Petter Måhlén's Blog

A bit more than a week ago, my brother sent me a couple of links about DCI. The vision paper, a presentation by James Coplien and some other stuff. There were immediately some things that rang true to me, but there were also things that felt weird – in that way that means that either I or the person who wrote it has missed something. Since then, I’ve spent a fair amount of time trying to understand as much as I can of the thoughts behind DCI. This post, which probably won’t make a lot of sense unless you’ve read some of the above materials, is a summary of what I learned and what I think is incomplete about the picture of DCI that is painted today.

The Good

I immediately liked the separation of data, context and roles (roles are not part of the acronym, but are stressed much more than interactions in the materials I’ve found). It was the sort of concept that matches your intuitive thinking, but that you haven’t been able to phrase, or perhaps not even grasped that there was a concept there to formulate. I have often felt that people attach too much logic to objects that should be dumb (probably in an attempt to avoid anemic domain models, something I, apparently like Mr Coplien, have not quite understood why it is such a horror). Thinking of objects as either containing data or playing some more active role in a context makes great sense, and makes it easier to figure out which logic should go where. And I second the opinion that algorithms shouldn’t be scattered in a million places – at least in the sense that at a given level of abstraction, you should be able to get an overview of the whole algorithm in some single place.

Another thing I think is great is looking at the mapping between objects in the system and things in the user’s mental model. That mapping should be intuitive in order to make it very easy to understand what to do in order to achieve a certain result. Of course, in any system, there are a lot of objects that are completely uninteresting for – and hopefully invisible to – end users, but very central to another kind of software users: programmers. Thinking about DCI has made me think that it is central both to have a good mapping between the end user’s mental model and the objects that s/he needs to interact with and the programmer’s mental model and the objects that s/he needs to interact with and understand. And DCI talks about dumb data objects, which I think is right from the perspective of creating an intuitive mapping. I think most people tend to think of some things as more passive and others as active. So a football can bounce, but it won’t do so of its own accord. It will only move if it is kicked by a player, an actor. It probably won’t even fall unless pulled downwards by gravity, another active object. It’s not exactly dumb, because bouncing is pretty complicated, it just doesn’t do much unless nudged, and certainly not in terms of coordinating the actions of other objects around it. The passive objects are the Data in DCI, and the active objects are the algorithms that coordinate and manipulate the passive objects.

The notion of having a separate context for each use case, and ‘casting’ objects to play certain roles in that context, feels like another really good idea. What seems to happen in most of the systems I’ve seen is that you do some wiring of objects either based on the application scope or, in a web service, occasionally the request scope. Then you don’t get more fine-grained with how you wire up your object graph, so typically, there are conditional paths of logic within the various objects that allow them to behave differently based on user input. So for every incoming request, say, you have the exact same object graph that needs to be able to handle it differently based on the country the user is in, whether he has opted to receive email notifications, etc. This is far less flexible and testable than tailoring your object graph to the use case at hand, connecting exactly the set of objects that are needed to handle it, and leaving everything else out.

The Not Great

The thing that most clearly must be broken in the way that James Coplien presents DCI is the use in some languages of static class composition. That is, at compile time, each domain object declares which roles it can play. This doesn’t scale to large applications or systems of applications because it creates a tight coupling between layers that should be separated. Coplien talks about the need for separating things that change rarely from those that change often when he discusses ‘shear layers’ in architecture, but doesn’t seem to notice that the static class composition actually breaks that. You want to have relatively stable foundations at the bottom layers of your systems, with a low rate of change, and locate the faster-changing bits higher up where they can be changed in isolation from other parts of the system. Static class composition means that if two parts of the system share a domain class, and that class needs to play a new role in one, the class will have to change in the other as well. This leads to a high rate of change in the shared code, which is problematic. Dynamic class composition using things like traits in Scala or mixins feels like a more promising approach.

But anyway, is class composition/role injection really needed? In the examples that are used to present DCI – the spell checker and the money transfer – it feels like they are attaching logic to the wrong thing. To me, as a programmer with a mental model of how things happen in the system, it feels wrong to have a spell checking method in a text field, or a file, or a document, or a selected region of text. And it feels quite unintuitive that a source account should be responsible for a money transfer. Spell checking is done by a spell checker, of course, and that spell checker can process text in a text field, a file, a document or a selected region. A spell checker is an active or acting object, an object that encodes an algorithm, as opposed to a passive one. Similarly, an account doesn’t transfer money from itself to another, or from another to itself. That is done by something else (a money transferrer? probably more like a money transaction handler), again an active object. So while I see the point of roles and contexts, I don’t see the point of making a dumb, passive, data object play a role that is cut out for an active, smart object.

