Now that everyone is talking about MVC, I notice that the business rules are not being addressed. In the old days of the 3-tier architecture, The business rules were in the middle layer. Where do they fall in the new MVC?
The reason you never see MVC address "Business Rules" is that MVC by and large is a presentation pattern. It's focused on how to structure your application. The Model, for example, can be thought of as a presentation model. The model of your application, which the view then renders.
However, in order to create the presentation model, you generally need to go to your domain models where all your business logic lives. At that point, MVC doesn't dictate where that code physically live. Is it on another tier? MVC doesn't care.
At first brush, I'd say they belong in the model. The MVC Entry on Wikipedia seems to agree: "In MVC, the model represents the information (the data) of the application and the business rules used to manipulate the data".
After all, by 'Business rules' we mean the functional algorithms and logic that encode the domain that your application is involved with, as opposed to input/output related logic. These core business-related logic does not - or should not- change based upon what is being displayed to the user (which is the domain of the View) or the user input (which is primarily received by the Controller).
In my experience, asking this sort of question has been very revealing during the software development process: we found a large number of things that were considered 'business rules' by some people, but turned out to be something else. If it is not a true business rule, it might not belong to the model.
Business rules always live in the model. The model is the bit that you could resuse with a completely different UI. The view is obviously completely dependent on UI choices and the controller has to take data from the model and tell the view to render it.
Putting business logic into the view is bad because it ties the structure to the presentation.
Putting business logic into the controller is bad because it splits your business domain between the data persisted by the model and the rules in the controller.
A quote from a Wikipedia Article:
MVC is often seen in web applications, where the view is the actual HTML page, and the controller is the code that gathers dynamic data and generates the content within the HTML. Finally, the model is represented by the actual content, usually stored in a database or in XML nodes, and the business rules that transform that content based on user actions.
Is there any reason why you cant mix MVC and Ntier? Our application does just that. Our controllers are used for data validation and decide which Business Layer calls to make.
OurApp.Web - Asp.net MVC Project
OurApp.Business - Business Layer Library
OurApp.DataAccess - Data Layer Library
OurApp.Entities - Basically all the 'models' shared by all layers
Business rules should be in the model, NOT the controller. The controller and view are part of the presentation layer.
The model represents the domain's entities and functionality ..
The controller is merely a manager for taking user input and requests, performing actions in/on the model and mapping that to views in the presentation layer. The controller is not just a mediator either, the view OR controller may act upon the model.
This is an anciently posted question, but I like a rules repository to be completely independent of any part of an application. Multiple applications, multiple implementations of a business tier, should be able to access a static rendering of a business rules repository. Simple separation decisions such as this make a migration from desktop -> web, for instance, trivial.
In my architecture, View -> Model -> Controller -> Business Tier -> Rules Repository, i.e. the controller accesses coarse data as presented by the business tier/layer, feeds it to the model which massages it into a presentable form, and the view passively displays it. The business tier, which is re-usable across any presentation format, will have explicit rules and access to a subsystem with implicit rules. By design, each component is ignorant of the details of a component above it.
I think the issue is a matter of definition. It seems to me that the logic for presenting the screens in the order needed is a controller issue and I have seen some projects that use a rules engine to determine the order and what is required input from the user. This is not the same as business rules imho.
You guys are wrong the business rules live within the controller and not the model...
Related
with nTier architecture it is common to create a data, business, workflow and ui layer. In this setup, your data layer and business layers are separated and can be reused by other layers.
In ASP.NET MVC it seems that the model is acting as both the business and data layer as clearly the model is the data and all documentation indicates that business logic belongs in the model.
How is this architecture promoting good separation of concerns when these two layers are mixed?
There is a difference between View Models and Domain Models. Domain Models is your application domain. These models can be used everywhere, in any tier and they are usually placed in a separate shared project. Your View Models are just for the UI. They are dependent on your page needs/structure. Let's say you want to create user management page, than your view model may be a class with 2 properties User and List<Role> where User and Role are domain models.
And finally, your Data Models usually are just database transfer objects. Entity Framework models are usually used as Data and Domain models at the same time.
