According to the book I am reading, I understand this facts. Entity objects is like model objects in MVC which are responsible for maintaining data. Boundary objects are those which interact with external users, handling both input and output. In MVC, user input is detected by control objects, but the handling of output is the responsibility of view objects. That are the difference I have found. But when I do googling, many said that MVC is typically used in user interface design whereas ECB is most often used in business logic. What does that mean? MVC is just used for user interface? If so, what is the responsibility of the Controller and view object in MVC?
The two architectural patterns have a different origin:
MVC was born from the effort to decouple business logic from user interfaces, in the early days of GUIs. It separates components by purpose, and user-interface was a big one.
ECB comes from use-case driven development. It separates components based on the use-cases, and these typically capture user/business goals.
The two patterns have some similarities. Indeed, the BCE-Entity components have the same purpose than the MVC-Model components: managing the domain objects.
However there are subtil differences: the MVC-Controller is meant to capture all the user input, and the MVC-View the outputs. The BCE-Boundary is meant to cover the full user interface (input and output) and relate to the BCE-Controler, which coordinates the other BCE components for the purpose of a use-case/goal. In other words, the BCE-Controller is related to the business logic (i.e. the model in the MVC).
If the application covers only one use-case, the main difference will be the controller. But if an application covers several different use-cases, you'll end-up with a different architecture.
In practice, BCE did not really made it through in modern user interfaces, whereas MVC is still very popular thanks to some frameworks.
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I've done some surface reading on mvc and mvp and they all talk about model, controllers and presenters. My looking up on those things didnt give me enough understanding so I want to know how important using an mvc or mvp is to website development, if it is compulsory to use either of them and lastly their benefits.
It is not like you can't develop web applications without the use of either of MVC or MVP. But they both are designing patterns and helps greatly in development an maintenance of your project and code.
At the heart of MVC is Separated Presentation. The idea behind Separated Presentation is to make a clear division between domain objects that model our perception of the real world, and presentation objects that are the GUI elements we see on the screen. Domain objects should be completely self contained and work without reference to the presentation, they should also be able to support multiple presentations, possibly simultaneously.
There are mainly 3 design patterns:
MVC (Model View Controller)
MVP (Model View Patterns)
MVVM (Model View View Model)
MVC (Model View Controller)
The MVC pattern is a UI presentation pattern that focuses on separating the UI (View) from its business layer (Model). The pattern separates responsibilities across three components: the view is responsible for rending UI elements, the controller is responsible for responding to UI actions, and the model is responsible for business behaviors and state management. In most implementation all three components can directly interact with each other and in some implementations the controller is responsible for determining which view to display.
Model View Presenter(MVP)
The MVP pattern is a UI presentation pattern based on the concepts of the MVC pattern. The pattern separates responsibilities across four components: the view is responsible for rending UI elements, the view interface is used to loosely couple the presenter from its view, the presenter is responsible for interacting between the view/model, and the model is responsible for business behaviors and state management. In some implementations the presenter interacts with a service (controller) layer to retrieve/persist the model. The view interface and service layer are commonly used to make writing unit tests for the presenter and the model easier.
Key Benefits
Before using any pattern a developers needs to consider the pros and cons of using it. There are a number of key benefits to using either the MVC or MVP pattern (See list below). But, there also a few draw backs to consider. The biggest drawbacks are additional complexity and learning curve. While the patterns may not be appropriate for simple solutions; advance solutions can greatly benefit from using the pattern. I’m my experience a have seen a few solutions eliminate a large amount of complexity but being re-factored to use either pattern.
Loose coupling – The presenter/controller are an intermediary between the UI code and the model. This allows the view and the model to evolve independently of each other.
Clear separation of concerns/responsibility
UI (Form or Page) – Responsible for rending UI elements
Presenter/controller – Responsible for reacting to UI events and interacts with the model
Model – Responsible for business behaviors and state management
Test Driven – By isolating each major component (UI, Presenter/controller, and model) it is easier to write unit tests. This is especially true when using the MVP pattern which only interacts with the view using an interface.
Code Reuse – By using a separation of concerns/responsible design approach you will increase code reuse. This is especially true when using a full blown domain model and keeping all the business/state management logic where it belongs.
