I'm starting on DDD and I have a doubt on application of DDD on a Web Project.
If I have multiple Bounded Contexts for every section of a web project. For example, "Catalog" and "Shopping Cart" on a E-Commerce project. ¿Where should be the code that implements the frontend for all the Web and presents concepts from many Bounded Contexts?
I have thought about creating the "Web" Bounded Context, but this Bounded Context won`t represent a specific Ubiquitous Language because this BC will use concepts of many Bounded Contexts and Subdomains.
What you do think about this?
Thanks.
Where this code goes depend on the structure of your application.
DDD is a set of patterns and rules that helps you model your business. This model should be ubiquitous, meaning different applications should share the same business logic. The main rule of DDD is what describes the business goes in the domain, everything else does not. DDD does not state anything about how to you should structure your application, it can be applied to any architecture.
What you describe is called presentation logic and does not describe your business logic. It describes how your system interacts with clients, which are external actors and is specific to your application: if you make a web or a mobile version of your app, chances are that you will have the same domain implementation but your presentation logic will slightly differ. So, there is no DDD answer to where the presentation logic goes, besides not in the domain.
If you make a traditional 3-layered application, this logic goes in the presentation layer.
While studying DDD I'm wondering why the Domain model need to define interfaces for the Infrastructure layer.
From my reads I got that a high level (domain model, application service, domain service) defines interfaces that need to be implemented by a lower layer (infrastructure). Simple.
From this idea I think it makes sense that an application level (in a high level) defines interfaces for a lower one (infrastructure) because the application level will use infrastructure components (A repository is a usual client of the applicaiton layer) but doesn't want to be tied to any special implementation.
However, this is confusing when I see in some books a domain level defining infrastructure interfaces because a domain model will not use ever a repository because we want our domain model "pure".
Am I missing something?
While studying DDD I'm wondering why the Domain model need to define interfaces for the Infrastructure layer.
It doesn't really -- that's part of the point.
The domain model defines the interfaces / contracts that it needs to do work, with the promise of happily working with any implementation that conforms to the contract.
So you can choose to implement the interface in your application component, or in the infrastructure component, or where ever makes sense.
Note the shift in language from "layer" to "component". Layers may be too simplistic to work -- see Udi Dahan 2007.
I came across the same question myself. From my understanding, the reason behind this is that you want the application to only consume objects/interfaces defined in the Domain Model. It also helps to keep one repository per aggregate since they sit next to each other in the same layer.
The fact that the Application layer has a reference to the Infrastructure layer is only for the purpose of dependency injection. Your Application layer services should call these interfaces exposed in the Domain layer, get Domain Objects (POCOs), do things and possibly call interfaces again; for example to commit a transaction. This is why for example the Unit of Work pattern exposes its action through a Domain Layer interface.
I am quite new to DDD and have come across a scenario that i'm not to sure how to handle.
I have an application that is used to track vehicles. This application is what will be implementing the "core" of the domain for the business i am working for. Not only is this application going to be used, there will be other utility programs that must be created and used in order to help this "core/main" application function.
for example:
there is an windows service needed that will perform configured queries on a database and return results to an external database that my routing application will use. This windows service has the concept of a QuerySettings class that can be created and is then executed by this application.
Question1:
What do you call utility applications like the above described in DDD? ( it definitely isn't the main core of the domain but it's needed in order for the core application to work )
QUestion2:
Is QuerySettings a domain model? if not what is it and where should it be placed within following the onion architecture?
For question1: You may have a look at Bounded context, I think Bounded context contains a group of Domain models that represent concepts in a subdomain(or a core domain). You may need to map or share domain models in different bounded contexts to handle your business, this depends on your bounded context strategy, share-kernal, anti-corruption-layer(to name a few).
For question2: I have little information of how QuerySettings works but in general it is a domain model but in a generic subdomain, not in your vehicle tracking core domain. In core domain's view, it maybe an infrastructure concept.
I'm somewhat new to Domain Driven Design and have coded up our core business processes and objects in our Domain Layer. The domain objects are being persisted through Repositories, and everything fits really well. The domain model and services are exposed to the front end via web / WCF services.
I'm starting to work on parts of the front end, and have some front end settings and concepts that need to be persisted, like saving user layouts, views, and preferences. Originally, I just had these models coded up in the front end assembly (I'm using WPF). I'm trying to figure out how these application objects fit into the existing domain model and repositories. Other than using the same User object, the application model concerns seem completely unrelated to the existing domain model. I'm wary of overengineering the solution, but it feels like I should create a sort-of mini-domain model and respository for the application objects. Should I create a separate set of assemblies/libraries to manage the application models or is there a better approach? Thanks!
Absolutely. Keep your core domain small and focused.
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?
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
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.
The only benefit I see in placing repository interfaces in the Application layer instead of the Domain is if you want to reuse the Domain DLL in another application where you would completely redefine the way you query your collections of domain entities - in other words, need completely different repositories.
However there are 2 major obstacles to that approach :
Domain layer Services. They represent processes that are at the crossroads of many entities, and thus need multiple references to do their job. It's a common and convenient thing for them to get these references by asking Repositories.
You can find an example of this in the RoutingService here.
Also, there are special cases when some entities could need the help of a repository to find a particular other entity. For instance, if you have a hierarchy of aggregate roots with parent/child relationships between them, one particular instance could say "I need the list of all my ancestors/descendants that satisfy these criteria". A repository seems like the best fit for that kind of query.