Architecture patterns like MVC,MVVM, MVP are only used in presentation tier?. Can we use in Business Logic layer or Data access layer? I previously thought that Presentation Tier is View;BusinessLogic Tier is Controller/Viewmodel and Data Access Layer is Model. please someone clarify this..
The patterns you mention provide concepts to couple business and presentation tier. Considering a classical three tier architecture with the tiers you've mentioned: Data Access Layer, Business Logic, Presentation Layer, then MVVM, MVC, MVP provide concepts for the coupling of Presentation Tier and Business Logic. As the main idea is a loose coupling between the two to avoid logical dependencies between the two layers as much as possible, I would think of them as a kind of mediator (in the meaning of the word, not the pattern) or glue between them.
Let me clarify this for MVVM, exemplary:
You can write a complete presentation tier in WPF without utilizing MVVM. Just as well you can write the business logic without knowing the concept of MVVM. You don't need ViewModels at all to build a working application.
But: You have no chance to cleanly separate the concerns of presentation/UI and the actual logic which performs the tasks your software has been written for. The border between the two is kind of fuzzy in the real world: You calculate data, now you want to transform the data to display it in a diagram. Is this business logic? Yes and no. Is this UI logic? Yes and no. There's a grey zone: Kind of logic, kind of UI. This is where MVVM (and the like) kick in: You add a layer between business and presentation which is dedicated to this grey zone only.
For MVVM: The View is the View, it's the presentation layer. And the ViewModel is the grey zone, which is neither real Presentation, nor real business logic, but the glue between them. The Model is the model, it's the business logic. It could even be written without knowing that it will be used in a WPF application (theoretically). And there can be business logic, which is not even a model in the MVVM meaning, because it is not UI related at all.
Below this, comes everything else with the DAL at the bottom. The DAL should really not care about how business logic and presentation are glued together and is beyond the scope of the discussed patterns.
Architecturally, I would say that MVC, MVP and MVVM is the presentation tier. The point of view between each components are:
View
This is very clear that it belongs to presentation layer. Not discussion about this
Controller / Presentation / View Model
If you take off the N-Tier principal, this is the business tier. Seems like this design pattern were invented without taking any coupling with the N-Tier.
Controller has Session and HttpContext utilization. It is web dependent. According to N-Tier principal, the BLL must not know any UI. Therefore it goes for presentation tier.
Presentation has events / event handlers / delegations and some UI-specific data. (CMIIW, I'm not too fluent with MVP). Therefore it goes for presentation tier.
As other said, ViewModel is rather hard to be classified as presentation tier. However, I find it better to put into presentation tier. In my experience using WPF, sometimes I need to use MVVM specific objects like ObservableCollection and INotifyPropertyChanged and ICommand to force data binding refresh. Sometimes it is needed for the ViewModel to access custom session state, such as login, etc. Other times, you need to specify some UI-specific parameter such as font color, etc. This may be avoided by handling the logic in View, however I find it is easier to do it in ViewModel.
Another thing to consider, using MVVM prevent me to use Service - Repository pattern.
Model
If you take the N-Tier off from MV- pattern, this is the entity model. It is described at Asp.Net MVC, where the model will be used in the view, as the container for data. However, if you take N-Tier into account, then this is the business tier, where you do insert/update/delete operation to the data, and the logic for it resides.
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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
Today I was getting some refresh about software design patterns.
In particular I was wondering about the difference between MVC and Three-tier-layer. Basically, from what I read on wikipedia and other sources, the main difference between the two is the components interaction:
A fundamental rule in a three tier architecture is the client tier
never communicates directly with the data tier;
whilst
...the MVC architecture is triangular: the view sends updates to
the controller, the controller updates the model, and the view gets
updated directly from the model
Now: if we take the apple docs regarding this matter we see this:
And they clearify that Views and Model shouldn't communicate directly:
view objects are typically decoupled from model objects in an MVC
application
and
When a model object changes (for example, new data is received over a
network connection), it notifies a controller object, which updates
the appropriate view objects
And so on.
So, what's the matter here? Is Cocoa adopting its own idea of an MVC, regardless of the common one? Or am I missing something in the common way of seeing an MVC architecture?
