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When you’re trying to design a service layer API, there are two things you always need to be very careful of. The first is making sure the API isn’t chatty. A service layer is usually deployed on another physical machine than the client layer, so you want clients to be able to do everything they need to do with a minimum of roundtrips, because those are after all rather expensive. The second thing you want to avoid is the implicit coupling between the client of the service and the service itself. In your quest to avoid the chatty interface, you might start grouping several service operations together in more coarse-grained operations to avoid the chatty communication that might otherwise occur. This can be good from a performance point of view, but it usually introduces a certain implicit coupling between the service and the client, because a specific grouping of operations might be beneficial to one client, but might be completely inappropriate to another type of client of the service.