficiently locating items
that have an uncommon attribute.