Some confusion with dependencies between Application and Infrastructure layers

Question

1. When the application is using a Repository ( which is an Infrastructure service ), then Repository interface is defined within a Domain layer, while its implementation is defined within an Infrastructure layer, since that way Domain model has no outgoing dependencies.

a) If we're 100 % certain that a particular ( non-Repository ) Infrastructure service InfServ won't ever be called by any code within Domain layer, then I assume InfServ's interface doesn't need to be defined within a Domain layer?

2. Assuming InfServ won't be called by code within Domain layer ( and as such we don't need to define its interface within Domain layer ):

a) if InfServ will be called by Application layer's Services, I assume Application layer should have an implicit dependency on Infrastructure layer and not the other way around? In other words, both InfServ's interface and its implementation should be defined within an Infrastructure layer?

b) And the reason why it is better for Application layer to have a dependency on Infrastructure layer and not another way around is that Infrastructure layer can be reused by many applications, while Application layer is in most cases specific only to a single application and usually can't be reused by other applications?

c) If we are 100 % certain that no code within Domain layer will ever use a Repository, then we wouldn't need to define its interface within Domain layer?

UPDATE:

1)

Yes, however, the definition of domain layer can include application services which act as a facade over the domain. The application service usually references a repository and other infrastructural services. It orchestrates domain objects and said services to implement use cases.

a)

... which act as a facade over the domain

Couldn't we argue that "regular" ( those I was referring to in my question ) Application Services also act as a facade over the domain?

b)

The application service usually references a repository and other infrastructural services.

From your reply, it seems as if you're suggesting ( though you're probably not ) that "regular" Application services don't normally reference Infrastructural services, which in fact they do ( as far as I know )?!

2)

a)

I usually merge application services, as described above, into the domain layer.

"Regular" Application layer is also part of a BLL layer, so couldn't we argue that it is merged with Domain layer ( it actually sits on top of Domain layer )?!

b)

I tend to adhere to the Hexagonal architecture style ...

It appears Hexagonal architecture doesn't have the explicit concept of Application layer ( ie of Application Services )?

c)

Part of the benefit of declaring a repository interface in the domain layer is that it specifies the data access requirements of the domain.

So we should include the Repository interface in Domain layer even if we domain code won't use it?

SECOND UPDATE:

2a)

If what you call a "regular" application service interacts with the domain, I think it is acceptable to make it part of the domain "layer". However, the domain should not directly depend on the surrounding application service and so it is possible to split them up into separate modules if desired.

• I'm not sure whether or not you're implying that the design of Application layer ( in traditionally layered architecture ) is any different from when you merge Application layer with Domain layer using, for example, Onion architecture?

• I'd say at least as far as modules are concerned, the two shouldn't be different, since in both cases we can separate Application and Domain layer modules? ( though I must confess I skipped the chaperon modules ( Evan's book ) since I didn't think I'd need that knowledge so early into learning DDD :O )

2b)

Yes because it can be contrasted with a layered architecture. A strict layered architecture does not gel with the idea of declaring repository interfaces in domain layer and implementing in infrastructure layer. This is because, on the one hand, you have a dependency in one direction, but in terms of deployment the dependency is in the other. Hexagonal addresses these concerns by placing the domain at the center. Take a look at the onion architecture - it is essentially hexagonal but may be easier to grasp.

I don't yet know the MVC pattern or Asp.Net MVC, but regardless, from reading the first three parts of the series ( which confused me to the point I stopped reading it ), it appears :

a) From Onion article:

Each layer is coupled to the layers below it, and each layer is often coupled to various infrastructure concerns.

The author is implying that in traditionally layered architecture TLA a Domain layer is coupled to Infrastructure layers, which certainly isn't true, since we normally define Infrastructure interfaces ( such as Repository interface ) within Domain layer?!

b) And if when using TLA we decide to define Infrastructure interfaces in Application layer, then Application layer also isn't coupled to Infrastructure layer?!

c) Isn't with Onion architecture an * Infrastructure layer* coupled to Application Core ( which includes Application layer ), since Infrastructure interfaces are defined in Application Core?

d) If yes to c), isn't it better to couple Application layer to Infrastructure layer ( for reasons I gave in my original question ( here I'm assuming Domain layer won't be calling Infrastructure services) )?

4)

From Onion article:

The first layer around the Domain Model is typically where we would find interfaces that provide object saving and retrieving behavior, called repository interfaces.

From Onion article:

The controller only depends on interfaces, which are defined in the application core. Remember that all dependencies are toward the center.

It appears the author is implying that since dependencies are only inwards and since Infrastructure interfaces are defined around Domain Model, that code within Domain Model shouldn't reference these interfaces. In other words, we shouldn't pass Repository reference as an argument to a method of a Domain entity ( which as you yourself has said is allowed ):

class Foo
{
           ...
       public int DoSomething(IRepository repo)
       {
              ...
            var info = repo.Get...;
              ...
       }
}

For reasons stated above I must confess that I fail to see the benefits of using Onion architecture or even how it is different from TLA ( assuming all Infrastructure interfaces are defined within Domain layer ) --> in other words, couldn't we depict TLA using Onion architecture diagram?!

FINAL UPDATE:

2)

b)

Yes. In TLA the domain layer would communicate directly with infrastructure in terms of classes declared by infrastructure layer not the other way around.

Just to be sure – you're saying that with TLA an Infrastructure interfaces would be defined in Infrastructure layer ( I know that with Hexagonal/Onion architecture they are defined in Application core )?

d)

The coupling goes both ways. The application core depends on implementations in infrastructure and infrastructure depend on interfaces declared in application core.

