Design a system

You need a plan.
You need some patterns.

Patterns

Web Hooks
http://apidocs.mailchimp.com/webhooks/
http://code.msdn.microsoft.com/site/search?f[0].Type=User&f[0].Value=Srigopal%20Chitrapu


CQRS - Command and Query Responsibility Segregation
http://msdn.microsoft.com/en-us/library/jj591573.aspx

Design Patterns in .NET Framework
http://msdn.microsoft.com/en-us/magazine/cc188707.aspx