So, answering your question: you choose your comfortable level of mixing models by yourself. The problem is if you don't want to mix, then you will have quite a bit of model duplications across the solution and you will have to do mapping from one type of model to the other manually or with help of libraries like AutoMapper. That's why developers choose some compromise.
Model–view–controller (MVC) is a software architectural pattern for implementing user interfaces. It divides a given software application into three interconnected parts, so as to separate internal representations of information from the ways that information is presented to or accepted from the user. (Wikipedia)
We should take into account that when we talk about ASP.NET MVC, we are talking about "User Interface" so, it is a user interface framework not an application one. In MVC, concerns are separated in three components: Model, View and Controller.
In multi-layer or multi-tier architecture, concerns are mostly separated in Presentation, Application, Business and Data access layers which is an application framework architecture and ASP.NET MVC belongs to the Presentation layer.
All in all, separation of concern is completely achieved if we distinguish between Application and Presentation frameworks.
Andrei M is spot on. I agree with the difference between View models and domain models. It may help to think of the M in MVC as a model for your views, and these "view models" should not have any knowledge of where they get their data from. Therein lies the separation of concerns. A controller will broker the exchanges necessary between your domain model and your view models to populate the view models with data. These data exchanges that the controller brokers are often achieved by using another layer such as a repository, infrastructure, or service layer... but not necessarily. In a one-project solution, your "Models" folder might contain a domain model, and then you might have a separate folder called "ViewModels". A controller would fetch data via "the Model" and then populate the view model with the data it "received" from the model. The view model would be the underlying model for your strongly-typed view. One may wonder why a developer might even bother with using a view model if there is a simple one-to-one mapping between domain object and a view. One example is that you may need to "flatten" a domain object and its relationships. Another example might be that for an index or list page, you may need to include search filter; view models greatly simplify accommodating changes to requirements.
I have heard that the controller should be kept light and models heavy.
I am somewhat confused about the best practice on what should be kept in the controller and what should be kept in the model.
In our organization, we use Entity Framework where and put the tables there.
For the controller, we use LINQ and then send the info over to the view.
Kind of confused on what code should be in the Controller and in the Model.
DisclaimerThe whole topic is a giant mess. Especially when it comes to Web MVC. For all practical purposes it is impossible to use classical MVC pattern for web, because the view should be observing model. Theoretically you could implement something like that with WebSockets, but keeping a persistent model for each user is not a realistic solution.
Here is what you must know about MVC
The most important idea in both classical MVC and MVC-inspired patterns Separation of Concerns. It divides the application in two major layers:
Presentation layer
Governs the user interface. It deals with both creation of the interface and reacts to the user's manipulation of this interface. This interface might be GUI for a desktop application or HTML web page, but it also can be REST API or receiver-responder on a Mars rover. This is why a web application can implement MVC pattern in both frontend and backend.
The mandatory parts are views and controllers, but, in context of web, fully realized views usually also use multiple templates to create the interface.
Model layer
This is where all the business rules and logic lives. The M in MVC is not a single entity. Instead it is a layer, which contains different structures. Some of those structures are also responsible for interaction with storage.
What are the responsibilities of controllers ?
Controllers are part of presentation layer, which deals with user input. In context of web-based implementations, you will usually have 1:1 relationship between views and controllers, where controller receives the requests from browser and, based on the content of said requests, alters the state of model layer and view.
If you are using classical MVC or Model2 MVC, then that is the extent of controllers responsibilities.
In MVP and MVVM patterns, where you have a passive view, controller-like structures are responsible for acquiring information from model layer and passing it on to the current view instance. This post might provide some additional details on the MVC-inspired patterns.
But the controller is in no way responsible for any form of business logic. If it was, it would mean, that you have a leaking abstraction, because the structures of presentation layer would be doing work, which should be in the model layer.
Usually the controllers will the be simplest structures in you application.
What about the model ?
As mentioned before, model is a layer, which encompasses all of the domain business logic and related functionality. This layer , just like presentation layer, is made up from multiple groups of structures:
Domain Objects[1]
These structures are usually what people mean, when talking about "models". They are also known as model objects or business objects. This is where most of domain business logic ends up.
Data Storage Structures
This group would contain all of the classes, which abstract the interaction with storage (SQL databases, caching systems, noSQL, remote SOAP or REST APIs). They will usually implement some variation of data mapper or repository pattern , but you could be using some other solutions too, like unit of work. The implementation details are not so important. What is important is that they let you store data from and retrieve information into your domain objects.
