Application databases and external integration points
Dave has an interesting requirements in his project:
We're not in control of where the data is located, how it's stored and in what configuration. In most cases employees need to be retrieved from a Active Directory (There's is no 'login', the Window Identity determines what a user can or can't do). Customer contacts are usually handled by the helpdesk department and each contact moment is logged in a helpdesk database. The customer (account information) itself often needs to be retrieved from an IBM DB2 database.
What you have is not one application that needs to access different data sources. That would be the wrong way to think about this, because it introduce a whole lot of complexity into the application.
It is much better to structure the application as an independent application with each integration point made explicit. Instead of touch the DB/2 database, you put a service on it and access that.
This isn’t just “oh, SOA nonsense again”, it is an important distinction. When you tie yourself directly to so many external integration points, you are also ensuring that whenever there is a change in one of them, you are going to be impacted. When you put a service boundary between you and the integration point (even if you have to build the service), the affect is much less noticeable.
Also, did you notice the blue lines going from the databases? Those are background ETL processes, replicating data to/from the databases. It allows us to handle situations where the integration points are not available.
In short, design you application so it doesn’t stick its nose into other people’s databases. If you need data from another database, put a service there, or replicate it. You’ll thank me when you app stays up.