I am trying to coach some guys on building web applications. They understand and use MVC, but I am interested in other common patterns that you use in building web apps.
So, what patterns have you found to fit nicely into a properly MVC app. Perhaps something for Asynchronous processes, scheduled tasks, dealing with email, etc. What do you wish you knew to look for, or avoid?
Not that it matters for this question, but we are using ASP.NET and Rails for most of our applications.
Once you get into MVC, it can be worthwhile to explore patterns beyond the "Gang of Four" book, and get into Martin Fowler's "Patterns of Enterprise Application Architecture."
The Registry pattern can be useful to make well-known objects available throughout the object hierarchy. Essentially a substitute for using global data.
Many MVC frameworks also employ the Front Controller and the Two-Step View patterns.
The "Model" in MVC is best designed as the Domain Model pattern, although some frameworks (led by Rails) conflate the Model with the ActiveRecord pattern. I often advise that the relationship between a Model and ActiveRecord should be HAS-A, instead of IS-A.
Also read about ModelViewController at the Portland Pattern Repository wiki. There is some good discussion about MVC, object-orientation, and other patterns that complement MVC, such as Observer.
This question is so open that it's hard to give a correct answer. I could tell you that Observer pattern is important in MVC (and for webapplication) and it would be a good answer.About all design pattern that exist are common in big web application. You will require to use some Factory to build complexe object and to access some section require some Facade.
If you want more "tips" or good practice instead of design pattern, I would suggest you to use IoC and the use of good Framework instead of starting from scratch. I can suggest you to explain the benefit of having a good ORM engine to drive you persistance layer faster too (usually can come from the Framework too).
Don't look at it from the aspect of what patterns to use with your development approach, but look at it more as how to apply patterns on a problem-by-problem basis. The architectural decisions made for the project provide just as much indication of what patterns to use as other people's experience will dictate.
That said, I have found that I am a fan of the Provider model for having multiple choices to accomplish a single task with ease of deployment added in. Also, the Unit of Work pattern is great for setting transactional boundaries. Largely, though, the architecture and business needs dictate the approach that is taken for any given code change or new development.
As much as I love patterns, I always fear seeing them overused. I have personally seen people that have used them just for the sake of using them, and it has actually made the code harder to maintain and more tightly coupled than it should have been. Also, it is good to know both sides of the patterns argument. A good pattern knowledge should be rounded out with (often considered a pattern, on its own) anti-pattern knowledge, as well.
I would most likely recommend some kind of Dependency Injection as well (Inversion of Control). Probably the single most important supplementary "pattern" to use.
Related
When developing an app with GUI, and database access, are there situations where the MVC architecture isn't relevant?
To me it seems that the Views and Controllers must only be different entities is one to upgrade the views, or to replace them with something else, namely mobile displays (or predicts such a possible change for the future of the app).
Also, I see the separation of the Model and Controllers only necessary if the Model is to be upgraded / replaced.
So is there any other purpose for the MVC architecture that the situations when components should be upgraded/ changed, or is this really it?
I like MVC because it makes it easier to think about how different parts of the app are going to work together. If everything is just lumped in together, I find it much harder to visualise in my head.
So it's not really a case of when you should use it, rather how do you prefer to think?
If you find it easier not using MVC then you should probably not use MVC.
I think, the root of you confusion is the scope at which you try to apply MVC design pattern.
MVC is not a pattern for small applications. Instead you are supposed to apply it, when your free-form OOP code starts to become unmanageable. Your codebase might be implementing all of the SOLID principles, but at some point you will start getting lost there.
That would be when you should be using MVC, because this design pattern applies additional constraints. It does not add anything new to application. Instead it limits what code can go in what parts of your application.
P.S. you also seem mistaken about what separation there is in MVC. The basic divide is between model layers and presentation layers. Those are two main parts MVC applications. And only then withing the presentation layer there is a separation between views and controllers. You might benefit from reading this article.
For me, it all comes down to testability. Automated testing of UI code is exorbitantly expensive compared to testing of model code. It is much easier to achieve test coverage in model and event controller layers compared to view layers.
If you have no need to test your application, and it is small enough that you can keep it all straight in your head, then MVC is probably a waste of time. Very few applications are truly small enough that these concerns are not at issue. But if the app is truly that small, MVC will add far more overhead than it will provide in value.
Don't think of MVC design pattern as you business architecture. Treat MVC as presentation architecture. There are many arguments about MVC usage in terms of business architecture. This stackoverflow question is one example.