My point is that since with Onion architecture an InfServ interface is declared within Application layer ( here the assumption is that InfServ is never called by Domain Layer and as such we decide not to define InfServ interface within Domain Layer – see original 1a question ), it means that Application layer controls the InfServ interface. But I'd argue that it would be better if Infrastructure layer controlled the InfServ interface, due to reasons stated in the original 2b question?!

4)

It appears the author is implying that since dependencies are only inwards and since Infrastructure interfaces are defined around Domain Model, that code within Domain Model shouldn't reference these interfaces.

In my opinion, your code sample is appropriate to code.

So I was correct in saying that Onion Architecture doesn't "allow" Domain Model to reference Infrastructure interfaces, since they are defined in layer InDe ( InDe of course also resides within application core ) surrounding DM strong textand referencing them from DM would mean that dependencies go upwards from DM to InDe?

DDD is typically presented in a hexagonal/onion architectural style which is why there may be some confusion. I think what you've probably been doing is already hexagonal which is why it seems like it is the same thing.

Yeah, I got that impression also. Though I do plan to look a bit deeper into the Hexagonal architecture ( especially since the new DDD book will base some examples on it ) after I finish reading Evan's book.

FOURTH UPDATE:

2)

d)

If infrastructure owned interface and implementation then the domain or application layer would be responsible for implementing persistence for itself.

I assume Application or Domain layers would need to implement it for themselves because referencing interfaces defined in Infrastructure layer would Onion's rule of inner layers not having dependencies on outer layers ( Infrastructure layer being the outer layer )?

4)

Domain model can reference infrastructure interfaces, such as a repository, because they are declared together. If application layer is split from the domain, as it is in the Onion diagram, then the domain layer can avoid referencing interfaces because they can be defined in the application layer.

But according to that article, Infrastructure interfaces are declared in a layer surrounding Domain Layer, which would mean it is closer to the outer edges of application core than Domain Layer – and as the article pointed out, the inner layer shouldn't have dependencies on outer layers>!

thank you

Answer 1

1a) Yes, however the definition of domain layer can include application services which act as a facade over the domain. The application service usually references a repository and other infrastructural services. It orchestrates domain object and said services to implement use cases.

2a,b)

I usually merge application services, as described above, into the domain layer. I tend to adhere to the Hexagonal architecture style, also called ports and adapters. The domain is at the center and is encapsulated by the application service and all external connections, including repositories, UI, and services act as ports.

2c) Part of the benefit of declaring a repository interface in the domain layer is that it specifies the data access requirements of the domain.

UPDATED

1a) I didn't intend for the application services that I mention to be distinguished from "regular" application services. If it is a facade over the domain then the rules apply.

1b) There may be services that can still be called application services, but aren't directly related to the domain and I wanted to exclude those.

2a) If what you call a "regular" application service interacts with the domain, I think it is acceptable to make it part of the domain "layer". However, the domain should not directly depend on the surrounding application service and so it is possible to split them up into separate modules if desired.

2b) Yes because it can be contrasted with a layered architecture. A strict layered architecture does not gel with the idea of declaring repository interfaces in domain layer and implementing in infrastructure layer. This is because on the one hand you have a dependency in one direction, but in terms of deployment the dependency is in the other. Hexagonal addresses these concerns by placing the domain at the center. Take a look at the onion architecture - it is essentially hexagonal but may be easier to grasp.

2c) Yes, but usually the repository interface would be referenced by an application service.

UPDATE 2

2a) They could be implemented differently, but the general responsibilities are the same. The main difference is in the dependency graph. While you can separate application services into their own modules with either architecture, the onion/hexagonal architecture emphasizes the use of interfaces to declare dependencies on infrastructure.

2ba) Yes and this is in fact a characteristic of the onion architecture.

b) Yes. In TLA the domain layer would communicate directly with infrastructure in terms of classes declared by infrastructure layer not the other way around.

c) Yes, infrastructure implements interfaces declared by domain layer.

d) The coupling goes both ways. The application core depends on implementations in infrastructure and infrastructure depends on interfaces declared in application core.

4) In my opinion, your code sample is appropriate code. There are cases where an entity needs access to a repository to perform some action. However, to improve the coupling characteristics of this code, it would be better to either define a specific interface that declares the functionality required by the entity, or if lambdas are available even better. The repository could then implement that interface and the application service would pass the repository to the entity when invoking the given behavior. This way, the entity does not depend on a general repository interface, instead it depends on a very specific role.

DDD is typically presented in a hexagonal/onion architectural style which is why there may be some confusion. I think what you've probably been doing is already hexagonal which is why it seems like it is the same thing.

UPDATE 3

2b) In TLA there would be no interfaces, or not the same kind. The domain would communicate directly with infrastructure, such as a persistence framework and so it would be responsible for persistence.

2d) If infrastructure owned interface and implementation then the domain or application layer would be responsible for implementing persistence for it self. In hexagonal/onion, the implementation of persistence is part of infrastructure - it "adapts" the abstract domain to database.

4) Domain model can reference infrastructure interfaces, such as a repository, because they are declared together. If application layer is split from domain, as it is in Onion diagram, then the domain layer can avoid referencing interfaces because they can be defined in the application layer.

UPDATE 4

2d) No that statement applies to a layered architecture with hierarchy like: UI -> Business Logic -> Data Access. The business logic layer depends on the data access layer and has to implement its data access based on the object model of the data access framework. The data access framework itself doesn't know anything about business layer.

4) The article specifies only one possible architecture. There are acceptable variations.