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Is the Controller in MVC considered an application service for DDD?
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Closed 6 years ago.
In an ASP.NET MVC world, could controllers act as the application layer, calling into my domain objects and services to get the work done (assuming the controllers just strictly calls the domain and does nothing more). In the particular case that am dealing there is very minimal application flow logic that I need to model, hence am thinking about doing away with application layer and calling the domain directly from within the controller.
Is this a fair approach?
I guess what you mean here is that your business logic is implemented using the Domain Model pattern. In such case, you application layer should be very simple, and by definition it shouldn't host any business logic. All business logic should reside in the domain layer; methods in your application layer should resemble the following steps:
Load instance of an aggregate
Execute action
Persist the updated aggregate
Return response to the user
If that's all you do in your application layer, I see no reason not to put it in the controller.
If, on the other hand, your domain model is anemic, and you do have business logic in the application layer, then I'd prefer to introduce a separate layer.
Treating your MVC controllers as your DDD application layer will present a few negative situations.
The biggest one is that your application layer (regarding DDD), is one of the key places where you model your domain/business processes. For example, you might in an application service have a method called:
PayrollService.CalculatePayslips();
This might involve, checking a domain service or external system for the current active employees, it might then need to query another domain service or external system for absences, pension deductions, etc. It will then need to call a domain service (or entity) to do the calculations.
Capturing domain/business logic like this in the MVC controllers would cause an architectural issue, should you want to trigger the task of calculating payslips from elsewhere in the system. Also if you ever wanted to expose this process as a web service, or even from another MVC controller, you would have references going across or up into your presentation layer. Solutions like these "might work", but they will contribute towards your application becoming Spaghetti Code or a Big Ball of Mud (both code smells in an architectural sense). You risk causing circular references and highly coupled code, which increases maintenance costs (both time, money, developer sanity, etc.) in the future.
At the end of the day, software development is a game of trade-offs. Architectural concerns become bigger issues, the bigger your application grows. For a lot of very small CRUD apps, architectural concerns are probably negligible. One of the key goals of DDD is to tackle complexity, so it could be argued that most DDD projects should be of sufficient complexity to be concerned about good enterprise architecture principles.
I would not do it.
The effort of creating a separate class for the application service and then invoke it from the controller is minimal. The cost is also only there when you build the application.
However, the price you have to pay once you start to maintain the application is much higher due to the high coupling between the business layer and the presentation layer.
Testing, reuse, refactoring etc is so much lower when you make sure to separate different concerns in an application.
Related
I am in the process of making project decisions on development patterns for a solution, which involves the use of Entity Framework 6 as the ORM choice,
and ASP.NET MVC 5.
I need insight on how transactions and business logic will be implemented. In respect to layers, I came to an initial assumption for the design where
Entity Framework on top of SQL Server can be considered the Data Access Layer (DAL). On top of Entity Framework, there will be a Service Layer, where business logic and validation will be implemented. On top of the Service Layer, I will have ASP.NET MVC Controllers consuming what the service layer offers.
Let me ellaborate on this initial conclusion drawn as a starting point for defining the architecture:
I want to follow principles to achieve the minimum complexity scenario possible, in respect to layers, abstractions and all the solution components responsibilities. As an excercise, with this "simplicity" in mind, I could just embrace the template "proposed" by Microsoft as when you just create a new Visual Studio ASP.NET MVC Web application, but I believe that does not fit the minimum design scenario needed for an enterprise application, since in Microsoft's template, the controller directly makes use of Entity Framework DbContext and consumes the Data Access Layer, besides the fact that no service layer is present. This leads to several issues, such as extremely tight coupling
between the presentation and data access layer, as well as the so called "fat controller" problem, where controllers become the bloated piece of the software with all the added responsibilities of business logic, transactions, and so on, making it truly a mess to software maintainability with, for example, the most basic principle of DRY (don't repeat yourself) being violated since you would get duplicated code and logic all over your fat controllers.
Ramping up the next stage on the path from simplicity to complexity, I assume it is fair to add a Service Layer to the design, because this way ASP.NET MVC controllers would talk only to this service layer, who would be responsible for all CRUD and validation of CRUD operations, and all other more complex business logic operations. This Service Layer then would talk to the data access layer being represented by Entity Framework.
