I deployed an ROR application on without using cache because in some API data was not update on API. Simply use gem 'thin' in Gemfile in my application i have more then 1000k to 2000k users and API response being slow day by day as user increase. some time API Timeout in 30 second , no to much queries , simple single query per API, and Using RDS server for Mysql
This is became headache
i solved this issue through this
http://nerian.es/articles/2014/08/30/how-to-handle-concurrent-requests-in-rails/