AWS introduced ElastiCache for this purpose. ElastiCache is fully compliant with MemCached and applications already using MemCached can easily migrate their code to Amazon ElastiCache. ElastiCache serves data from RAM, and execute all the simple operations (such as SET and GET) with O(1) complexity. Amazon ElastiCache large node can do 40k+ RPS throughput easily, this approach is far more performing than relying on database for everything.Refer the following benchmark from garantiadata to understand how memcached/elasticache can add performance to your architecture. http://garantiadata.com/blog/its-true-even-modest-datasets-can-enjoy-the-speediest-performance.
Using Amazon ElastiCache distributed caching tier in your architecture, you reduce the overall read load in your database and increase the overall performance of your application. Request/response latency can be reduced to few milliseconds using Amazon ElastiCache in your architecture.Since Amazon ElastiCache is a volatile in nature and the items have TTL's it is recommended to fall back to DB or source data stores in event of cache miss.
To know more about Amazon ElastiCache please deep dive into this article here : http://harish11g.blogspot.in/2013/01/amazon-elasticache-memcached-internals_8.html