Services
Or you could call the "components". There are high level abstractions in your model layer, which facilitate the interaction between domain objects and storage structures. The usually represent large chunks of model layer, like "recognition service", "mailers", "article management", and will provide the interface for presentation layer to interact with.
That's something of a religious debate.
Some like as little as possible code in their controller and other as little as possible in their model.
Do what feels natural to you in the project, but be consistent within it.
All else is dogma you can pick an example either way and make a case.
Model is the core of your application. It is best to think of models as of your business entities. Do you want to create a view of an invoice? Then Invoice be your model, it represents the underlying object.
Controller is just a way to handle requests from a client, retrieving the data from database (or updating them) and flushing out the responses.
Your thoughts about the application design should be model-centric, that's the important part.
In simple terms, Model represents the underlying data that your application will be using. It is to be designed in a way that it can be used across different applications.
For example, A model to represent News data can be used by console commands, Web service etc.
It is in model where you will have ur business logic defined, independent of the view.
Controller can be thought of as glue that binds Model and View together. They deal directly with client requests and accordingly interact with views and models.
In a well designed application, you data structure and business logic will be designed in a
Model, making it "heavy". While Controller will just interlink your model and view with the client requests, making it "light".
In the classic Model-View-Controller MVC pattern, your Model is essentially a "headless" application, with no UI and is completely UI-agnostics. It offers an API that is the functional core of the application.
The View is the user interface, however you choose to define it (web page? elevator control panel? something else?). A given application might have 1 view or it might have many views.
The Controller (or Controllers — like Views, you might have one to many Controllers for a given application) relays and transforms events, notifications and data between View and Model so as to preclude the Model from needing to know anything that is View-specific.
The idea is to isolate the core application (the Model) from the user interface (the View). From that some things follow:
The View is aware of and communicates with both Controller and Model (you're unlikely to try to wire up the View to another Model) and expected for the Controller to be aware of both View and Model.
The Controller is aware of and communicates with both View and Model.
The Model knows nothing of Controller or View.
Code that performs business logic should be in the Model, not in the Controller.
I'm no MVC expert, but try and concentrate on the fact that the controller should use the input from the user to direct them to the correct view.
I don't know if NerdDinner is an ideal example, but you can see Scott Hanselman et al does a little bit of data access from his EF context but pushes most of the other logic to service classes or helpers on the model.
I don't know if I agree with the 'models heavy' part, as I don't use the models as 'business objects'. If I really need a lot of 'business' logic, I will typically create that in a separate 'Domain' layer and may even have a separate Data Access layer on top of this. But for a lot of simple (see: non-enterprise) projects, this is overkill in my experience.
First, before anyone screams dupe, I had a hard time summarizing it in a simple title. Another title might have been "What is the difference between a domain model and MVC model?" or "What is a model?"
Conceptually, I understand a Model to be the data used by the views and controller. Beyond that, there seems to be a great deal of differing opinions on what makes up the model. What's a domain model, versus an app model, vs a view model, vs a service model, etc..
For example, in a recent question I asked about the repository pattern, I was told point blank that the repository is part of the model. However, I have read other opinions that the model should be seperated from the persistence model and the business logic layer. After all, isn't the Repository pattern supposed to decouple the concrete persistence method from the model? Other people say there is a difference between the Domain model and the MVC model.
Let's take a simple example. The AccountController that is included with the MVC default project. I've read several opinions that the Account code included is of poor design, violates SRP, etc.. etc.. If one were to design a "proper" Membership model for an MVC application, what would that be?
How would you seperate the ASP.NET services (Membership provider, role provider, etc..) from the model? Or would you at all?
The way I see it, the model should be "pure", perhaps with validation logic.. but should be seperate from business rules (other than validation). For example, let's say you have a business rule that says someone must be emailed when a new account is created. That doesn't really belong in the model in my view. So where does it belong?
Anyone care to shed any light on this issue?
The way I have done it - and I'm not saying it is right or wrong, is to have my View and then a model that applies to my view. This model only has what is relevant to my view - including data annotations and validation rules. The controller only houses logic for building the model. I have a service layer which houses all business logic. My controllers call my service layer. Beyond that is my repository layer.
