MVC in the frontend makes perfect sense. But why do we need MVC in the backend as well? Where is the "view" in this case given backend doesn't provide anything visual.
MVC is a design pattern that promotes separation of concern where three participating concerns are
•
The Model a data structure which holds business data and is
transferred from one layer to the other.
The View which is responsible to show the data present in the
application or think of it as a data structure (complete decoupled
from the model) solely used for presentation purpose (not necessary a
presentation output itself) e.g. view template in below diagram
The Controller act as a mediator and is responsible to accept a
request from a user, modify a model (if required) and convert it into
the view.
MVC as a pattern can exist completely on the backend or completely on frontend or in its common form of backend and frontend combined.
One has to think relatively and see how to keep all these three concerns sperate for better application design.
The whole idea behind MVC pattern is a very clear separation between domain objects which represents real-world entities and the presentation layer data structure. Domain objects should be completely independent and should work without a View (data representation) as well. Or other way to think of it is, in MVC context views are isolated from the model. It allows to use the same model with different views.
Spring MVC is a good example of a backend MVC framework.
Below diagram depicts how all the three components exist on the server side only (inside the application container). Taken from official blog only.
MVC is about separating concerns in applications that accept user input, perform business logic, and render output. It does not say where that logic resides. Nor does it specify that all the logic must live in a single process.
A fairly traditional use of MVC is with a spreadsheet. Let's look at both a single process application and a multi-process application to see how they might implement this simple spreadsheet:
A B C
----- ----- ---------
1 | 1 2 =A1+B1
Let's say the user enters the number 4 into cell A1. What happens?
SINGLE PROCESS APPLICATION (e.g. Microsoft Excel): The user input is handled by the view logic, up until the user leaves the cell. Once that happens, the controller receives a message to update the model with the new value. The model accepts the new value, but also runs some business logic to update the values of other cells affected by the change. Once complete, the model notifies the view that its state has changed, and the view renders the new state. That notification can happen via pub/sub as #jaco0646 suggests, but it could also be handled with a callback.
MULTI-PROCESS APPLICATION (e.g. Google Sheets): The user input is handled by the view logic (in the client), up until the user leaves the cell. Once that happens, the controller (on the server) receives a message (via HTTP, or a socket) to update the model (also on the server) with the new value. The model accepts the new value, but also runs some business logic to update the values of other cells affected by the change. Once complete, the model notifies the view that its state has changed, and the view renders the new state (in the client). That notification can happen via the controller's HTTP response, or via a socket.
In other words, the MVC pattern is applicable to both scenarios.
Furthermore, it is also perfectly valid to treat the client and the server as two entirely separate applications, both of which might implement MVC. In this scenario, both the client's model, and the server's view are less traditional. The client's model is likely making AJAX requests to the server, as opposed to running complex business logic itself or persisting the data locally. And, the server's view is likely a serializer that produces some form of structured output that the client understands, like JSON, XML, or even a CSV.
MVC is a perfectly valid pattern anytime an application needs to accept user input, perform some business logic, and render some output -- regardless of whether that application lives on one or more processes -- and regardless of whether the view is something a human will consume.
Related
What exactly are those controllers? We were asked to build an ATM in Java for school project, and part of our design is:
We have accounts that stores most of informations
We have users that can make operations for their own accounts and stores some minor informations.
(Also we have atm class to store users and make some toplevel changes)
We have userInterface to catch inputs and use controllers.
Am I right that our accounts are models, interfaces are views and users are controllers?
Thanks a lot to address my problem,!
You say: "accounts are models". Actually no, they are not.
A domain model (also called model layer, or model) is not a single component, but a layer, composed of multiple components. It abstracts a real-life process and the needed resources. In other words, it models a business logic (represented by data and, especially, behavior).
The model layer can be part of an application, or can be shared by multiple applications.
Each of the model components has a certain role. It can be an entity (e.g. a domain object), a value object, a service, a data mapper abstraction, a repository abstraction, an abstraction of an external service (like an email or paying service), etc. By abstractions, I mean interfaces or abstract classes. The concrete implementations should not be part of the domain model, but of a different, external space vis-à-vis model, serving as infrastructure constructs.
So, your User, Account and Atm classes are just components of the model. More exactly, they are entities.
On the other hand, the controllers and the views are components of the UI layer.
The controllers (should) take the responsibility of only deferring (e.g. dispatching) the execution of the user request to the model layer. More precisely, to the service layer - which is defined as a boundary layer of the domain model and is represented by its service components. So, a certain service instance is injected into the controller as dependency. The controller passes it the current user input (data) and calls a certain method of it, in which the required user request processing steps are defined. The service instance, in conjunction with other model layer objects, performs all these steps to update the model with the user input. Having the dispatching task in mind, a controller method should therefore be slim, containing 1-3 lines of code.