Hide Data Access – Using these patterns forces you to put the data access code where it belongs in a data access layer. There a number of other patterns that typical works with the MVP/MVC pattern for data access. Two of the most common ones are repository and unit of work. (See Martin Fowler – Patterns of Enterprise Application Architecture for more details)
Flexibility/Adaptable – By isolating most of your code into the presenter/controller and model components your code base is more adaptable to change. For example consider how much UI and data access technologies have changed over the years and the number of choices we have available today. A properly design solution using MVC or MVP can support multi UI and data access technologies at the same time.
Key Differences
So what really are the differences between the MVC and MVP pattern. Actually there are not a whole lot of differences between them. Both patterns focus on separating responsibility across multi components and promote loosely coupling the UI (View) from the business layer (Model). The major differences are how the pattern is implemented and in some advanced scenarios you need both presenters and controllers.
Here are the key differences between the patterns:
MVP Pattern
View is more loosely coupled to the model. The presenter is responsible for binding the model to the view.
Easier to unit test because interaction with the view is through an interface
Usually view to presenter map one to one. Complex views may have multi presenters.
MVC Pattern
ontroller are based on behaviors and can be shared across views
Can be responsible for determining which view to display
Further More Research on topic to choose best pattern
Further research and also using the term "twisting the triad" will result in a couple of interesting articles to read that always addresses your question.
The most often heard result is this:
Do you develop a web application? Learn about MVC.
Do you develop a winform application? Learn about MVP.
Do you develop a WPF application? Learn about MVVM.
You can follow it on MVC, MVP and MVM Architectures for web development
You're definitely not required to use an MVC or MVP framework when writing a web app. People still write apps using nothing more than jQuery. However, many popular frameworks like angular 1, backbone, and knockout do make use of models, views, and/or controllers or some subset of them so it's probably good to know.
As for the benefits, it comes down to maintainability. As applications grow larger they become difficult to modify successfully without some sort of overarching structure to keep things consistent and to ensure good practices are used. Frameworks like angular provide you with this out of the box by using proven and well understood concepts like MVC. Without this you'll eventually have to come up with your own patterns and subsystems, which while doable, can take a lot of time and effort. Either that, or your app buckles under its own weight.
If you're new to development it's difficult to get an appreciation for this without working on a large project and seeing how crazy things get without some structure. I'm not sure it's something you can learn in a vacuum or that you need to concern yourself with too much right now. Eventually it will become painfully obvious to you why it's important.
As mentioned above, it has a lot to do with the ease of maintaining the code as it grows but that's not all.
A framework such as AngularJS allows you to change the way you build sites by applying the MVC methodology. When working with AngularJS is important to shift paradigms from manipulating the DOM to describing the desired effect and watching it happen.
With MVC each component is kept separate which makes testing them a lot easier. The separation and modular nature of this patterns allows for greater flexibility and allows bigger apps to grow at a faster rate without loosing quality.
A great tutorial on AngularJS can be found at : https://m.youtube.com/playlist?list=PLYxzS__5yYQmX2bItSRCqwiQZn5dIL1gt
I've been reading a lot of blogs which advocate the fat models and skinny controllers approach, esp. the Rails camp. As a result the routers is basically just figuring out what method to call on what controller and all the controller method does is call the corresponding method on the model and then bring up the view. So I've two concerns here which I don't understand:
The controller and router are really not doing much different tasks other than just calling a method on the God-like model based on the route.
Models are doing too much. Sending emails, creating relationships, deleting and modifying other models, queuing tasks, etc. Basically now you have God-like objects that are supposed to do everything that may or may not concern with modeling and dealing with data.
Where do you draw the line? Isn't this just falling into the God pattern?
It might not be the best idea to look at Rails as a staple of MVC design pattern. Said framework was made with some inherent shortcomings (I kinda elaborated on it in a different post) and the community only just now has begun addressing the fallout. You could look at DataMapper2 development as the first major step.
Some theory
People giving that advice seem to be afflicted by a quite common misconception. So let me begin by clearing it up: Model, in modern MVC design pattern, is NOT a class or object. Model is a layer.
The core idea behind MVC pattern is Separation of Concerns and the first step in it is the division between presentation layer and model layers. Just like the presentation layer breaks down into controllers (instances, responsible for dealing with user input), views (instances, responsible for UI logic) and templates/layouts, so does the model layer.