My domain objects are housed separately (in their own project, actually). They have their own data annotations and validation rules. My repository validates the objects in my domain before saving them into the database. Because every object in my domain inherits from a base class which has validation built in, my repository is generic and validates everything (and requires it inherits from the base class).
You might think that having two sets of models is duplication of code, and it is to an extent. But, there are perfectly reasonable instances where the domain object is not appropriate for the view.
Case in point is when working with credit cards - I have to require a cvv when processing a payment, but I cannot store the cvv (it is a $50,000 fine to do so). But, I also want you to be able to edit your credit card - change of address, name, or expiration date. But you aren't going to give me the number or the cvv when editing it, and I certainly am not going to put your credit card number in plain text on the page. My domain has these values required for saving a new credit card because you give them to me, but my edit model doesn't even include the card number or cvv.
Another benefit to so many layers is that if architected correctly, you can use structuremap or another IoC container and swap out pieces without detrimentally affecting your application.
In my opinion, controller code should only be code targeted at the view. Show this, hide that, etc. The service layer should house the business logic for your app. I like having all of it in one place so it's easy to change or tweak a business rule. The repository layer should be relatively dumb - devoid of business logic and only query your data and return your domain objects. By separating the view models from the domain model, you have much more flexibility when it comes to custom validation rules. It also means you don't have to dump every piece of data into your view in hidden fields and push it back and forth between the client and server (or rebuild it on the backend). Your view model will then house only the information relevant to the view - and it can be customized to have bools for view logic or counts or enums so that the view itself isn't cluttered up with complicated logic statements like
<% if (!String.IsNullOrEmpty(Model.SomeObject.SomeProperty) &&
Model.SomeObject.SomeInt == 3 && ...) { %>
While everything seems spread out and over-layered, it has a purpose for being architected this way. Is it perfect? not really. But I do prefer it to some past designs of calling repositories from the controller and having business logic mixed in the controller, repository, and model.
I too often wondered how exactly the MVC elements fit in a traditional web application structure, where you have views (pages), controllers, services, and data objects (model). As you said, there are many versions of that.
I believe the confusion exists because of the above stated, widely accepted architecture, which uses the "anemic domain model" (alleged)-anti pattern. I won't go into much details about the "anti-patternness" of anemic data model (you can look at an effort of mine to explain things here (Java-based, but relevant for any language)). But in short, it means that our model holds only data, and business logic is placed in services/managers.
But let's assume we have domain driven architecture, and our domain objects are the way they are expected to be - having both state and business logic. And in this domain-driven perspective things come into place:
the view is the UI
the controller gathers the inputs of the UI, invokes methods on the model, and sends back a response to the UI
the model is our business components - holding the data, but also having business logic.
I guess that answers your main questions. Things get complicated when we add some more layers, like the repository layer. It is often suggested that it should be invoked by the business logic placed in the model (and hence each domain object has a reference to a repository). In the article of mine that I linked I argue that this is not quite a best practice. And that in fact it is not a bad thing to have a service layer. By the way, domain-driven design does not exclude the service layer, but it is supposed to be 'thin', and only coordinating domain objects (so no business logic there).
For the anemic data model paradigm, which is widely adopted (for good or for bad), the model would be both the service layer and your data objects.
In my opinion,
Model -
Should not contain business logic, it should be pluggable(WCF like scenario). It is used to bind to view so, it should have properties.
Business Logic -
It should be placed at "Domain Services Layer", it is separate layer altogether.
Also, will add one more layer here "Application Services".
App Services talks to Domain Services layer to apply business logic and then lastly return the Model.
So,
Controller will ask Application Service for Model and the flow will go like,
Controller->Application Services(using domain services)->Model
The MVC pattern and the Asp.net framework makes no distinction on what the Model should be.
MS's own examples include persistence classes in the model. Your question about membership being in the model. This depends. Are classes in your model owned by something? Is there a link between who logs in and what data is displayed? Is there filtering of data part of a permissions system that is editable? Is who last updated or edited an object part of your domain as in somebody else needs to see it or something for backend support?
