Doing DCI with ASP.NET MVC

The DCI - Data, Context, Interaction - paradigm (intro article, an earlier post) brings the roles domain objects play at runtime to center stage. DCI also fits right into the gaps of MVC. What? Which gaps?

MVC Gaps
Back in the day, when MVC was originally described by Trygve Reenskaug as an architecture approach it was all about the users mental model, giving the user the sense of just working directly with the domain objects. Now dont confuse this with the dreaded CRUDy forms-over-data battleship grey enterprise app. Those apps force the user to manipulate the data directly; respecting, fighting and being forced into submission by the techniocal details of databases. MVC on the other hands puts that the model is the users mental model - the programmer, then has to deal with hiding the technicalities of persistence and such. As illustrated below the users works as if manipulating the domain objects directly. The view simply reflects the model state, the controller reacts to user actions, and translates them to model updates, and that's it.

Direct manipulation metphor as illustrated by Trygve Reenskaug

This is fantastic for fairly simple apps. But as soon we introduce business rules with a certain amount of complexity, the direct manipulation metaphor does not suffice anymore. We need somewhere to put those business rules.  -The controller already has clear responsibility, so in keeping with single responsibility principle that's not the place for the behaviour. -The traditional OO answer is to put the behaviour into the model, but that leads to hard-to-follow logic jumping back and forth in the model objects, and (gark) up and down long inheritance chains. Moreover it turns out that a given domain object will play different roles over times. I.e. the object doesn't need just one behaviour, it needs several, and even worse it needs different behaviours over time. There goes SRP again, only this time in the "intelligent model".

Roles and  Contexts to Fill the Gap
So we need something more than MVC: The view is not the place for the behaviour, the model is not the place for the bahivour, and the controller is not the place for it either. This is where roles as separate things come into play. We want explicit roles. We want those business rules implemented in the roles. And we want to assing those roles to domain objects as needed.

The role a given object plays at a given moment depends on the context at that moment. So why not introduce contexts as something explict along with the explicit roles? Together roles implementing behaviour and contexts assigning roles to objects fill the gap in MVC.

Back to APS.NET MVC
Lets get practical: Where do the contexts and the roles go then? Since the contexts manage assigning roles and kicking off the behaviuour the contexts and roles fit well together. In the ASP.NET MVC + DCI demo I did for ANUG I organized the code like this:

 The Contexts folder has a subfolder for the use case I demoed. Other use cases would have their own subfolders. Inside the subfolder is the context for executing that use case, and the associated roles.

What is a role in C#? A role in C# is a set of extension methods on an interface (see this and that for more on roles in C#):

    1     public static class TransferMoneySourceTrait

    2     {

    3         public static void TransferTo(this TransferMoneySource self, TransferMoneySink recipient, decimal amount)

    4         {

    5             // The implementation of the use case

    6             if (self.Balance < amount)

    7             {

    8                 throw new ApplicationException(

    9                     "insufficient funds");

   10             }

   11 

   12             self.Withdraw(amount);

   13             self.Log("Withdrawing " + amount);

   14             recipient.Deposit(amount);

   15             recipient.Log("Depositing " + amount);

   16         }

   17     }

How does the context track the role assignments? Simple:

    8     public class TransferMoneyContext

    9     {

   10         public TransferMoneySource Source { get; private set; }

   11         public TransferMoneySink Sink { get; private set; }

   12         public decimal Amount { get; private set; }

   13 

   14         public TransferMoneyContext(TransferMoneySource source, TransferMoneySink sink, decimal amount)

   15         {

   16             Source = source;

   17             Sink = sink;

   18             Amount = amount;

   19         }

   20 

   21         public void Execute()

   22         {

   23             Source.TransferTo(Sink, Amount);

   24         }

   25     }

   26 

Where are the objects mapped to roles? Right in the seam between the controller and the context (lines 35 to 37):

   11     public class TransferMoneyController : Controller

   12     {

   13         AccountRepository accountRepo = new AccountRepository();

   14 

   15         public ActionResult Index()

   16         {

   17 

   18             ViewData["SourceAccounts"] =

   19                 accountRepo.Accounts.Select(a => new SelectListItem {Text = a.Name, Value = a.Id.ToString()});

   20             ViewData["DestinationAccounts"] =

   21                 accountRepo.Accounts.Select(a => new SelectListItem {Text = a.Name, Value = a.Id.ToString()});

   22             return View();

   23         }

   24 

   25         [AcceptVerbs(HttpVerbs.Post)]

   26         public ActionResult Index(FormCollection form)

   27         {

   28             var sourceAccountId = form["SourceAccounts"];

   29             var destinationAccountId = form["DestinationAccounts"];

   30             var amount = form["Amount"];

   31 

   32             var sourceAccount = accountRepo.GetById(int.Parse(sourceAccountId));

   33             var destinationAccount = accountRepo.GetById(int.Parse(destinationAccountId));

   34 

   35             new TransferMoneyContext(sourceAccount as TransferMoneySource,

   36                 destinationAccount as TransferMoneySink,

   37                 decimal.Parse(amount)).Execute();

   38 

   39             ViewData["Amount"] = amount;

   40             ViewData["Source"] = sourceAccount.Name;

   41             ViewData["Destination"] = destinationAccount.Name;

   42 

   43             return View("Result");

   44         }

   45     }

Apart from lines 35 to 37 this is all just standard ASP.NET MVC stuff: Getting data in and out of the ViewData, and returning the view to render back to the user.

Summing Up
MVC is great for simple situations. It doesn't quite scale to complex business scenarios, but DCI does, and the two are a great match.
Leveraging the power of C# to do DCI and the niceness of ASP.NET MVC to go along with it, makes for a - IMHO -very compelling architecte style for an enterprisy web app.