While it can be said that Cocoa's version of MVC is a sort of subset of the actual definition of MVC, they are not separate entities. The Cocoa version of MVC typically revolves around the use of a View (typically an NSWindow and/or an NSView), a controller (typically an NSWindowController), and a model layer (anything from a simple array to a Core Data stack). The separation of powers in this model is clear, but where in the 'tier' structure that Wiki defines should each of these belong?
I would argue that the Controller and the View are a part of the client layer. Not only is the controller responsible for the delegation between the model and the view, but it is responsible for responding to user events and determining the correct course of action to take during non-framework error handling. By taking this approach to MVC, you can now begin to see how Cocoa does, in fact, satisfy the broader definition of the pattern.
A fundamental rule in a three tier architecture is the client tier never communicates directly with the data tier;
This one's probably the hardest to reason about of the 3, and it involves delving into what "communication" actually means in the context of the pattern. When we say communication, what we mean is that the controller has no direct involvement in the actions taken by the model. That's not to say that the controller cannot order a change in the contents of the model, but rather that the controller does not have a hand in how the model updates itself. The controller acts as a director, not an implementer, which vastly simplifies the creation of a database layer, and is one of the reasons that Core Data and SQLite3 exist as external frameworks rather than as Foundation classes.
view objects are typically decoupled from model objects in an MVC application
That brings up one of the age-old taboos when programming with the pattern: making your views too smart. The controller provides a solid barrier between the model and view, such that the controller acts as a director and a filter for content from the model layer. Without any such barrer, say a tableview, would have to ensure that every cell had a copy of the data from the database, and that each cell knew when and how to update itself when a change is propagated in another cell. In Cocoa, this is where our NSWindowControllers come in. They manage the display of a root view and act as a barrier between some model and the content of the view it manages. Though, it is important to note that the controller objects in Cocoa are view-biased, mostly because it would be nearly impossible to provide a generic outlet to any kind of model layer without quite a bit of unnecessary glue.
When a model object changes (for example, new data is received over a network connection), it notifies a controller object, which updates the appropriate view objects.
That's the way it should be, for the reasons I've laid out above. But, to build on the networking example you've given, consider this:
Given an NSOperation that fetches data, and a controller that manages a tableview, you would probably not like the controller sticking its fat fingers into the operation, nor would you like the tableview to receive raw NSData and have to spend valuable rendering time processing the result and displaying it.
And so on. So, what's the matter here? Is Cocoa adopting its own idea of an MVC, regardless of the common one? Or am I missing something in the common way of seeing an MVC architecture?
I guess the conclusion I would draw from this is that your definition of the separation of powers in MVC and in how Cocoa does it is off. Cocoa is fairly rigid about adhering to the pattern, though there is an interesting contemporary movement within the Objective-C community towards MVVM.
You are correct the MVC practiced in most cocoa apps is not the MVC as it is defined in the text books. There are many variations of MVC employed by different frameworks. The MVC employed by tools with visual designers are heavily influenced by their visual designer implementation. With XCode you have story boards and nibs. The cocoa libraries and the way concerns are separated are influenced by this. If you want to take advantage of these tools, I would recommend understanding how concerns are separated by Xcode and work within this approach. Your code will coexist with it more smoothly. Apple documentation will help with this.
That being said, MVC is about separation of concerns. Separating concerns is hugely important in designing and maintaining software. Separating concerns properly can reduce dependency, reduce cyclomatic complexity, and make your code much more testable and maintainable. I think it is good that you are paying attention to this and whatever way you structure MVC should look to the reason why you are separating concerns as the guide to implementation.
I just cannot seem to get my head around what exactly is the MODEL in MVP.
If I have a layered architecture PRESENTATION / APPLICATION / DOMAIN / INFRASTRUCTURE, what exactly is the MODEL?
DOMAIN objects accessed through
lower layers?
A separate object defined in the
PRESENTATION layer that maps to the
UI and uses data obtained from a
lower layer?
If someone could clear my understanding on what is the MODEL it would be greatly appreciated.
The Model is normally the group of classes/types/components that represent the core domain (business or otherwise) that your application operates within. These are the classes that perform the key logic required, often in the form of business rules, and also consume/manipulate data.