The major parts that the model layer consists of are:
Domain Objects
Also known as domain entities, business objects, or model objects (I dislike that latter name because it just adds to the confusion). These structures are what people usually mistakenly call "models". They are responsible for containing business rules (all the math and validation for specific unit of domain logic).
Storage Abstractions:
Usually implemented using data mapper pattern (do not confuse with ORMs, which have abused this name). These instances usually are tasked with information storage-from and retrieval-into the domain objects. Each domain object can have several mappers, just like there are several forms of storage (DB, cache, session, cookies, /dev/null).
Services:
Structures responsible for application logic (that is, interaction between domain objects and interaction between domain objects and storage abstractions). They should act like the "interface" through which the presentation layer interacts with the model layer. This is usually what in Rails-like code ends up in the controllers.
There are also several structures that might be in the spaces between these groups: DAOs, units of work and repositories.
Oh ... and when we talk (in context of web) about a user that interacts with MVC application, it is not a human being. The "user" is actually your web browser.
So what about deities?
Instead of having some scary and monolithic model to work with, controllers should interact with services. You pass data from user input to a specific service (for example MailService or RecognitionService). This way the controller changes the state of model layer, but it is done by using a clear API and without messing with internal structures (which would cause a leaky abstraction).
Such changes can either cause some immediate reaction, or only affect the data that the view instance requests from model layer, or both.
Each service can interact with any number (though, it's usually only a handful) of domain object and storage abstractions. For example, the RecogitionService could not care less about storage abstractions for the articles.
Closing notes
This way you get an application that can be unit-tested at any level, has low coupling (if correctly implemented) and has clearly understandable architecture.
Though, keep in mind: MVC is not meant for small applications. If you are writing a guestbook page using MVC pattern, you are doing it wrong. This pattern is meant for enforcing law and order on large scale applications.
For people who are using PHP as primary language, this post might be relevant. It's a bit longer description of the model layer with a few snippets of code.
If the "model" classes are implemented poorly yes, your concern is relevant.
A model class shouldnt be doing Email (infrastructure tasks).
The real question is what does model in MVC imply.
It isnt restricted to POCO classes with a few methods.
Model in MVC means Data and Business logic. Treat it as a superset of classic core POCO models.
View ==== Controller ==== Model ---> Business Process layer --> Core models
Throw in Infrastructure assemblies and Data Access layers and use injection to hand that into the BPL then your a process is using MVC as intended.
BPL may invoke UoW / Respository patterns, and execute business rules and call Infrastructure features by way of injected Objects or interface patters.
So the recommendation to keep a controller skinny doesnt mean the "person" class in a classic Core model should have 50 methods, and call Email directly. You are right to think this is wrong.
The Controller May still be required to instantiate and inject Infrastructure classes into the BPL or core layer if called directly. There should be a business layer or at least classes orchestrating calls across Classic Object model classes.
Well thats my "view" anyway ;-)
For generic take on MVC the wiki description http://en.wikipedia.org/wiki/Model%E2%80%93view%E2%80%93controller
A Little Blog that talks about the "M" in MVC. http://www.thedeveloperday.com/skinny-controllers/
I think you can make a distinction between a single fat model (possibly named App or Application), and several fat models broken down into logical groups (Business, Customer, Order, Message). The latter is how I structure my apps, and each model roughly corresponds to a database table in a relational database or collection in a document database. These models handle all aspects of creating, updating, and manipulating the data that makes up the model, whether it is talking to the database or calling an API. The controller is very thinm responsible for little mor that calling the appropriate model and selecting a template.
Wikipedia describes the Single Responsibility Principle this way:
The Single Responsibility Principle states that every object should have a single responsibility, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility.
The traditional use of the controller in MVC seems to lead a programmer towards a violation of this principle. Take a simple guest book controller and view. The controller might have two methods/actions: 1) Index() and 2) Submit(). The Index() displays the form. The Submit() processes it. Do these two methods represent two distinct responsibilities? If so, how does Single Responsibility come in to play?
Yes it does.