Options for DCI on .NET

Here's a short summary of the technical/language options for doing DCI on the .NET platform. The list below is loosely ordered in order of increasing "mainstreamness" - according to me :-).
The list notes the language, how roles and contexts are handled and how the solution is run on .NET.

• Scala "cross-compiled" to MSIL/CLR:
  In Scala contexts are simply objects instantiated at runtime from context classes. The contexts have the responsibility of instantiating data objects and mapping roles to them. Roles are traits that are mixed into data objects at instantiation time. So typically the contexts receive ids for data objects, pull the corresponding data from e.g. a database, and instantiate the data object with a role trait mixed in. In Scala this is all strongly typed and looks like this:
  
  

  class MoneyTransferContext(sourceAccountId: int,
                              sinkAccountId: int){
     val source =
          new SavingsAccount(sourceAccountId) with TransferMoneySource
     val sink =
           new CheckingAccount(sinkAccountId) with TransferMoneySink
     def execute = {
       source.deposit(100000)
       source.transferTo(200, sink)
       }
  }

  
  To run this in a .NET setting the Scala code must "cross-compiled" to MSIL, and must use the .NET standard library along with the Scala APIs. For details on this see my earlier post, or the introduction on the Scala language web site.


• Python running on IronPython:
  In Python contexts are objects, that take the responsibility of mixing roles into data objects dynamically. The data objects can be instantiated outside the context or inside the context depending on taste. When the context is done the roles can be yanked back out of the data objects. I dont know a whole lot of Python, so I wont go into details, but refer you to Serge Beaumonts sample.
  To run this on .NET use IronPython.
• Ruby running on IronRuby:
  In Ruby contexts are also objects, that take the responsibility of mixing roles into data objects dynamically. Data objects can be instantiated inside the context or outside the context. Roles in Ruby are implemented as modules, which using the Ruby standard library can be mixed into any Ruby object. The Ruby code looks like this:
  

  class TransferMoneyContext
     attr_reader :source_account, :destination_account, :amount
     include ContextAccessor
     
     def self.execute(amt, source_account_id, destination_account_id)
       TransferMoneyContext.new(amt, source_account_id,
                                destination_account_id).execute
     end
   
     def initialize(amt, source_account_id, destination_account_id)
       @source_account = Account.find(source_account_id)
       @source_account.extend MoneySource
       @destination_account = Account.find(destination_account_id)
       @amount = amt
     end
  
     def execute
       in_context do
         source_account.transfer_out
       end
     end

  end

  
  To run this on .NET just use IronRuby. It runs fine out of the box.


• C# and Dynamic:
  Using the dynamic features of C#, contexts are objects that receive either data objects or IDs of data objects. In case of an ID the context instantiates the data object and populates it based on the ID and some data source. In both cases the data object has a role reference which will point to an expando object, into which the roles methods are injected. The code looks roughly like this:
  

  dynamic transfer = new ExpandoObject();
   transfer.Transfer = (Action)
    ((source, sink, amount) =>{ 
      dynamic mSource = source;
      dynamic mSink = sink;
      mSource.DecreaseBalance(amount);
      mSink.IncreaseBalance(amount);});
   Account sourceAcct = new Account { CashBalance = 100m };
   sourceAcct.Role = transfer;
   Account sinkAcct = new Account { CashBalance = 50m };
   sourceAcct.Role.Transfer(sourceAcct,sinkAcct,10m);

  Running on .NET? Well, it's C#, so just do it.


• C# and extention methods:
  Using C# extension methods for roles contexts are objects that receive either data objects or IDs of data objects. In either case the context maps the data objects to roles. Roles are implemented as static classes containing extension methods for role interfaces. Data objects capable of playing a given role implement that role interface. The code looks like this:
  
  public class TransferMoneyContext { public TransferMoneySource Source { get; private set; } public TransferMoneySink Sink { get; private set; } public decimal Amount { get; private set; } public TransferMoneyContext(TransferMoneySource source, TransferMoneySink sink, decimal amount) { Source = source; Sink = sink; Amount = amount; } public void Execute() { Source.TransferTo(Sink, Amount); } } public interface TransferMoneySink { void Deposit(decimal amount); void Log(string message); } public interface TransferMoneySource { decimal Balance { get; } void Withdraw(decimal amount); void Log(string message); } public static class TransferMoneySourceTrait { public static void TransferTo(this TransferMoneySource self, TransferMoneySink recipient, decimal amount) { // The implementation of the use case if (self.Balance < amount { throw new ApplicationException("insufficient funds"); } self.Withdraw(amount); self.Log("Withdrawing " + amount); recipient.Deposit(amount); recipient.Log("Depositing " + amount); } }
  
  How to run this on .NET is ... well ... pretty obvious :-)

RHoK'ing for Unicef

I was at the RHoK 2.0 Aarhus event this weekend, and got the opportunity to do a simple little web app for Unicef. The app itself is not that interesting for people outside of Unicef, but it will save real money for Unicef that they can use on something else...like helping children. The app is something like 80% done, and we did it in about 28 hours. Wow, it feels great to have been able to actually help Unicef just by doing a saturday to sunday hackathon along with a few other geeks.

The app was done in ASP.NET MVC 2, with Enitity Framework 4 and SQL Express underneath, and the experience was pretty good: The architecture just came out of the box with MVC. EF code first made thinking about the database schema a non-issue (and the load on the app is expected to be low, so I don't think will need to go in a tweak for performance later). We were able to get by with some of the auto generated CRUD views that Visual Studio can make for ASP.NET MVC model objects. All in all the development was pretty quick.
The code is on google code.