In your layered example, the Model would mostly be found in the Domain layer but could also be in the Application layer.
I think you're having difficulty understanding it because you are trying to combine two separate architectural patterns, or ways of looking at the application, being n-tier/n-layer versus MVP.
It's completely reasonable (and quite common) to use some sort of Model/View approach while at the same time applying layering in your application.
Maybe you should focus on them one at a time to start with and then overlay them when you are more familiar with both.
In any of the Model-View-* architectures, the Model is what describes the data in your application (and, if they fit the need, are passed in to the View for rendering).
If your application already has Domain objects, it very well may be the case that you could use them for your Model.
It doesn't matter what architectural guidelines you're following, M is always going to be the same thing. The Model is the piece that is specific to your domain. It's the part that really is what you're application is trying to do. The Model is supposed to represent your business domain. This goes for MVP, MVC, MVVM, etc.
If you were making a inventory system, then an Inventory class would most likely be in your Model, a Product would probably be there, an Order, you get the idea. These are the things that compose your domain logic.
The model is the data. This might just be data out of a database in DataSets, or it might be a complete domain model with objects representing your field of business.
The view is the UI, whether web pages or a Windows application or a mobile device application.
The presenter is the glue between the two and the brains of the whole outfit. Actions initiated by the view take place in the presenter. Generally in a WinForms application, for instance, a Button.Click event in my View simply calls a method on the Presenter, which then takes whatever action is necessary (and it may just be doing something back in the View).
The presenter holds a reference to the view (through an interface), and to the model. The view has a reference to the presenter (usually I strongly-type this, but it can be an interface as well). The model doesn't know about the presenter or the view.
I am new to the concept of MVC and multi-tiered web architecture. I developing a PHP application and am using one of the available MVC frameworks. My question is as follows:
From what I understand, MVC in and of itself is not considered a multi-tier architecture. I can understand how using MVC alone is a step up from taking an unstructured approach, but I was contemplating how a simple 3-tier architecture would fit in? Would MVC reside in the presentation layer? What are the merits of adding a tiered approach? From what I gather, with MVC alone there are no explicit data objects responsible for retrieving data from the database and this is usually stuffed into the model. Likewise the business logic, which in a 3-tiered architecture would reside in a 'business-layer' (or whatever you want to call it), can be stuffed into the controller.
Is my understanding somewhat correct? I know I asked a lot of questions, but I would like to hear you discuss how you incorporated an n-tier architecture into your MVC framework (PHP or otherwise) as I assume that the two are not mutually exclusive. Thanks!
M) M is your model. This is generally living in your business layer or the layer just behind your presentation layer. Many people don't like the presentation layer to have any knowledge of the business layer though and so they further abstract that by having what is called a ViewModel. These frequently are DTO (data transfer objects) that loosely map to your Domain model. For me (.net guy) there are tools such as AutoMapper to make the conversion from Domain Model to View Model.
V) V is your view. The view is your presentation layer. This is the actual HTML or PHP code that the user directly touches and interacts with. The view should be as light as possible (meaning no logic if possible). Try to keep any sort of if/then type scenarios out of the view and stick to just displaying and collecting data. Present a ViewModel to your web designers so that they don't contaminate your DomainModel.
C) C is your controller. This is much like a co-ordinator. It takes data from your view and makes sure it gets to the right back end function/method for processing that data. It also co-ordinates data from the back end on it's way to the front end.
Where multi-tier design concepts come in is behind the Presentation layer (where is where MVC is primarily). When the controller is taking data from the view and passing it back to the back end it (if you follow DDD or Domain Driven Design) would pass the data to an application service (a class that co-ordinates back end matters). The service might further push the data into a Repository layer (which is a class that speaks to the database, filesystem, web services, etc. - any infrastructure stuff). DDD is a big topic but will get your head around the n-tier approach and how it works with MVC.
While researching this topic take a look at IoC (inversion of control), DI (dependency injection), TDD (test driven development), and as many patterns as possible (facade, factory, etc.).
In general, business logic should not be in the Controller - you'd end up with massive controllers if you followed this pattern. The Model basically houses all your non-presentation layers ... data access, business logic, and business entity objects. Your controller prepares the business data for the view.
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.