The email example is also it depends. Are you familiar with domain eventing or eventing in particular? Do you have a separate service to send emails? Is the act of sending an email part of your domain or is it a application level concern outside of the scope of your system? Does the UI need to know if an email was sent successfully or not? Do emails that fail to send need retries? Does the content of the email sent need to be stored for support or customer service requirements?
These types of questions are overly broad and subjective but I'm answering so you and everybody who voted you up can understand this.
Your requirements/timelines/resources all bleed into your system's architecture. Even the revenue model can have an effect. You also have to consider the pattern you are shooting for. DDD is much different than persistence-as-model applications and all the slop in between are also valid for certain apps. Are you shooting for testing the app? All of this has an effect.
What are the responsibilities of one vs the other?
What kind of logic should go in one vs the other?
Which one hits services and databases?
How do I decide if my code should go in the viewmodel or the controller?
For the record, I am using ASP MVC, but since the question is architectural, I do not believe it matters what language or framework I am using. I'm inviting all MVC to respond
The ViewModel is a Pattern used to handle the presentation logic and state of the View and the controller is one of the fundamentals parts of any MVC framework, it responds to any http request and orchestrates all the subsequent actions until the http response.
The ViewModel Pattern: More info
In the ViewModel pattern, the UI and
any UI logic are encapsulated in a
View. The View observes a ViewModel
which encapsulates presentation logic
and state. The ViewModel in turn
interacts with the Model and acts as
an intermediary between it and the
View.
View <-> ViewModel <-> Model
The Controllers (Comes from the Front Controller Pattern): More Info
It "provides a centralized entry point
for handling requests."
HTTP Request -> Controller -> (Model,View)
--Plain Differences:--
While the ViewModel is an optional
pattern the Controller is a must, if
you are going the MVC way.
The ViewModel encapsulates
presentation logic and state, The
Controller orchestrates all the
Application Flow.
The ViewModel can be on the client side as well as server side.
Wherever it may be, the sole purpose of viewmodel is to play the
presentation data.
In MVC architecture Viewmodel is not mandatory but with out controller the request from the client cannot be processed.
Controller can be visualised as the main interface between client and server to get any response from the server. It processes the client request, fetches data from repository and then prepares the view data. Viewmodel can be visualised as view data processor/presenter thus an interface to manage the view more eloquently.
In the overall context of a web application we can say the controller is the application request handler whereas viewmodel is only the UI handler.
The Model-View-Controller (MVC) is an architectural design pattern which exists, primarily, to separate business logic from the presentation. Basically, you don't want your back-end touching your front. It typically looks like this:
The reasons for doing this is because, by separating the back-end and the front, you don't tie your user-interface directly to your data/work. This allows you to put new interfaces onto your business logic without affecting said logic. In addition, it also improves the ease of testing.
A simple example of where MVC comes in handy - let's say you have an application that manages your company's finances. Now, if you are correctly using MVC, you can have a front end that sits at some financier's desk and lets him handle transactions, manage the finances, etc. BUT, because the business logic is separate, you can also provide a front-end to your CEO's Blackberry that lets him see the current status of the business. Because the two front-ends are different, they can do different things while still providing (different types of) access to the data.
EDIT:
Since you updated your question a bit, I'll update my answer. There is no perfect science to the separation of MVC. There are some good rules of thumb, however. For example, if you are talking about GUI components, that is probably a view. (Are you talking about look and feel, usability, etc.) If you are talking about data and the "business" side of the house (databases, logic, etc), you are probably referring to a model. And, anything that controls the interaction between the two is most likely a controller.
In addition, it should be noted that, while Views and Models are typically "physically" separated, a controller can exist with a view when it makes sense.
You are correct when you say the framework (or even language) for MVC doesn't matter. The pattern itself is language agnostic and really describes a way to architect your system.
Hope that helps!
I think there's some value to learning received doctrine. But there is also value in understanding how the doctrine came to be the way it is.
Trygve Reenskaug is widely credited with inventing MVC. N. Alex Rupp's article Beyond MVC: A new look at the servelet architecture includes a History of MVC. In a section on Reenskaug's 1978 work at Xerox Palo Alto Research Center, there's a link to his paper Thing-Model-View-Editor: an Example from a planningsystem. There the pieces are described like this.
