title: Domain queries in CQRS
body: <p>We are trying out <a href="http://codebetter.com/blogs/gregyoung/archive/2009/08/13/command-query-separation.aspx" rel="noreferrer">CQRS</a>. We have a validation situation where a CustomerService (domain service) needs to know whether or not a Customer exists. Customers are unique by their email address. Our Customer repository (a generic repository) only has Get(id) and Add(customer). How should the CustomerService find out if the Customer exists?</p>