Actually, you are looking for N-Tier architecture. Where it is separated as DAL, BLL and PL:
Data Access Layer (DAL):
A layer responsible to inteact with the Storage (insert/update/delete)
BLL:
A layer where the business logic resides. This layer is the core of your application. Some people often use the term Middleware (please correct me if wrong) to represent the BLL. BLL does not know the UI, means that it can be used by Desktop app, web app, mobile app, etc.
PL:
This is your presentation layer or UI layer. MVC, at least the View and Controller resides here.
There are benefits to the MVC architecture, and there are some disadvantages to it. You have to weigh them for your project to see which would be the most appropriate for you.
Advantages for MVC:
More maintainable because it's compartmentalized (separation of
concerns).
It's more testable because you can unit test the controllers.
You typically have more control over the HTML that gets generated
(Yes, you can accomplish the same with webforms, but only if you give
up all of the advantages of webforms as well).
Your webpages will be smaller and faster because you won't be
carrying around page/view/control state.
Integrates better with client-side lifestyle and libraries
(Bootstrap, jQuery and it's many plug-ins, AJAX, etc)
Advantages of webforms:
More 3rd party controls (webforms relies heavily on either 3rd party controls, or custom usercontrols to achieve rapid application development).
If you need viewstate, then it takes less work, but this is pretty
rare if designed right.
Integrates better with server side control libraries.
Of course, someone is going to say, why did you list xyz as an advantage for whatever, because you can do that in the other one too! Well, you can achieve the same thing in both frameworks, it's just a matter of ease. What is easy for one may be more difficult in the other, but both of them, given enough time and resources can do it too.
MVC is about separation of concerns and making these concerns testable.
Someone said 'MVC is not a pattern for small applications.'. I disagree. Why? It only dictates how you separate concerns, I don't understand why this should be different for small applications. I would argue it's even simpler because every developer uses the same pattern and is used to it. It's not overhead, it's consistency. Also look what this guy has to say.
Another thing: MVC is a presentation layer pattern (Separated Presentation), it means it logically separates your UI in a models, views and controllers. Controllers are responsible for managing the flow, interacting with the backend system to query and save data, and converting that data to models (or view models) that are used by the views.
The backend in itself is another system, which has its own independent architecture, with services, domain and data layer (as for example the onion architecture, of which an example can be found here).
I am sorry for my question may looking a old repetitive questions but I as I am starting Linq to SQL I want to discuss how many layers (architecture) should I use ?
I am working on web mostly web sites and small to medium scale web applications. I understand dividing application into layers help its maintainanace and enhancement but frankly I want some balance way which give me rapid development and code reuse-ability as well. I cannot spare so much time on unwanted management of layers.
Before I was using 4 layers (business objects, BLL,DAL and user itnerface.) I became confuse on it as different people have described different layers. Please guide me what and how many layers I should use ? Thanks
Don't use the layer architecture. Use the onion architecture.
The most important aspect from architecture perspective is the separation of concerns. Separation of concerns leads to clean code, easy maintenance, extensible, etc. That said, I recommend to base your decisions based on the following criteria:
Try to architect your system in a loosely coupled way. Use messaging instead of RPC as messaging is reliable, scalable, asychronous, loosely coupled, etc. You can either use MSMQ or NServiceBus (for service bus based architecture).
Create layers based on the separation of concerns concept. For e.g. you can go for typical 3 layered architecture which will have just UI layer, business logic layer (business rules+workflow) and data access layer or more granular such as UI layer, services layer (facade), business logic layer, data access layer, etc. Using IoC / Dependency injection will make life easy as none of the layers will have direct dependency. Moreover, it promotes unit testing easy as you inject mocks instead of the real implementations for the unit test. There are so many IoC frameworks available (NInject, Autofac, Castle Windsor, Structure Map, etc...)
Try to use EF instead of Linq to SQL as the later works only with SQL, while the EF works with any database. Moreover, in my opinion, EF is where microsoft is innovating and I would assume that Linq to SQL may be retired one day.
Great question haansi. This is something that I have wrestled with quite often when building small to medium sized sites. Finding that balance of creating a architecture that gives you the greatest flexibility and allows for rapid deployment is what I think we all need to strive for in all of our work.
With that being said, I have found that using the Repository Pattern to be quite helpful with LINQ to SQL projects. I couple that with the Model View Presenter pattern (for WebForms or other projects) and it provides a great foundation for reuse with minimal layers.