The views (should) obtain (the updated) data from the domain model - by querying it (e.g. by pulling data from it) itself - and display it. Analog to the controllers, the views communicate with the model through certain service component(s).
I used the verb "should" to emphasize the fact, that there are different implementation models of the UI layer. An implementation could use the controllers and the views as described above - the original MVC design. Another implementation would use the controllers not only to update the model (through services), but also to query it (through services), in order to pass the received data to the view for displaying to the user. Or an implementation could even not use services at all, forcing the steps of processing the user request to be defined in the controllers and/or the views. And so on. So, it's up to you, how you choose to implement the UI layer.
Note that you can also have controllers and/or views named like the model components (User, Account, Atm, etc). But then you must use namespaces to differentiate between all of them - the recommended way to go. In Java, namespaces are managed by packages.
Some resources with more details (mainly web MVC related, with examples in PHP):
How should a model be structured in MVC
PHP - Structuring a Slim3 web application using MVC and understanding the role of the model
MVC, controller - use cases
GeeCON 2014: Sandro Mancuso - Crafted Design
MVC, Delivery Mechanism and Domain Model
Catalog of Patterns of Enterprise Application Architecture
Controllers in this context will receive user's request via interfaces and call service to perform any action, call Database layer to fetch data and populate into models, integrate model with view to create desired view and return back the combined view to the user. The User and Account will be different but related entities which have their representation in the database.
You didn't get it quite right, but don't worry -- it's actually not complicated to figure out what the parts are when you know why they exist in the first place.
Here it is:
MODEL: MVC is an architectural pattern for separating concerns in applications with user interfaces. The model is simply the part of the application that is not concerned with the user interface. It has no code in it at all that depends on the UI, the specific operations that the UI provides. Together the view and controller define the user interface, and the model is just everything else, although when we talk about it in the context of a MVC application, we usually just refer to everything else the UI might see or affect. Sometimes this leads us to make an object in the application that is called the model, but that is not really required and often not useful.
It's important to understand that the arrows in those MVC diagrams that go in a circle show data flow. When you draw a diagram that shows dependency, the view and controller depend on the model. The model does not depend on the view.
Now that we've defined the model as "the model is not the user interface", obviously the user interface consists of the view and the controller.
We separate these, and put the user between them:
VIEW: This is the part of the user interface that presents information to the user. It determines what the user will see, hear, etc. It takes information from the model and provides information to the user.
CONTROLLER: This is the part of the user interface that gets information from the user and implements the actions that he wants to perform.
The canonical example is that the view creates the button, and the controller handles the click. The separation between the view and controller, with the user (and time) in between, is the whole purpose of MVC. They do not connect in software except via the model.
The fundamental assumption of MVC is just this: We can separate the view code from the controller code, because they run at different times. The view code runs after the model changes, before the user can act on those changes, and the controller code runs after the user decides to act.
It's also important to understand that this is never entirely accurate. MVC is an over-simplification and there are actually always connections between the controller code and the view code that don't go through the model. But we try. Most of the design work in MVC frameworks or applications is an attempt to manage and properly design the ways in which they cheat in this.
Learning about MVC pattern (the original pattern developed in 1978, not the MVC web application framework developed by Microsoft) I realised there's plenty of class diagram and another diagrams on the Internet, but there is not a component diagram, with their interfaces etc. I found only this one, and I wanted to know how accurate would be, and why there is almost 0 components diagram of the MVC pattern on the Internet:
Well, to be honest there is a lot of information of MVC which is confusing (as no one specifies which "iteration/version" of MVC they are talking). The components are the same: a model, a view, a controller.
But, multiples sources establish their relationships as they please. In the original MVC, as i understood, the controller arranges the views for the user to see and interact. The user input is processed by the controller, which sends updates to the model. The model, after changing state, sends signals to update the view accordingly (which means that the view is attached to the model).
That component diagram you posted is not completely inaccurate. It is best to take a case study and analyze that component diagram. For example:
Some other questions might rise, like, how can the controller handle input if the user always interact with the view? Check Details of implementation of MVC/ M(VC)
Some other sources establish the controller as a "bridge" between model and view (web MVC) and the interactions are bidirectional (that is, the controller might also modify the views, the model might also send signals to the controller).
I want to try to write (amateur here!) a multiplayer game, and now at designing I decided to use the MVC-pattern.
Now my question is: where should I put my networking code? In the Model or the Controller? (Obviously not the View)
EDIT:
Sorry, for the hundredst time my question was unclear.
The game itself will be MVC, and it will firstly communicate with a server (find player), and later with that player (send your turn and get other's turn). So where should I do that?
The MVC design pattern is actually a combination of two layers: presentation layer and model layer. Presentation layer usually deals with user interface (updates it and reacts to user's interaction). The model layer deals with domain business logic and persistence.
