Hello, Here is what I mean regarding having some common types (Provider interface): For example, let's see how Reservation message looks like (it applies to reservation, provision, release, terminate and query messages): reservation reservationRequestType correlationId replyTo reservation and compare it to provision message: provision provisionRequestType correlationId replyTo provision So my question is, do we need these reservationRequestType and provisionRequestType? Couldn't it look like the following: reservation correlationId replyTo reservation provision correlationId replyTo provision or we could have some common header for correlationId and replyTo fields: reservation commonRequestType (contains correlationId, replyTo) reservation provision commonRequestType (contains correlationId, replyTo) provision Regarding the Requester interface: All confirmation/failed messages (except queryConfirmed, queryFailed and reservationConfirmed) contain correlationId and GenericConfirmedType/GenericFailedType. For example, terminateConfirmedType looks like: terminateConfirmedType correlationId terminateConfirmed (contains GenericConfirmedType) Question is do we need to have terminateConfirmed type here (or in case of terminateFailedType do we need terminateFailed)? Couldn't it look like: terminateConfirmedType correlationId GenericConfirmedType Br michal