To give you a quick overview of my application architecture, I have the following layers in my application:
Domain Model - Models the problem domain and business rules.
Service Model - Models the service contract consumed by the application.
Data Access Layer - Persistence of domain models, in this case using EF.
Services - Implementations of the service model.
Website - MVC web application.
Web API - RESTful web service API.
Now the problem is that the Web API came later; the MVC web application was the first thing that was built on top of the architecture.
I find myself having to replicate business logic and ViewModels (MVC) and Messages (Web API)...According to the DRY principle, I shouldn't be doing this, so the natural solution is to push the business logic into its own layer which is independent of the application layers.
But...this means that the business logic layer needs it's own set of models which sit between the application layer and the business logic layer. In this respect, it would not be applicable to use domain models, since these really shouldn't ever hit the application layer.
So in short, I'm looking for an industry accepted standard for business logic modelling. Is there an accepted standard for how this should work, and if so, what type of model (i.e. it's not a domain model, and it's not a view model) belongs to the business logic layer?
You have stumbled upon a contested point of discussion. I myself have struggled with this for a long time.
Object bigots will argue that you shouldn't even have a service layer. Your domain model could then be directly consumed which would eliminate the duplication of logic. Granted, those were the good old days of software modeling. With the advent of the web, apis and SOAs, we are forced to look at alternative ways of modeling our software. Enter Anemic Domain models.
Anemic Domain models essentially consist of light weight objects that resemble DTOs more than anything, with underlying services that do the heavy lifting.
The architecture you have described above seems to be a hybrid design. I am guessing by now you must have run into the issue of mapping EF classes to domain objects, which creates yet another layer of objects, unless you use POCOs, in which case you run into namespace issues and what not.
Anyways, I don't think there is an industry standard. What I can tell you is that I have seen a few patterns emerge here and there leaning towards Anemic Domain Models and it's not hard to see why. In disconnected environments (e.g. web, API), complex objects don't work very well, hence the affluence of services.
Since you have layered your application nicely and do not wish to expose your domain model objects, I would suggest using Anemic Models in your service implementations. These would essentially function as DTO objects that can be reused and serialized if the need be, but can also implement basic logic that may even map back to functionality implemented in the services.
On a final note, there is no one-size-fits-all. Use the right tool for the job. Patterns are meant to be guidelines, not step-by-step instructions, so make sure you feel free to tweak them in order to fit your particular needs while retaining the general idea.
You can read more about Anemic Domain Models here:
https://en.wikipedia.org/wiki/Anemic_domain_model
Make sure to check out Martin Fowler's objections to Anemic Domain Models as well:
http://www.martinfowler.com/bliki/AnemicDomainModel.html
Related
I am a new comer in DDD concepts and I am trying to design a solution structure strictly follow DDD & Prism (WPF).
I am stacking at where to create DTO project (Data Transfer Object). As I know, DDD will have 4 general layer:
Presentation
Application
Domain
Infrastructure
So that, would you please tell me which layer for the DTO project belong to?
I have refered at: DDD - which layer DTO should be implemented
Some one told that we should implement it at Service layer, but I think It is not make sense, cause of follow DDD concept, we don't have which called Service layer.
Thank you so much,
Regards
Generally speaking, the location of the component's code should stay besides the owner of that component. The owner has full control of its components. This idea follows the dependency inversion principle.
So,in your case, who is the owner of the DTO? Who controls the structure and the purpose of that DTO?
Using DDD (in any architecture!) you should, however, take into account that domain code should not depend on infrastructure, application or UI code. So, if you decide to put the DTO class into the application layer then you cannot refer to that DTO from your domain code (i.e. a domain entity or a domain service cannot have any references to the DTO class - no import or use directive or new statements)
You might be disturbed by my answer, but I have to repeat again:
Domain-Driven Design is not about layers. It is about Domain and Design.
Domain is your problem space. This is what your users need. This is what your domain experts tell you to do.
Design is how you model your solution. It starts from the knowledge you got by speaking to domain experts. It continues with deciding what are your bounded contexts and how they are placed on the context map. Then you decide how to implement each context - would this be a Domain Model, Active Record, CRUD or some off-the-shelf software.
When you decided on the implementation, you can think of the internal structure and DDD does not dictate anything there.
If your question is where to put DTO for the data - well, they might be located in a separate project if more than one other project uses them, or you put them to the DAL (which is typical) since your presentation layer will most probably use it directly, without involving the domain model, otherwise you won't be mentioning DTOs at all.
Concerning using the term DDD for your question, I wrote a blog post some time ago, targeting this issue, which you might want to read.
I see that we should add DTO classes in the layer will create objects of them to send their to another layer.
In DDD model the Application layer and Infrastructure layer already can access the Domain layer so they don't need DTOs in the infrastructure layer because it will return entity class.
Based on the above we need DTOs to represent the data retrieved from the DB(through Infrastructure layer) in the Application layer and send these data using DTOs to the Presentation layer without making Presentation layer get access the domain entities which are contained in the Domain model layer.