The networking code should go in the model layer.
To be exact, in the part of model layer, that deals with persistence, because there, from the standpoint of business logic, there is no difference where the data comes from. It can be from the SQL database, from opened network socket or detector on the mars rover. Those all are just data sources, which, often implemented as data mappers, are part of the model layer.
You could put the actual game itself in a new project and reference that between your MVC application, that way your game is entirely separated from your web application. This could be useful if you ever wanted to port it to WPF for instance. Another alternative is to have the game as Web Service which the MVC application requests information from and would provide scalability for additional languages to plugin in.
However, if you decide to keep everything as a whole in MVC then I would suggest the Model.
As a breakdown:
The controller takes care of all the web requests, i.e. GET and POST. It can also populate a model and return the appropriate view for that request.
The model contains the domain objects and logic to perform (i.e. extracting information from the repository and manipulating the data to be passed to the view).
The view returns the markup which is based upon the data stored within the model.
In certain implementations additional logic such as checking conditions and Repository calls also take place at the controller level, which is a technique known as Fat Controller Thin Model.
Edit:
You should be sending a request to the controller. I.e. In your games controller have a HTTPPost method that connects to the server and then sends the players turn info and gets the new information. For example:
[HttpPost]
public ActionResult SendPlayerTurnInformation(PlayerObject player)
{
// logic to connect to the Game Network
// connection.UpdatePlayerTurn(player);
//return success/fail
}
You could then do the same to get a specific players turn information and then update your model to be passed to the view which would contain the new information.
I was wondering what exactly is the difference between MVC(which is an architectural pattern) and an n-tier architecture for an application. I searched for it but couldn't find a simple explanation. May be I am a bit naive on MVC concepts, so if anyone can explain the difference then it would be great.
N-tier architecture usually has each layer separated by the network. I.E. the presentation layer is on some web servers, then that talks to backend app servers over the network for business logic, then that talks to a database server, again over the network, and maybe the app server also calls out to some remote services (say Authorize.net for payment processing).
MVC is a programming design pattern where different portions of code are responsible for representing the Model, View, and controller in some application. These two things are related because, for instance the Model layer may have an internal implementation that calls a database for storing and retrieving data. The controller may reside on the webserver, and remotely call appservers to retrieve data. MVC abstracts away the details of how the architecture of an app is implemented.
N-tier just refers to the physical structure of an implementation. These two are sometimes confused because an MVC design is often implemented using an N-tier architecture.
If a 3-tier design were like this:
Client <-> Middle <-> Data
the MVC patter would be:
Middle
^ |
| v
Client <- Data
Meaning that:
in the 3-tier equivalent, communication between layers is bi-directional and always passes through the Middle tier
in the MVC equivalent the communication is in unidirectional; we could say that each "layer" is updated by the one at the left and, in turn, updates the one at the right –where "left" and "right" are merely illustrative
P.S. Client would be the View and Middle the Controller
This is what say about n-tier architecture
At first glance, the three tiers may
seem similar to the MVC (Model View
Controller) concept; however,
topologically they are different. A
fundamental rule in a three-tier
architecture is the client tier never
communicates directly with the data
tier; in a three-tier model all
communication must pass through the
middleware tier. Conceptually the
three-tier architecture is linear.
However, 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.
The only similarity is that the two patterns have three boxes in their diagrams. Fundamentally they are completely different in their uses. If fact, it is not usually a choice between which pattern to use, but both patterns can be use together harmoniously. Here is a good comparison of the two: http://allthingscs.blogspot.com/2011/03/mvc-vs-3-tier-pattern.html
This diagram shows how both patterns can be used together, with MVC used solely within the Presentation/UI Tier:
Give yourself a break. And don't restrict yourself to certain patterns when solving real-world problems. Just remember some general principles, one of which is SEPARATION OF CONCERNS.
A fundamental rule in three-tier architecture is the client tier never communicates directly with the data tier; in a three-tier model all communication must pass through the middleware tier.
It’s liner architecture. This addresses the question of how to pass information between a user and a database. Where as MVC is a triangular architecture: the View sends updates to the Controller, the Controller updates the Model, and the View gets updated directly from the Model. This addresses questions of how a user interface manages the components on the screen.
#Cherry
Middle ware works more like a request handler or redirector in MVC Pattern.
I would like to explain a bit about MVC, According to me Model View Controller works like this.
Client initiates the session by requesting for any service.
This request is received and handled by Controller (Request handler, redirector etc)
Controller process a basic info on the request and redirect it to the relevant Model which can fill up the data request.
Model fill up the request according to the parameters passed by Controller and send back the results to Controller. (Note: Here i like to clear that data is not directly returned to client in true MVC architecture, rather it fills up and returned to controller.)
Controller than send that data to View(Client).
Client has the requested service in front of him.