My Webform calls a Presenter class, which in turn is responsible for populating the View. To populate that View the Presenter can call N number of Repositories. The Repository is where you encapsulate your DataContext class and your LINQ to SQL calls. These calls return the model classes.
One huge benefit to this regardless of the size of the app is that get great re-use out of your Repository, you get to maximize the use of LINQ and you have used some patterns that other software developers could easily read and support.
Another big benefit is that you now have created a simple architecture that can benefit from using Unit Testing to test from the Presenter back to the Repository without a ton of effort.
Good luck!
I decided to use one layer (DAL + BLL) for small projects and for large applications will use Different layers for DAL & BLL. I will use Linq in DAL and funtiosn will return IQueryable.
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What are some best practices to keep in mind when developing a script program that could be integrated with a GUI, probably by somebody else, in the future?
Possible scenario:
I develop a fancy python CLI program that scrapes every unicorn images from the web
I decide to publish it on github
A unicorn fan programmer decides to take the sources and build a GUI on them
he/she gives up because my code is not reusable
How to prevent the step four letting the unicorn fan programmer build his/her GUI without too much hassle?
You do it by applying a good portion of layering (maybe implementing the MVP pattern) and treating your CLI as a UI in it's own right.
UPDATE
This text from the wikipedia article about the Model-View-Presenter pattern explains it quite well.
Model-view-presenter (MVP) is a user
interface design pattern engineered to
facilitate automated unit testing and
improve the separation of concerns in
presentation logic.
The model is an interface defining the data to be displayed or
otherwise acted upon in the user
interface.
The view is an interface that displays data (the model) and routes
user commands (events) to the
presenter to act upon that data.
The presenter acts upon the model and the view. It retrieves data
from repositories (the model),
persists it, and formats it for
display in the view.
The main point being that you need to work on separation of concern in your application.
Your CLI would be one implementation of a view, whereas the unicorn fan would implement another view for a rich client. The unicorn fan, would base his view on the same presenters as your CLI. If those presenters are not sufficient for his rich client he could easily add more, because each presenter is based on data from the model. The model, in turn, is where all the core logic of your application is based. Designing a good model is an entire subject in itself. You may be interested in reading, for example, about Domain-Driven Design, even though I don't know how well it applies to your current application. But it's interesting reading anyway.
As you can see, the wikipedia article on MVP also talks about testability, which is also crucial if you want to provide a robust framework for others to build on. To reach a high level of testability in your code-base, it is often a good idea to use some kind of Dependency Injection framework.
I hope this gives you a general idea of the techniques you need to employ, although I understand that it may be a little overwhelming. Don't hesitate to ask if you have any further doubts.
/Klaus
This sounds like a question about how to write usable code.
When considering reusablility of code, generally speaking, one should try to:
separate functionality into modules
have a well-defined interface
Separating functionality into modules
One should try to separate code into parts that have a simple responsibility. For example, a program that goes out to the internet to scrape pictures of unicorns may be separated into sections that a) scrapes the web for images, b) determines if an image is a unicorn and c) stores the said unicorn images into some specified location.
Have a well-defined interface
Having a well-designed interface, an API (application programming interface), is going to be crucial to providing a way to reuse or extend an application.
Providing entry points into each functionality will allow other programmers to actually write a new user interface for the provided functionality.
The solution to this kind of problem is very simple, but in practice, a lot of junior programmers have trouble with this pattern. Here's the solution:
You design a unicorn-scraping API. This is the hard step; good API design is insanely hard, and there aren't many examples to study. One API that I think is worth studying is the one in Dave Hanson's book C Interfaces and Implementations.
Then you design your command-line interface. If the functionality you are exposing is not to complicated, this is not too hard. But if it's complicated, you may want to think seriously about managing your API using an embedded scripting language like Lua or Tcl and designing an interface for scripting rather than for the command line.
Finally you write your command-line processing code and glue everything together.
Your hypothetical successor builds his or her GUI in one of two ways: using the embedded scripting languages, or directly on top of your API.
As noted in other answers, model/view/controller may be a good pattern to use in designing your API.
You'll be taking input, executing an action, and presenting output. It might be a good idea to use a callback mechanism (such as event handlers, passing a method as a parameter, or passing this/self to the called class) to decouple the input and output methods from the execution of the action.
