DDD Anti-Pattern #3: Not Taking Bounded Contexts Seriously

My team had the challenge of building a field service automation application that would run on Windows Mobile devices. Availability of the Compact Framework meant that most code that would run on the server would also run on the device. This meant that we could build one domain object model and deploy it on the server and on the client. But in Jurassic Park fashion, we were so preoccupied with the fact that we could, we didn’t spend enough time asking if we should.

The Advantage We Got

We used the Repository pattern and built our domain model out of plain old CLR objects (POCOs). This meant that we could have the same rich domain model both on the client where the user is interacting through a UI and on the sever where changes from the field and from host system integrations are being processed. This allowed us to keep our code DRY. There was no repetition among the entities, behaviors, relationships between entities, and tests. We never had to worry about model inconsistencies between the client and the server. It seemed like a big win, and in some ways it was.

The Price We Paid

As we built out the product, we saw that the object model’s usage patterns were not the same on the client as on the server. We ended up with behavior that only applied on the client or on the server.

In an effort to keep from putting code into our domain that would not be relevant on both the client and server, we started putting logic into little services that work with the domain. Most of this logic was on the client side and got applied through use of the Event Aggregator pattern. This worked, but tended to impoverish the domain model itself.

Fixing It

The problem largely fixed itself when Windows Mobile became irrelevant almost overnight. We rebuilt the client in JavaScript that runs on most modern browsers. This gave us a do-over opportunity and removed the option of sharing model code between the server and client.

If I had the Windows Mobile situation to do over again, I would be more careful to define a common structure that the client and server could share, but compose behaviors relevant to the client and server within each of those contexts.