Thing
Something that is of interest to the user. It could be concrete, like a house or an integrated
circuit. It could be abstract, like a new idea or opinions about a paper. It could be a whole,
like a computer, or a part, like a circuit element.
Model
A Model is an active representation of an abstraction in the form of data in a computing
system
View
To any given Model there is attached one or more Views, each View being capable of
showing one or more pictorial representations of the Model on the screen and on hardcopy. A
View is also able to perform such operations upon the Model that is reasonabely associated
with that View.
Editor
An Editor is an interface between a user and one or more views. It provides the user with a suitable command system, for example in the form of menus that may change dynamically
according to the current context. It provides the Views with the necessary coordination and
command messages.
Rupp identifies Reenskaug's Editor as a Controller or Tool.
MVC Triads emerged in SmallTalk-80. The model was an abstraction of the real-world concept, the view was its visual representation, and the controller was the buttons and slider bars that allowed the user to interact with it (thereby "controlling" the view). All pieces in the triad were interconnected and could communicate with the other two pieces, so there was no layering or abstraction involved. Since then, Reenskaug has "preferred to use the term Tool rather then Controller." According to his notes, these are the terms he has used in later implementations
Some logic and model should be invoked to generate some data (structured or semi-structured). From this data the returned page/JSON/etc. is created, typically with only rudimentary outlining logic.
The first part (creating the data) is done by the controller (via the model usually). The second part - by the View. The ViewModel is the data structure being passed between controller and view, and usually contains only accessors.
Model represents your data and how it's manipulated. Thus, model touches the DB.
View is your UI.
Controler is the glue between them.
MVC stands for Model, View, Controller.
Model = Data (Database tables)
View = HTML, CSS, JavaScript, etc
Controller = Main logic, a contract between Model & View.
In simple and graspable terms,
MVC allows you to develop your applications in a way that your business data and presentation data are separated. With this, a developer and designer can work independently on a MVC app without their work clashing. MVC makes your app avail OOP too.
I need a simple pattern to do the above.
Few things to note:
1) I have a class that I am obliged to use that does the actual data retrieving and it return DataTable
2) I am not concerned with the generic interfaces that support all possible database types, we are sticking with one database type.
3) How do I elegantly trap the error and inform the user that error occured.
4) Do not offer me to learn MVC - it is not an option right now.
I am interested in the actual pattern design.
You want to use the MVC pattern to separate business logic from presentation (note that I am NOT talking about the ASP.NET MVC Framework) and the DAO pattern to separate business logic from data access.
Your class that returns a DataTable becomes part of your model. You write a layer (a DAO for each domain object) that takes your DataTable and translates it into your various domain objects. Your UI shouldn't contain any logic that does more than presenting output to the user; anything that is actual logic for retrieving data from your model is handled in a controller layer, that gets the data the user is requesting from the model and sends it to the appropriate view to present it to the user.
Exceptions should be handled at layer boundaries; either catch and do something about it (which might just be to log it and send the user to an error page), or wrap in a higher-level exception as appropriate.
Do not offer me to learn MVC - it is not an option right now.
You can do a simple MVC or MVP pattern without using the ASP MVC Framework. It is pretty simple to introduce. Here is a pretty simple example MVP Example
There are a number of patterns that approach what you describe. I would recommend getting a copy of Martin Fowler's excellent book Patterns of Enterprise Architecture specifically Chapter 14. Web Presentation Patterns. You will find that any serious attempt to separate the presentation, domain (business logic), and data source will lead you to one of many variations of the same theme.
MVC, MVP, visual proxy, etc all break down to three layers what differs is the responsibilities of each layer and how layers communicate with each other.
For instance, the Passive View pattern basically strips the UI layer of anything not directly related to presentation. The typicall example being a field you want to highlight if a certain condition is true. In a Passive View the form would only only contain the logic to determine the whether the field should be highlighted. The business rule that trigger's this state would be in a presenter/controller layer which does not directly depend on the actual view.
As far as the data source layer, the main benefit isn't being able to switch databases at the drop of a hat. The main benefit is that changes to the database schema only affect the data source layer and don't extend into the rest of the application. If your stuck with datasets a good approach is the Table Data Gateway pattern.