Finally, from my experience and using the DDD model I prefer to add DTOs in a separate project inside the Application layer. which means it belongs to the Application layer.
You can see the sample of my project and the "Core layer" in it means the "Domain Layer"
Also you can read Excellent detailed explanation for DDD model from Microsoft
can you give me some hit or give me explanation how to create Enterprise App in .net, and how projects type use or how should be structure of this projects? I newbie in EE and I read about it, but for me is the best explanation on real world example. My idea about EE solution structure in .net is that:
Data Tier (project type => class library)
database access classes
some mappers (I am not sure if I could use data mapper pattern or else? Is good idea?)
Bussiness Tier (project type => class library)
entities which wil lbe mapped in data mapper in Data Tier
and some application logic
service tier (I am not sure if it should be individual tier, or subtier of bussiness tier - I want to use WCF)
Client (project type => WebForms / Android / WPF / ....)
will be communicate with bussiness tier over WCF
Is my idea good? I will be gratefull for any explanation or hint how patterns could I use with respect my low knowledge. I have requirements to use 2-3 patterns, becouse is school project. Thanks for answers
A implementation I have found quite useful is:
Data Access Layer:
Entity Framework: Unit of Work
Repository Pattern
Business Layer
This layer maps entities and db calls to DTOs
Presentation Layer
Here you present your DTOS using MV? or any other model
There you are using at least 4 design patterns.
For the data layer I would go with repository pattern and unit of work pattern. This is really good way to abstract data layer and create testable code which can be easily unit tested.
Business layer it depends, by DDD business should be encapsulated inside rich data model. Anemic data model is considered as anti pattern. But personally rich model can lead to the ruin of the separation of concern paradigm. Sometimes it is useful to have anemic data model and business layer on top of that model. Like handlers where every handler does exactly one action...
On top of business layer is usually application layer which expose interface to the outside, to your clients. It should be really thin without any business logic in it. Maybe Restful API which will enable you to connect various clients like Android, wpf, Javascript...
In my n-tier .Net Application I got next layers:
Data Access Layer (EF)
Business Layer (Validation & Business Logic)
Presentation Layers (1 MVC Controller and many API Controllers)
I found, that my Business Services only validate business objects, call CRUD DAO methods and return results to Api Controllers.
So, I doubt: may be Web Api Controllers should be used as Business Services?
Interesting, just answered a similar question...
So I woudn't do it I were you.
Here's just a few disadvatages of the approach from the top of my head:
Performance - a redundant HTTP roundtrip in Web MVC project.
Separation of concerns - most of the time the functionality provided
by API differs greatly form UI for the same project/application. You
might want to limit the API to a few methods with a strict contract.
In case you want Web API to be a layer between Web MVC and your DAL
you will have to expose all functionality you need for UI as well.
Also you might want to have different authorization and
authentication mechanisms. Very often API exceptions handling is
also different as well as input validation.
Maintanance - everytime you need to make a change required for UI
only you have to make sure it doesn't brake your API clients. Also
API versioning is a very important topic and mixing it with most UI
changes makes this process even more difficult.
Probably for now you application is not that complex but from the design perspective your solution is much more flexible now than it will be if you decide to put Web API between your UI and DAL layers.
N-Tier applications and multi-layer are not popular among the new wave of developers. Keep in mind, that just because something is not popular, among a group, does not mean that it does not have merit.
Pros of MVC:
Separation of Concerns
Unit Testing
Does a multi-layer MVC application using a Web.API have merit:
I know this will be met with some discontent and disagreement. However, my concern is that single purpose application developers are not giving consideration to enterprise development. Enterprise development, load balancing, manageable code maintenance, and true Separation of Concerns are only possible with multi-layer applications that can easily lend themselves to N-tier.
Many developers are operating in environments that demand that they design and implement data structures in SQL, create and maintain models and CRUD functionality, develop controllers and design good looking and friendly views. The MVC model utilizing Entity Framework makes this a manageable task for these small to moderate platform developers.
In the Enterprise, separating the Business and Data Access layers from the User Interface makes real good sense. Right now MVC is a popular and very functional platform for efficient and usable User Interface development. What will be the UI platform in ten years? Separating the UI from the other layers gives more life to the work spent developing the business logic. Not to mention, that it allows for accessing data on multiple platforms today.
Multi-layer MVC using Web.API has these advantages:
True Separation of Concerns
Supports Unit Testing
Makes the Business logic and Data Access logic more scalable and reusable than MVC alone.
Supports data processes without page refresh. (REST)
CONS:
- Does not easily support use of Entity Framework. (Which is not ready for the enterprise yet anyway)
You could have your code as part as your model, and that would even be considered as good separation of concerns since MVC is build for that.
But what my preferred thing to do is keep logic in a Business Layer of it's own. The reason for that is, I think, better separation of concerns. I like using IoC, so there might be different configurations that I route thought different running solutions. Or, I might have another UI/API/Project that uses the same logic layer.
As for the overhead, it has a little overhead, but I think worth the trouble comparing to the actual overhead in code it creates.