Aside from this, program to an interface, not to an implementation - the essence of MVC/MVP, as klausbyskov mentioned. e.g., Don't directly call file.write(); make myModel.saveMyData() which calls file.write, so someone else can make a somebodysModel.saveMyData() that writes to a database.
I'm new to MVC structure and feel it's harder to get things done because it's a new way of doing things. Are there anybody who has experience of MVC vs. pages way of doing things. Is the MVC way of doing it the holy graal now or is there still value doing ordinary object-oriented development (or even procedural pages webdev)? Is MVC a fad?
A brief list of pros and cons for MVC
Pros
Testablily
Separation of Concerns - Promotes decoupling between major components
Helps you focus on one task/area at a time
Natural fit for Web and desktop interactions
Fits well with other design patterns, Single Responsibility
Principle etc.
Cons
Can require more code and effort
Can reduce clarity for simple pages (in these cases try to continue using simple pages)
Will take more learning
MVC is much more than a fad. It is a very pragmatic way of separating multiple concerns of a web application into manageable and reusable sections. Granted it does take some getting used to at first, but with some conscious effort at breaking your application up the MVC-style can be very rewarding. Often solutions are more succinct as they only need to concentrate on a single operation or task.
It is not a new idea either. It has been around in one form or another since 1979 (#Sarfraz Ahmed's link) and has been used in various web and desktop platforms.
If you are finding yourself having trouble using an MVC-style implementation then try to break it down into the individual parts or actions that are being used, and their corresponding area, model view or controller. As you probably expected, this will come easier over time.
Good luck
To MVC or not is mostly up to the needs of your project. Sometimes I have simple stuff that doesn't need the Model, but can still benefit from a controller/view arrangement. In those cases I might go with a micro-framework ( for PHP that would be limonade ) or even less. Still the majority of my web projects are MVC applications.
I started making websites around 1996-1997, everything was kind of new and there was no sense to what might be a good or bad idea in the long run. Around 2005 when I left the military and got back into web development it wasn't that enjoyable to fight through rat's nest of organically designed code. So When I was introduced to Ruby on Rails and the concept of MVC, I instantly recognized for myself that this was a game changer.
As MVC is increasingly adopted in more and more web projects, its paving the way for the next generations of developer's lives to be a little easier and more productive by providing common ground across companies and projects. Yes MVC is a little tough to swallow when going from the upfront simplicity of cooperative script to page projects but anything worth doing isn't going to be that easy right?
With all that said, if you get a good grounding in MVC in one language, it can provide a mental common ground for you to learn other languages. As for how long MVC will last as a dominant framework design pattern, the software industry is like a desert, everyday things change and great ideas become eclipsed by better ones (XHR for websockets, embedded objects for native audio/video tags) but I think whatever finally dethrones MVC will be somewhat similar because it doesn't really matter how great an idea is if only a very small portion of people understand it.
If we speak about ASP.NET MVC, the framework page itself has a discussion on this point:
The ASP.NET MVC framework offers the following advantages:
It makes it easier to manage complexity by dividing an application into the model, the view, and the controller.
It does not use view state or server-based forms. This makes the MVC framework ideal for developers who want full control over the behavior of an application.
It uses a Front Controller pattern that processes Web application requests through a single controller. This enables you to design an application that supports a rich routing infrastructure. For more information, see Front Controller on the MSDN Web site.
It provides better support for test-driven development (TDD).
It works well for Web applications that are supported by large teams of developers and Web designers who need a high degree of control over the application behavior.
The Web Forms-based framework offers the following advantages:
It supports an event model that preserves state over HTTP, which benefits line-of-business Web application development. The Web Forms-based application provides dozens of events that are supported in hundreds of server controls.
It uses a Page Controller pattern that adds functionality to individual pages. For more information, see Page Controller on the MSDN Web site.
It uses view state or server-based forms, which can make managing state information easier.
It works well for small teams of Web developers and designers who want to take advantage of the large number of components available for rapid application development.
In general, it is less complex for application development, because the components (the Page class, controls, and so on) are tightly integrated and usually require less code than the MVC model.
I think this exhaustive list provides answers to all your questions about MVC.
Since you have not mentioned language and if you mean mvc in php then this is also a great
resource for those new to mvc:
http://phpro.org/tutorials/Model-View-Controller-MVC.html
This is from a ASP.NET WinForms -> ASP.NET MVC perspective, however, ASP.NET MVC was the first time I heard of the MVC pattern (noob programmer) and I think if I discovered MVC in another language before ASP.NET MVC, I would've left ASP.NET/C# and went to that framework.
