I think the system OP is describing is a little bit more complex. You're not just describing message types, you also have message templates; a template declares

I think the system OP is describing is a little bit more complex. You're not just describing message types, you also have message templates; a template declares a message type and a set of prefilled fields. You save data by just sending the subset of fields that are actually changing, which is a very good abstraction for market data. The template is hydrated on the protocol parsing layer so your code only has to deal with message types itself.