In our current kdepim code we use some classes throughout the codebase. I’m going to line out the problems with that and propose how we can do better.
The Application Domain
Each application has a “domain” it was created for. KOrganizer has for instance the calendar domain, and kmail the email domain, and each of those domains can be described with domain objects, which make up the domain model. The domain model of an application is essential, because it is what defines how we can represent the problems of that domain. If Korganizer didn’t have a domain model with attendees to events, we wouldn’t have any way to represent attendees internally, and thus couldn’t develop a feature based on that.
The logic implementing the functionality on top of those domain objects is the domain logic. It implements for instance what has to happen if we remove an event from a calendar, or how we can calculate all occurrences of a recurring event.
In the calendaring domain we use KCalCore to provide many of those domain objects and a large part of the domain logic. KCalCore::Event for instance, represents an event, can hold all necessary data of that event, and has the domain logic directly built-in to calculate recurrences.
Since it is a public library, it provides domain-objects and the domain-logic for all calendaring applications, which is awesome, right? Only if you use it right.
KCalCore provides additionally to the containers and the calendaring logic also serialization to the iCalendar format, which is also why it more or less tries to adhere to the iCalendar RFC, for both representation an interpretation of calendaring data. This is of course very useful for applications that deal with that, and there’s nothing particularly wrong with it. One could argue that serialization and interpretation of calendaring data should be split up, but since both is described by the same RFC I think it makes a lot of sense to keep the implementations together.
A problem arises when classes like KCalCore::Event are used as domain objects, and interface for the storage layer, and as actual storage format, which is precisely what we do in kdepim.
The problem is that we introduce very high coupling between those components/layers and by choosing a library that adheres to an RFC the whole system is even locked down by a fully grown specification. I suppose that would be fine if only one application is using the storage layer,
and that application’s sole purpose is to implement exactly that RFC and nothing else, ever. In all other cases I think it is a mistake.
The domain logic of an application has to evolve with the application by definition. The domain objects used for that are supposed to model the problem at hand precisely, in a way that a domain logic can be built that is easy to understand and evolve as requirements change. Properties that are not used by an application only hide the important bits of a domain objects, and if a new feature is added it must be possible to adjust the domain object to reflect that. By using a class like KCalCore::Event for the domain object, these adjustments become largely impossible.
The consequence is that we employ workarounds everywhere. KCalCore doesn’t provide what you need? Simply store it as “custom property”. We don’t have a class for calendars? Let’s use Akonadi::Collection with some custom attributes. Mechanisms have been designed to extend these rigid structures so we can at least work with it, but that only lead to more complex code that is ever harder to understand.
Instead we could write domain logic that expresses precisely what we need, and is easier to understand and maintain.
Zanshin for instance took the calendaring domain, and applied the GettingThingsDone (GTD) methodology to it. It takes a rather simple approach to todo’s and initially only required a description, a due date and a state. However, it introduced the notion that only “projects” can have subtodo’s. This restriction needs to be reflected in the domain model, and implemented in the domain logic.
Because there are no projects in KCalCore, it was simply defined that todo’s with a magic property “X-Project” are defined as project. There’s nothing wrong with that itself, but you don’t want to litter your code with “if (todo->hasProperty(X-Project))”. So what do you do? You create a wrapper. And that wrapper is now already your new domain object with a nice interface. Kevin fortunately realized that we can do better, and rewrote zanshin with its own custom domain objects, that simply interface with the KCalCore containers in a thin translation layer to akonadi. This made the code much clearer, and keeps those “x-property”-workarounds in one place only.
A useful approach to think about application architecture are IMO layers. It’s not a silver bullet, and shouldn’t be done too excessively I think, but in some cases layer do make a lot of sense. I suggest to think about the following layers:
- The presentation layer: Displays stuff and nothing else. This is where you expose your domain model to the UI, and where your QML sits.
- The domain layer: The core of the application. This is where all the useful magic happens.
- The data access layer: A thin translation layer between domain and storage. It makes it possible to use the same storage layer from multiple domains and to replace the storage layer without replacing all the rest.
- The storage layer: The layer that persists the domain model. Akonadi.
By keeping these layer’s in mind we can do a better job at keeping the coupling at a reasonable level, allowing individual components to change as required.
The presentation layer is required in any case if we want to move to QML. With QML we can no longer have half of the domain logic in the UI code, and most of the domain model should probably be exposed as a model that is directly accessible by QML.
The data access layer is where akonadi provides a standardized interface for all data, so multiple applications can shared the same storage layer. This is currently made up by the i.e. KCalCore for calendars, the akonadi client API, and a couple of akonadi objects, such as Akonadi::Item and Akonadi::Collection. As this layer defines what data can be accessed by all applications, it needs to be flexible and likely has to be evolved frequently.
The way forward
For akonadi’s client API, aka the data access layer, I plan on defining a set of interfaces for things like calendars, events, mailfolders, emails, etc. This should eventually replace KCalCore, KContacts and friends from being the canonical interface to the data.
Applications should eventually move to their own domain logic implementation. For reading and structuring data, models are IMO a suitable tool, and if we design them right this will also pave the way for QML interfaces. Of course i.e. KCalCore still has its uses for its calendaring routines, or as a serialization library to create iTip messages, but we should IMO stop using it for everything. The same of course applies to KContacts.
What we still could do IMO, is share some domain logic between applications, including some domain objects. A KDEPIM::Domain::Contact could be used across applications, just like KContact::Adressee was. This keeps different applications from implementing the same logic, but of course also introduces coupling between those again.
What IMO has to stay separate is the data access layer, which implements an interface to the storage layer, and that doesn’t necessarily conform to the domain layer (you could i.e. store “Blog posts” as notes in storage). This separation is IMO useful, as I expect the application domain to evolve separately from what actual storage backends provide (see zanshin).
This is of course quite a chunk of work, that won’t happen at once. But need to know now where we want to end up in a couple of years, if we intend to ever get there.
One thought on “On Domain Models and Layers in kdepim”
maybe you already know this design approach, if not hopefully it is useful: http://alistair.cockburn.us/Hexagonal+architecture
IMHO building complex software with this approach paired with domain driven design makes building and especially maintenance a lot more pleasure