And if you want to follow the SRP, you disaggregate your Controller into a Dispatcher and Actions; the Dispatcher dispatches control to its actions, and at compile-time (C++ templates) or at runtime (Java XML, whatever), you'd compose Dispatchers and Actions.
Why don't we see this more often? Because Controllers are often "ad hoc" implementations, leaf-level concrete classes that aren't generalized and aren't meant to be subclassed. Here, the class is used more to conveniently group code, the actions are almost certainly non-public (likely private, maybe protected), "merely" internal implementation details.
The choice of how to decide what action to dispatch to, the number and diversity of possible actions, is high, and dispatching and action are tightly-coupled. So in practice, it's often easier to just put the code together in one place.
No, it doesn't.
There is nothing inherent to the MVC pattern or its variations which lead to a violation of the Single Responsibility Principle. Whether the implementation of a controller violates SRP or not is based upon whether the encapsulated behavior has more than one reason to change (just as any other class), not because of any presupposed prescriptive use of the pattern.
The example you set forth is a subset of a basic forms over data application, where the controller is merely providing CRUD operations for a given model. CRUD operations are fairly cohesive in nature, so this generally doesn't constitute a violation of SRP. Where having multiple methods on a single controller start to become suspect is when the methods represent different behavioral interactions across the domain.
That said, even if someone where to argue that CRUD represents four separate non-cohesive concerns, there is nothing inherent to the MVC pattern that forces you to facilitate each of these actions within the same controller.
For a little history on the MVC pattern as well as some discussion of its application in Web development, checkout Interactive Application Architecture Patterns.
I read a text recently saying the MVC pattern describes the layers in an application. But personally I see MVC showing several key roles in an application.
Which word do you think is better, layer or role, to describe the three main pieces of MVC?
Layers should imply a very narrow coupling between the respective sets of code. MVC involves relatively tight coupling between the model, view, and controller. Therefore, if you characterize this as a layering pattern, it becomes problematic in terms of defining an API between the layers. To do this properly, you would have to implement some unintuitive patterns.
Because of this, I would agree with your tendency to view it as a pattern that defines roles within a single layer.
I think roles is a better description. The view and the controller are both in the same "layer" and usually the model is described as a layer but is used between layers.
Usually my applications are centered around the domain model with stuff like presentation, persistence and file-io around it. Thinking about an architecture as layered doesn't really work for me.
MVC clearly defines ROLES. these are 3 roles you can implement in any number of layers. For example u can have a multi layer controller
Roles, not layers. Layers are completely dependent on the underlying implementation of the MVC pattern. For instance, a service layer may be a single layer on one implementation, but it could have a web service remoting layer and a database layer (for two differing service layers) on another implementation. The concept of layers is just to help you organize it, as is the pattern, but layers are not as easy to spot as patterns, and layers can change, whereas the pattern remains the same despite the layers changing due to different implementations.
You cannot compare those two words, because they describe different concepts.
To me, a layer is something opaque that offers some functions I can use to do things. For example, a good hardware layer for a wireless transmitter would just give me a send and a receive-function (based on bytes, for example), hiding all the ugly, ugly details from me.
A role is a way an object will behave. For example, a transformation in one of my compilers is going to take an abstract syntaxtree and return an abstract syntaxtree or an affection in my current project is going to take a state-difference and return a specifically altered state-difference.
However, coming with those two definitions, I do not see the need to chose a single "correct" term and burn the other as wrong, because they don't conflict much. A part of a layer has a certain role, and a set of objects conforming to certain roles form a layer. Certainly, the controller forms a certain layer between the UI and the model (at least for input), however, ot also has a role - it turns certain event into certain other events (and thus, it is some sort of adapter).
I think either can be reasonably argued for, but I think describing the parts as "layers" is more consistent with other conventions, like the OSI model. Since the View, Controller, and Model get progressively closer to your data, it's more of a layered structure. It seems that "roles" would apply to different parts of an application on the same layer.
Why not Both? I see it as 3 separate layers implementing 3 different roles.
It's all terminology, but I think the correct software architecture term would be "layer", as in logical layer. You could use the term "architectural layer" if it is clearer.
The thing is, it's just a different way of slicing an application: a classic n-layer app would be:
UI
Business Logic
Persistence
You could have the following logical layers in a simple MVC application:
UI
Controller
Model
Persistence
But you could still talk about the "UI" and "Controller" together as forming the User Interface layer -- I usually split out the Controller into a separate layer when describing and diagramming these architectures, though.
