title: gRPC repeated field vs stream
body: <p>Hi im currently looking into grpc and im curious about the use usage of a repeated field vs a stream. 
  For example let's say i want to implement a reservation service for movie seats. The issue im facing is, that i want to inform the service for which movie i want to reserve the seats for.
  I can think of 2 solutions, first: 
  I send the id of the movie with every seat i want to reserve, or with oneof at the beginning of the stream 
  like this: </p>
  
  <pre><code>rpc ReserveSeatsForShowing(stream SeatReservationRequest) returns(Reservation);
  
  message SeatReservationRequest{
      oneof reservationOneOf{
          int32 showingId = 1;
          SeatReservation seatReservation = 2;
      }
  }
  </code></pre>
  
  <p>Or using a repeated field like this</p>
  
  <pre><code>rpc ReserveSeatsForShowing(SeatReservationRequest) returns(Reservation);
  
  message SeatReservationRequest{
      int32 showingId = 1;
      repeated SeatReservation seatReservation = 2;
  }
  </code></pre>
  
  <p>Since i haven't really worked with grpc before im not quite sure which option to choose or if other options are available.</p>
  
  <p>Looking forward for your recommendations</p>