You have seen the basic principles. They may seem simplistic, but it's surprisingly difficult to be disciplined enough to follow them. One place where experienced programmers have continually broken these simple rules is in the area of persistence frameworks. Sometimes, persistence solutions like EJB CMP are too invasive, requiring extensive code changes to support. Other times, persistence frameworks do not allow for extension, so common legacy models like stored procedures cannot be supported. More often than not, though, they're simply overbuilt.