My Thoughts

First, I haven’t yet understood how Dependency Injection, DI, is ‘one of the blind men that only see a part of the elephant’. To me, possibly still being blind, it seems like DI can work just as well as class composition. Here’s what the money transfer example from the DCI presentation at QCon in London this June looks like with Java and DI:

interface SourceAccount {...}

interface DestinationAccount {...}

class SavingsAccount implements SourceAccount, DestinationAccount {...}

class PhoneBill implements DestinationAccount {...}

public class MoneyTransactionHandler {
   private final GUI gui;

   public MoneyTransactionHandler(GUI gui) {
       this.gui = gui;
   }

   public void transfer(Currency amount,
                        SourceAccount source,
                        DestinationAccount destination) {
      beginTransaction();

      if (source.balance() < amount) {
         throw new InsufficientFundsException();
      }

      source.decreaseBalance(amount);
      destination.increaseBalance(amount);
      source.updateLog("Transfer out", new Date(), amount);
      destination.updateLog("Transfer in", new Date(), amount);

      gui.displayScreen(SUCCESS_DEPOSIT_SCREEN);

      endTransaction();
   }
}

This code is just as readable and reusable as the code in the example, as far as I can tell. I don’t see the need for class composition/role injection where you force a role that doesn’t naturally fit it upon an object.

Something that resonates well with some thoughts I’ve had recently around how we should think about the way we process requests is using contexts as DI scopes. At Shopzilla, every page that is rendered is composed of the results of maybe 15-20 service invocations. These are processed in parallel, and rendering of the page starts as soon as service results for the top of the page are available. That is great, but there are problems with the way that we’re dealing with this. All the data that is needed to render parts of the page (we call them pods) needs to be passed as method parameters as pods are typically singleton-scoped. In order to get a consistent pod interface, this means that the pods take a number of kitchen sink objects – essentially maps containing all of the data that was submitted as a part of the URL, anything in the HTTP session, the results of all the service invocations, and things that are results of logic that was previously executed while processing the request. This means that everything has access to everything else and that we get occasional strange side effects, that things are harder to test than they should be, etc. If we would look at the different use cases as different DI scopes, we could instead wire up the right object graph to deal with the use case at hand and fire away, each object getting exactly the data it needs, no more, no less. It seems like it’s easy to get stuck on the standard DI scopes of ‘application’, ‘request’, and so on. Inventing your own DI scopes and wiring them up on the fly seems like a promising avenue, although it probably comes with some risk in terms of potentially making the wiring logic overly complex.

In some of the material produced by DCI advocates, there is a strong stress of the separation between ‘object-oriented’ and ‘class-oriented’ programming, and class-oriented is bad. This is a distinction I don’t get. Or, the distinction I see feels too trivial to be made into a big deal. Of course objects are instances of a class, and of course the system’s runtime state in terms of objects and their relationships is the thing that makes it work or not. But who ever said anything different? There must be something that I’m missing here, so I will have to keep digging at it to figure out what it is.

DCI + DI

I think the spell-checker example that James Coplien uses in his presentations can be used in a great way to show how DCI and DI together make a harmonious combo. If you need to do spell checking, you probably need to be able to handle different languages. So you could have a spell checker for English, one for French and one for Swedish. Exactly which one you would choose for a given file, text field, document or region of text is probably something that you can decide in many ways: based on the language of the document or text region, based on some application setting, or based on the output of a language detector. A very naive implementation would make the spell checker a singleton and let it figure out which language to use. This ties the spell checker tightly to application-specific things such as configuration settings, document metadata, etc., and obviously doesn’t aid code reuse. A less naive solution might take a language code as a parameter to the singleton spell checker, and let the application figure out what language to pass. This becomes problematic when you realise that you may want to spell check English (US) differently from English (UK) and therefore language isn’t enough, you need country as well – in the real world, there are often many additional parameters like this to an API. By making the spell checker object capable of selecting the particular logic for doing spell checking, it has been tied to application-specific concerns. Its API will frequently need to change when application-level use cases change, so it becomes hard to reuse the spell-checker code.

A nicer approach is to have the three spell checkers as separate objects, and let a use-case-specific context select which one to use (I’m not sure if the context should figure out about the language or if that should be a parameter to the context – probably, contexts should be simple, but I can definitely think of cases where you might want some logic there). All the spell checker needs is some text. It doesn’t care about whether it came from a file or a selected region of text, and it doesn’t care what language it is in. All it does is check the spelling according to the rules it knows and return the result. For this to work, there needs to be a few application-specific things: the trigger for a spell checking use case, the context in which the use case executes (the casting or wiring up of the actors to play different roles), and the code for the use case itself. In this case, the ‘use case’ code probably is something like:

public class SpellCheckingUseCase {
   private final Text text;
   private final SpellChecker spellChecker;
   private final Reporter reporter;

   // constructor injection ...

   public void execute() {
      SpellCheckingResult result = spellChecker.check(text);
      reporter.display(result);
   }
}

The reporter role could be played by something that knows how to render squiggly red lines in a document or print out a list of misspelled words to a file or whatever.

In terms of layering and reuse, this is nice. The contexts are use-case specific and therefore at the very top layer. They can reuse the same use case execution code, but the use case code is probably too specific to be shared between applications. The spell checkers, on the other hand, are completely generic and can easily be reused in other applications, as is the probably very fundamental concept of a text. The Text class and the spell checkers are unlikely to change frequently because language doesn’t change that much. The use case code can be expected to change, and the contexts that trigger the use case will probably change very frequently – need to check spelling when a document is first opened? Triggered by a button click? A whole document? A region, or just the last word that was entered? Use different spell checkers for different parts of a document?

I think this is a great example of how DCI helps you get the right code structure. So I like DCI on the whole – but I still don’t get the whole classes vs objects discussion and why it is such a great thing to do role injection. Let the algorithm-encoding objects be objects in their own right instead.