I would stop there and say the design with these proposed layers is enough, but that's where I need more insight on how to proceed. I need to resolve the question on how would transactions be implemented, if you think of them as a wrapper for a series of individual operations performed by methods responsible for validation and business logic residing in classes inside the service layer. In terms of implementation using Entity Framework, if I get every individual operation performed by a service layer method to issue a .SaveChanges(), I would lose the ability of having DBContext to behave like a Unit of Work, wrapping up a single .SaveChanges() for many individual DBSet operations. In this case, the DBSets may behave like repositories. Many people argue that Entity Framework's DBContext and DBSet are implementations if Unit of Work and Repository pattern, respectively.
In a more objective question then, how can I implement these patterns using directly DBContext and DBSet, without any further abstraction into new generic or specific entity repository classes and unit of work generic class? The implementation needs to rely on the consumption of a service layer for the reasons I have already stated.
I think an answer to that would be just the last complexity leap I feel necessary to get my "least complex viable design".
Let me put a more concrete example to illustrate:
In my service layer, I have 2 methods to implement validation logic for 2 insert operations, with a programmer defined method Insert such as:
EntityOneService.Insert
EntityTwoService.Insert
Each of these methods in their corresponding service layer classes would have access to a DBContext and use DBSet.Add to signal they should be persisted,
in case all validation and/or business logic passes. The desired scenario is that I can use each service layer method call in an isolated way, and/or in groups, such as in a new different service layer class method, such as:
OperationOnePlusTwoService.Insert
This new method would implement calls to EntityOneService.Insert and EntityTwoService.Insert, IN A TRANSACTION-LIKE FASHION.
By transaction-like I mean that all calls must succeed, not violating any validation or business rule, in order to have the persistence layer to commit the operations.
DBContext.SaveChanges() apparently would have to be called only once for this to happen, OUTSIDE of any service layer Insert method implementation. In the
context of an ASP.NET Controller consuming service layer methods, how could that be achieved, without actual implementation of a Unit of Work and Repostory abstraction over DBContext and DBSet?
Any advice please would be very much appreciated. I am not posting this to argue the value of a real abstraction and implementation of Repository and Unit of Work patterns, or if Entity Framework's DBContext and DBSet are or are not equivalent to proper Repository and Unit of Work patterns, that's not the point. My project requirements do not involve in any way the need to decouple the application from Entity Framework, or to ideally and fully promote testability.
These are not concerns and I am well aware of consequences and future maintainability impacts on not adopting full fledged implementations of half a dozen layers and all design patterns possible to make a big world-class enterprise solution.
desired scenario is that I can use each service layer method call in an isolated way ... but that behave IN A TRANSACTION-LIKE FASHION.
This is rather simple with EF, assuming your services are not remote (in which case transactions are not advisable in the first place).
In the simplest implementation, each service instance requires a DbContext to be passed in its constructor, and contributes its changes to that DbContext. The orchestrating controller code can control the lifetime of the DbContext and control its use of transactions.
In practice interfaces, rather than concrete service types are typically used, and Dependency Injection is often used instead of constructors. But the pattern is the same.
eg
using (var db = new MyDbContext())
using (var s1 = new SomeService(db))
using (var s2 = new SomeOtherService(db))
using (var tran = db.Database.BeginTransaction())
{
s1.DoStuff();
s2.DoStuff();
tran.Commit();
}
David
I am trying to implement an Onion architecture for a ASP.Net MVC 5 project. I have seen opinions that services should be injected rather than instantiated even though, correct me if I am wrong, the idea expressed by Jeffery Palermo (http://jeffreypalermo.com/blog/the-onion-architecture-part-3/) was that any outer layer should be able to directly call any inner layer. So my question is
Can the onion architecture work without IOC, and if yes, is it ideal?
Let's say we go with IOC, if the UI should not know about the actual implementation of
domain services, should we apply the same principle to the domain models
themselves e.g. injecting models into the UI instead of referencing
them directly?
I am understand why some solutions apply IOC on domain services but are accessing the domain models directly in the controllers.
OA can be thought of as n-tier architecture + Dependency Injection--so you would be hard-pressed to implement OA without IOC.
Regarding outer layers using any inner layer, I personally disagree with Palermo on this point. I think that outer layers should be constrained to working with the next layer in (to restate: outer layers should not be allowed to bypass a layer). I asked him about this on twitter once and he said that it's probably not a good idea for the data access implementation code to work with the presentation layer (remembering that the implementation code is on the outer rim of his architecture).
