I am using Elastic Beanstalk and Amazon RDS. Now I want to cache this RDS.
According to this documentation, I just have to create an ElastiCache cluster and give it access for the security group of the Elastic Beanstalk cluster. I have done that. But I am not sure, whether this Redis cache cluster is caching the Beanstalk's RDS. How can I link the two and also ensure that they are indeed linked?
ElastiCache doesn't (automatically) cache RDS. You can implement code that does that yourself but that's not what it does.
ElastiCache provides you with Memcached or Redis in-memory databases as a service. You have to write to and read from the database yourself