Learning and using MVC has made me a better programmer. I've always felt ASP.NET placed a transparent barrier between myself and the code (HTML or C#).
I've always wanted to break through that barrier, and MVC gives me complete control of everything in my app while emphasizing separation of concerns (something that makes coding infinitely more fun and less of a headache).
For me, MVC was the right choice.
The big advantage of MVC (in general) is that you are far less likely to create monolithic applications. You have a single request-act-response pathway, vs. something like ASP.NET webforms or JSF (although JSF is not as bad as webforms). It's very easy for a small site in webforms to become a big unmanageable tangled mess in webforms, then you have to start whipping out the custom controls and your team is lost. If you understand how the web work, MVC is easy ... although it is then in your hands to manage state.
After struggling with MVC for some time, I have to said that I now much prefer MVP (Model-view-presenter) architecture. Separation of concerns, decoupling and asynchronization are much easier to achieve with MVP than MVC, IMHO, because each MVC implementation doesn't split the concepts the same way, each controller doesn't handle the same things.
I've been reading through a couple of questions on here and various articles on MVC and can see how it can even be applied to GUI event intensive applications like a paint app.
Can anyone cite a situation where MVC might be a bad thing and its use ill-advised?
EDIT: I'm specifically talking about GUI applications here!
I tried MVC in my network kernel driver. The patch was rejected.
I think you're looking at it kind of backwards. The point is not to see where you can apply a pattern like MVC, the point is to learn the patterns and recognize when the problem you are trying to solve can naturally be solved by applying the pattern. So if your problem space can be naturally divided into model, view and controller then it is a good candidate for MVC. If you can't easily see which parts of your design fall into the three categories, it may not be the appropriate pattern.
MVC makes sense for web applications.
In web applications, you process some data (on SA: writing questions, adding comments, changing user info), you have state (logged in user), you don't have many different pages, but a lot of different content to fit into those pages. One Question page vs. a million questions.
For making CMS, for example, MVC is useless. You don't have any models, no controllers, just a pages of text with decorations and menus. The problem is no longer processing data - the problem now is serving that text content properly.
Tho, CMS Admin would build on top of MVC just fine, it's just user part that wouldn't.
For web services, you'd better use REST which, I believe, is a distinct paradigm.
WebDAV application wouldn't benefit greatly from MVC, either.
The caveat on Ruby for Web programming is that Rails is better suited for building Web applications. I’ve seen many projects attempt to create a WebDAV server or a content management system CMS with Rails and fail miserably. While you can do a CMS in Rails, there are much more efficient technologies for the task, such as Drupal and Django. In fact, I’d say if you’re looking at a Java Portal development effort, you should evaluate Drupal and Django for the task instead.
Anything where you want to drop in 3rd party components will make it tough to work in the MVC pattern. A good example of this is a CMS.
Each component you get will have their "own" controller objects and you won't be able to share "control" of model -> ui passing.
I don't necessarily know that MVC is ever really a bad idea for a GUI app. But there are alternatives that are arguably better (and also arguably worse depending on whose opinion you're asking). The most common is MVP. See here for an explanation: Everything You Wanted To Know About MVC and MVP But Were Afraid To Ask.
Although I suppose it might be a bad idea to use MVC if you're using a framework or otherwise interacting with software that wasn't designed with MVC in mind.
In other words, it's a lot like comparing programming languages. There's usually not many tasks that one can say that one is better than the other for. It usually boils down to programmer preference, availability of libraries, and the team's experience.
MVC shouldn't be used in applications where performance is critical. I don't know if this still applys with the increase of computing power but one example is a call center application. If you can save .5 seconds per call entering and updating information those savings add up over time. To get the last bit of performance out of your app you should use a desktop app instead of a web app and have it talk directly to the database.
When is it a bad thing? Where ever there is another code-structure that would better fit your project.
There's countless projects where MVC wouldn't "fit", but I don't see how a list of them would be of any benefit..
If MVC fits, use it, if not, use something else..
MVC and ORM are a joke....they are only appropriate when your app is not a database app, or when you want to keep the app database agnostic. If you're using an RDBMS that supports stored procedures, then that's the only way to go. Stored procs are the preferred approach for experienced application developers. MVC and ORM are only promoted by companies trying to sell products or services related to those technologies (e.g. Microsoft trying to sell VS). Stop wasting your time learning Java and C#, focus instead on what really matters, Javascript and SQL.