That's all about MVC that i know.
An N-Tier architecture is best defined using a Deployment Diagram.
An MVC architecture is best defined using a Sequence Diagram.
The 2 are not the same and are not related and you can combine the two architectures together. A lot of companies have taken the steps to create N Tier'd architecture for not only deployment and scalability, but for code reuse as well.
For example, your Business Entity objects may need to be consumed by a desktop app, a web service exposed for a client, a web app, or a mobile app. Simply using an MVC approach will not help you reuse anything at all.
Besides being linear, another major difference that was not emphasized enough here is that in the N-tier model, N is not necessarily 3-tiers! It is most often implemented as three tiers (presentation, app, data) with the middle layer having two sub-tiers (business logic and data access). Also, the model in MVC can contain both data and business logic for data manipulation, whereas these would be in separate tiers in n-tier.
Conclusion : N-tier is an architecture, MVC a design pattern. They are the same metaphore applied in two different fields.
Jerry: Here's a simple example of how the two are related:
Tier 1 - Consists of Models that communicate with Tier 2 through some sort of network service or similar, controllers to handle input validation, calculations and other things relevant for the views. And it contains the views themselves, ofcourse - which can be the GUI in a desktop-app, or the web-interface in a web-app.
Tier 2 - Contains some sort of service or other way of recieving messages from Tier 1. Does not/should not know about Tier 1, so can only answer to calls from above - never ask for things by itself. Also contains all business-logic.
Tier 3 - Contains the domain model, object representation of the database and all logic to communicate and update database-entries.
N-tier architecture never communicates directly to the data access layer. In a 3-tier architecture:
Presentation layer presents the related UI,
Business layer contains related logic and
then the data access layer.
All the data communicates through the middle tier. Presentation <-> Business <-> Data.
MVC (Model-View-Controller) architecture is triangular.
The View sends updates to the controller,
The Controller updates the Model and
then the View directly gets updates from the model.
Model(Data), View(UI), Controller(Logic).
in the MVC/MVP/MVPC design pattern where do you put your business logic? No, I do not mean the ASP.NET MVC Framework (aka "Tag Soup").
Some people say you should put it in the "Controller" in MVC/MVPC or "Presenter". But, others think it should be part of the Model.
What do you think and why?
This is how I see it:
The controller is for application logic; logic which is specific to how your application wants to interact with the domain of knowledge it pertains to.
The model is for logic that is independent of the application. i.e. logic that is valid in all possible applications of the domain of knowledge it pertains to.
Hence nearly all business rules will be in the model.
I find a useful question to ask myself when I need to decide where to put some logic is "is this always true, or just for the part of the application I am currently coding?"
The way I have my ASP.NET MVC application structure the workflow looks like this:
Controller -> Services -> Repositories
The Services layer above is where all the business logic takes place. If you put your business logic in your Controller layer, you lose the ability to re-use that business logic in other controllers.
I don't believe it belongs in the controller, because once it's embedded there it can't get out.
I think MVC should have another layer injected in-between: a service layer that maps to use cases. It contains business logic, knows about units of work and transactions, and deals with model and persistence objects to accomplish its tasks.
The controller has a reference to the service that it needs to fulfill its use case. It worries about unmarshalling requests into objects the service can deal with, calls the service, and marshals the response to send back to the view.
With this arrangement, the service is usable on its own even without the controller/view pair. It can be a local or remote object, packaged and deployed any way you wish, that the controller deals with.
The controller now becomes bound more closely to the view. After all, the controller you'll use for a desktop is likely to be different than the one for a web app.
I think this design is more service-oriented.
Put your business logic in domain and keep your domain separte. I prefered Presenter -> Command (Message command use NServiceBus) -> Domain (with BC Bounded Context) -> EventStore -> Event handler -> Repository (read model). If logic is not fit in domain then use service layer.
Please read article from Martin fowler, Eric Evan, Greg Young and Udi dahan. They have define very clear picture.
Here is article written by Mark Nijhof : http://elegantcode.com/2009/11/11/cqrs-la-greg-young/
By all means, put it in the model!
Of course some of the user interaction will have to be in the view, that will be related to your app and business logic, but avoid this as much as possible. Ironically following the principle of doing as little as possible as the user is 'working' in your GUI and as much during 'submit' or action requests makes for a better user experience and usability, not the other way around. At least for line-of-business apps.
You can put it in two places. The Controller and in the Presentation layer. By having some of the logic in the Presentation layer, you can limit the number of requests back into the architecture which adds load to the system. Yeah, you have to code twice, but sometimes this is what you need for a responsive user experience.
I kinda like what was said here (http://www.martinhunter.co.nz/articles/MVPC.pdf)
"With MVPC, the presenter component of the MVP model is split into two
components: the presenter (view control logic) and controller (abstract purpose control logic)."