I think Palermo makes room for bypassing a layer precisely because he wants to be able to manipulate a Domain Models and Domain Services in the Controller. As far as I understand Domain Driven Design, Domain Services are only created when logic does not neatly fit into a Domain Model. If that is the case, then Domain Services and Domain Models are not really 2 separate layers. Rather, it's better to think of them as a single Business Layer. If they are both the same layer, then the question of whether you can use both in a Controller resolves itself. Then you can say without contradiction that outer layers should be constrained to talking to the next layer in the onion.
First, remember that the Onion Architecture (OA) is agnostic to application design styles, so there is nothing stopping you from injecting your domain into the UI. Second, the linked article points out that IoC is core, so you won't want to try to go without it. I'm also surmising you're talking about your domain entities - those things with data/state in your domain.
OA is about shielding your domain (business rules, entities, etc) from the vagaries of infrastructure changes, not the other way around. All you're buying yourself by using interfaces to get to your domain is extra code and indirection. Ask yourself this - If my domain changes (the core of your business), is it realistic to expect my user interface won't? Put another way, if your business rules change, the users are probably going to want to see that reflected in the UI if it involves adding or removing fields or whole lines of business.
Now ask the inverse of that question - If I change from Oracle to NoSQL, will my Domain code change? Injection of your infrastructure laden services is meant to give you the answer "No". This presumably means easier to maintain and more stable code.
So, in conclusion, unless you need to hide logic on your domain entities, or there is a compelling architectural reason against it (e.g., you're flinging these out of your server to a remote client or you want to add UI-specific attributes to your properties to affect validation or label display), you should go ahead and reference your concrete domain entities directly in your "application/UI" layer.
I am developing an application using MVC3 and Entity Framework. Its a three layered approach with Presentation Layer hosted in Web Server and Business Layer and Data Access Layer in Application Server. We are not exposing the Object Context to Presentation Layer or Business Layer. Object Context is wrapped in Data Access Layer only and exposing data access and data persistence as functionalities as Data Access Layer methods(ie data access logic is separated and implemented in Data Access Layer only). Business Layer is calling data access layer methods and returns data to Presentation Layer.
My concern is most of the Business Layer Methods are for just to access the data and it is just forwarding the call to Data Access Layer without any operation. So we are repeating the code in both layers. Do we have any other better approach to avoid this duplication.
Is it a good practise to implement the data access logic in the business layer in Layered approach?
Can someone suggest a good implementation approach for Layered application with 3-tier architecture?
If you find yourself that all that your business layer is doing is simply delegating the calls to the data access layer then this is a strong indication that this business layer is probably not necessary for your application as it brings no additional value. You can get rid of it and have the controllers talk directly to the data access layer (via an abstraction of course) - that's what most of the tutorials out there show with the EF data context.
In simple applications that are consisting mainly of CRUD actions, a business layer might not be necessary. As your application grows in complexity you might decide to introduce it later, but don't do too many abstractions from the beginning especially if those abstractions don't bring you any additional value.
As previously mentioned there is no "right" way to set things up. I have found a few things over the years that help me decide which approach to take.
Two-Tiered
If your shop is stored procedure heavy with a lot of database programmers and tends to put business logic in the database then go with a two-tiered approach. In this situation you will find that your business layer is generally just calling the data layer. Also, If your code base and pages tend to be small and you never repeat functionality then go with a two tiered approach. You will save a lot of time.
Three-Tiered
If your shop likes to have a lot of business logic in code and that logic is repeated everywhere then go with three-tiered. Even if you notice a lot of your business layer just calling the data layer you will find over time when you add functionality that it is a whole lot easier to just add code to the business layer. Also, If you have a large code base with a ton of pages with a lot of repeated logic then I would definitely go with this approach.
I have found a mixture of both in our enterprise level application at my work. The problematic areas are the ones where dynamic sql (gag) was used and business logic was repeated over and over. I'm finding as I refactor I pull this code into a 3-tiered architecture and save myself a ton of time in the future because I can just reuse it.
I don't think its necessarily bad to duplicate the code for logical separation. Time will come when this will pay off. Say you will replace SQL server by Oracle. Or Microsoft will come up with Linq 2.0 and some implementations will change. You will be thanking yourself for having it separate, while those who called database right from the business layer will have to do modifications in both places - DAL and BLL.