I agree with others here, looking into using strongly typed views, the controllers would only create an instance of the viewmodel and send it on to the view. The view model then is the intermediary to the data services layer. I personally create all my entities using EF in a different project just to separate function. The view model can either call EF directly or you can add another layer of pre-canned EF queries which the Viewmodel uses just to populate the view collections. This would look like this:
[View]-[Controller]-[Viewmodel]-[Optional repository and interface to EF]---[EF]
In the interface to EF you would catch all DB errors when trying to get information and post back to the view according to your design.
The beauty of strongly typed views is that they post back and forth from the View and can contain methods which you can call at will. Because they are a pseudo model for the view, they can also have properties specific to the view which may be used at the business layer. I pass view models around quite a bit were warranted. (After all they are just a pointer)...
You can implement business logic/validation/calucations in the actual model/entity classes rather than ApiControllers, otherwise you will end up with so much code in your controller which is not really a good thing.
Also you can use DataAnnotations Attributes to perform your validation which sits outside of your controller. for.e.g. http://msdn.microsoft.com/en-us/library/ee256141(v=vs.100).aspx
I am using ASP.NET MVC in my application. I have divided my application into three layer architecture. 1) Data Access Layer (using Entity Framework), 2) Application/Business Layer, 3)Presentation Layer (ASP.NET MVC).
Because I am using MVC framework on my presentation layer, I am confused about the business logic. I need to know where do I put my business logic in MVC pattern. In other words we can say from where I need to call my middle layer. From the Models or from the Controllers?
If I call my business logic from Controller then It seem like the Models are useless. Other wise if I call business logic from the model then it seems like unnecessary bidden on the system because Business Object maps with model and then model is passed to the Controller. Model does exactly what DTO is doing.
Any help will be appreciated
ASP.NET MVC layer or tier doesn't contains neither business logic nor business model. M in MVC stands for UI model, not the model of your application core and MVC (as well as other MV* patterns) is generally pattern for separating UI concerns. You should send messages (call) your Business Layer (BL) from Controllers, aggregate data, create or map it results into UI model and pass it to view. Your UI model should know nothing of BL model - this distinction makes loosely coupled layers of your application.
In other words we can say from where I need to call my middle layer.
From the Models or from the Controllers?
Definitely from Controllers. You inject dependencies to it and call them from your Action methods. ASP.NET MVC provide lots of mechanisms of injection dependencies into controllers and integrates nicely with NInject, StructureMap and some other IoC containers.
Dependencies between components in MVC is given below
Picture is takes from Martin's Fowler article on GUI Architecture, which by the way, very nice reading regarding MVC and MVP.
Also, Pluralsight has set of videos on Software Practices, which covers Design Patterns. I've learned a lot from their definition of MVVM and MVP.
Reading this materials should increase your understanding not only on patterns itself, but also in how they fit into application environment and interact with it.
This is purely design/architecture decision you have to make based on your requirements.
If you want to scale up your application to support other services/applications, it is advised not to write any business logic in Controller/Model. You could write it in business/application layer. This will help you to scale up your architecture in future. Say if you want to create restful service for you mobile application, you could just write services as wrapper to re-use existing business/application layer.
Also just have a look at Domain Driven Design, Eric Evans book is worth reading.
http://dddsample.sourceforge.net/architecture.html
Almost all examples of DDD within the Alt .NET community are applied to web development; what I would like to see are some examples of DDD applied in scenarios where the client handles most of the business logic and the server is simply a web service for CRUD using DTOs [1].
I'm currently working on a product that has an anemic domain model. Coming from years of web development, this sort of thing would typically make me cringe; however, I'm not sure how I would go about structuring this code around the DDD principles. In a web app, I have direct access to the database at the point where I handle business logic (implemented within my domain model, invoked by the controllers), which means that I can easily have a rich domain model and use an ORM for CRUD. But, in my current project, DTOs are the only sane way to transfer data. 80% of the business logic is implemented in View Models (this is a WPF app), the other 20% percent is located in stored procedures - and 100% of the DAL is hand written ADO .NET. There's really isn't a lot of business logic implemented within the web services, so there aren't any "entities" to be found in the solution - they're all DTOs.
How could I introduce DDD into this sort of client/server architecture? What sort of experiences have you had, and what approach did you take? What patterns would you suggest? I haven't had much time to grok CQRS, but something tells me it could related - is that so?
Foot notes
Perhaps this is the wrong question to be asking. In this type of client/server WPF scenario, would it make sense to follow the web app paradigm, where 99% of the client's focus is on display logic?
This is funny, because I often wonder how DDD can be applied to web development.
I don't mean this to be a comprehensive answer to your question, but you might find something useful among the responses to an old question of mine: Should I map a DTO to/from a domain entity on both client and server sides?
As a very general answer to your question, I'd say that you could move logic out of the view models and into a client-side domain model (since it sounds like the server side is working just fine without any concept of a domain model). You'd use the DTOs coming out of the web service to "hydrate" these entities.
If you're creating a smart client, then you want to leverage the ability to maintain state and perform logic directly in the client (so, "no" to the footnote).