In many leading DDD projects, especially MVC style, I see the UI using display objects that mirror domain entities, rather than using those domain objects directly. This style is obviously for decoupling and separation of concerns, and I personally prefer this style.
But what I'm not sure of, is whether this a strict tenet of DDD, or whether this is more just different developers' interpretation of it.
Can you use your domain objects directly in the UI, and still be following the DDD methodology in that act?
Or is it a DDD best practice to always use display objects?
Note: While I mention MVC, I'm really interested in whether display objects must be used in almost all DDD compatible UI patterns in a DDD project.
I didn't really start to understand why or how you would decouple the domain model from presentation concerns until I started following the work of Greg Young and Udi Dahan (via Martin Fowler).
They have been teaching a principle known as Command and Query Responsibility Segregation (CQRS).
My interpretation of CQRS is that there are two sets of responsibilities that can pull a domain model in different directions, resulting in a model that does a mediocre job of both. The two responsibilities are commands (i.e. behavior that would cause a write to the database) and queries (i.e. reading from the database in order to fetch data for UI consumption). An example would be adding getters and setters to your entities to support databinding in the UI. If your model has getters and setters, it will probably do a poor job of modeling state changes that need to happen transactionally or encapsulating business logic. It will also have no way of modeling the business context of state changes (see Event Sourcing).
In DDD terms, you might say that the domain model and the presentation model are usually in separate Bounded Contexts.
The solution as prescribed by CQRS is to create one model for commands and another for queries. If your current model has methods for changing state (i.e. the behavior in the model), and getters that expose state to a UI for databinding, you would refactor these two responsibilities into separate models for commands and queries. The query model will not be mapped to your domain entities, but instead directly to the database. If your database doesn't capture derived state from your domain model, check out a pattern called Eager Read Derivation.
If your system is simply CRUD and has no behavior, try out a scaffolding system that can be automatically built off of your database schema, like ASP.NET Dynamic Data
If you're doing an MVC pattern, you need view objects; the DDD is just your model. That doesn't mean you must always use MVC; a DDD could be built, say, as a simulator, where all you look at is log messages emitted. But in MVC you really should have separate view objects.
Now, ask yourself why that would be? The answer is that the view can change, even though the business model doesn't. The DDD model should express, in the business's terms, what is essential to the business.
DDD is a way of thinking while designing a software that starts with modelling the domain. As the webpage puts it:
Domain-driven design is not a
technology or a methodology. It is a
way of thinking and a set of
priorities, aimed at accelerating
software projects that have to deal
with complicated domains.
One thing that follows naturally out of this design pattern is a layered architecture. As it is said in DDD Pattern Summaries
Partition a complex program into
LAYERS. Develop a design within each
LAYER that is cohesive and that
depends only on the layers below.
Follow standard architectural patterns
to provide loose coupling to the
layers above. Concentrate all the code
related to the domain model in one
layer and isolate it from the user
interface, application, and
infrastructure code. The domain
objects, free of the responsibility of
displaying themselves, storing
themselves, managing application
tasks, and so forth, can be focused on
expressing the domain model. This
allows a model to evolve to be rich
enough and clear enough to capture
essential business knowledge and put
it to work.
Whether you need to have display objects to do that? That is just one way of implementing this, but might not even be the best to provide loose coupling. As for an example: maybe the view layer is but a webbrowser and xlt files to visualize xml files emmitted by the business layer? If anybody has more fitting examples, please add them. My point is that DDD stresses a layered architecture, but does not limit this to one possible solution.
Within an MVC design you would typically have a mapping from Repository -> Domain Models and then from Domain Models -> View Models. The View Models often contain Domain Objects though.
The answer is quite straight :
The domain objects have domain oriented design and methods.
The view objects have view/control oriented design and methods.
If you can use your domain objects in the view, they are maybe not quite domain oriented.
Domain objects are really the internal logic inside your business logic layer. They shouldn't be directly exposed to your clients (UI layer). Instead, encapsulate the usage of your domain model into application services. The application services can utilize lightweight DTOs and/or command objects to pass data and intent between the client and the domain model.