For what its worth. But again, there's no right answer, up to utility, usability, convenience, and most importantly having it matching its purpose.
Hope this is of help.
I will designing a couple of web applications shortly. They will probably be done in asp.net mvc.
In my existing web apps, done in delphi, the data access layer is seperated out into a completely separate application, sometimes running on a different server. This is done more for code reuse than for architectuaral reasons. This won't be a factor in the next app as it will be all new.
Is having a separate data access application overkill in a mvc app? I will already be separating out the business classes by virtue of using MVC, and I will be using an ORM to do the db persistance.
Edit: Just to clarify; I use the term tier to refer to separate physical applications, something more than just a logical separation or layer.
The term "Tier" in my experience is generally referring to physical application seperations e.g. Client Tier & Server Tier.
MVC - refers to 3 "Layers" with the concern being separation around the 3 concerns it details Model (Data), View (UI), Controller (App Logic).
Now that I have made that distinction regarding my terminology..
Is having a separate data access application overkill in a mvc app?
I would say No(again depending what you mean by application), it is not overkill, as it may in actual fact result in a more maintainable system. Your ORM will possibly allow for new data access options to be plugged in, but what if you wish to add a new ORM? Having a clearly separated Data Access Layer (DAL) will allows for much greater future flexibility in this aspect of your application.
On the other hand, depending on the scale and vision for the application creating a completely stand alone Data Access option may be overkill, but in a nutshell separation of the DAL into different assemblies is very much a recommended practice in the composition of an application implementing the MVC pattern.
Hope this helps, If you need more depth do comment.
Well I gues it depends a little on whether you're talking about tiers (pysical) or layers (logical/projects).
Regarding the layering - you could take a look at something like s#arp architecture (code.google.com/p/sharp-architecture/ ) for an example of how they do it (they have taken a pretty maximal approach to layering).
For an example of more minimalistic views here, take a look at Ayende's blog: ayende.com/Blog/
Regarding tiers - I think needlessly adding extra tiers and putting everthing over the wire will just hit your performance, unless you need to do this for capacity reasons. Get the layers right, and them seperate them into teirs as you need to adjust for capacity (should not take too much refactoring if you've seperated your concerns well).
Great comment Tobias.
I say add enough layers so that it makes sense to you and makes it easier to maintain. Also to keep a seperation of concerns.
This is a general question about design. What's the best way to communicate between your business layer and presentation layer? We currently have a object that get pass into our business layer and the services reads the info from the object and sets the result into the object. When the service are finish, we'll have a object populated with result from business layer and then the UI can display according to the result of the object.
Is this the best approach? What other approach are out there?
Domain Driven Design books (the quickly version is freely avaible here) can give you insights into this.
In a nutshell, they suggest the following approach: the model objects transverse from model tier to view tier seamlessly (this can be tricky if you are using static typed languages or different languages on clinet/server, but it is trivial on dynamic ones). Also, services should only be used to perform action that do not belong to the model objects themselves (or when you have an action that involves lots of model objects).
Also, business logic should be put into model tier (entities, services, values objects), in order to prevent the famous anemic domain model anti pattern.
This is another approach. If it suits you, it depends a lot on the team, how much was code written, how much test coverage you have, how long the project is, if your team is agile or not, and so on. Domain Driven Design quickly discusses it even further, and any decision would be far less risky if you at least skim over it first (getting the original book from Eric Evans will help if you choose to delve further).
We use a listener pattern, and have events in the business layer send information to the presentation layer.
It depends on your architecture.
Some people structure their code all in the same exe or dll and follow a standard n-tier architecture.
Others might split it out so that their services are all web services instead of just standard classes. The benefit to this is re-usable business logic installed in one place within your physical infrastructure. So single changes apply accross all applications.
Software as a service and cloud computing are becoming the platform where things are moving towards. Amazons Elastic cloud, Microsofts Azure and other cloud providers are all offering numerous services which may affect your decisions for architecture.
One I'm about to use is
Silverlight UI
WCF Services - business logic here
NHibernate data access
Sql Server Database
We're only going to allow the layers of the application to talk via interfaces so that we can progress upto Azure cloud services once it becomes more mature.