I have been reading "Clean Architecture" by Robert Martin and have been struggling to understand one thing: How should we pass data between layers in the "Clean Architecture"?
According to Robert Martin:
The important thing is that isolated, simple data structures are passed across the boundaries. We don’t want to cheat and pass Entity objects or database rows. We don’t want the data structures to have any kind of dependency that violates the Dependency Rule.
If I understand it correctly, we should always pass DTOs between layers. For example, Business Logic layer should never return Domain Entity back to UI layer, it should map Domain Entity to DTO and return it instead.
Similarly, Business Logic layer maps Domain Entity to DTO and passes it to Data Access layer, so Data Access layer knows nothing about Domain Entity.
It is kind of scary, because we have to perform 8 mappings for one request (4 in -> and 4 out <-) Please see the picture below.
Please note that arrows on the diagram show the data flow, not the dependencies.
I tried to find good examples but they only confused me. For instance, some examples have Domain Entity which is directly used by Data Access layer, so that Domain Entity dictates the structure of the table in the database (Domain entity used as an Entity Framework Entity).
But what if one day I decide to change the table structure? It means I'll have to make changes not only in my Data Access layer, but also in my Business Logic layer as well. It defeats the "Plugin Architecture".
What is the correct way?