How Redis(Elasticache) Cache & Persistence have to be used with AWS RDS for CI?

I'm on infinite loop of implementing Redis Cache(Service: AWS ElastiCache) for SELECT queries (database hosted at AWS RDS) and CodeIgniter hosted at EC2. 

I had explored many many of docs from AWS, CI and blogs. But, really confused about architecture of implementing with best practice. I have to achieve faster response with DB queries, and my project involves/hits maximum of SELECT queries. And, these READ queries pulled from slave database(Read Replica) with AWS RDS.

Can community experts  adds their expertise to this thread?

 from Aaimaa Web Solutions