Unit tests and the Layers Pattern (part 2)

I have used the Layers Pattern on my last project. This was a OPC UA client for data acquisition module in an Industrial Continuous Data Acquisition suite of software. This suite of applications was already using a proprietary plug-in framework to allow integration of different analyzers. The idea was to take the generic approach one step further and use a standard technology (OPC UA) for interfacing to analyzers.
On this project OPC UA can be seen as providing the low-level data access module. Using this the program could read/write parameters synchronously or use a subscription mechanism to get notifications when parameters were updated. Starting the project I also faced the problem of not having much documentation available and not much in terms of sample code. It turned out that the only source of significant code example was the C# framework. For a Java application this is a problem. While I started the project thinking that I would use an ANSI C library and JNI, I ended up having to use JNI, C++/CLI and a C# framework. In the end I had something like the following layers for my new module:

============================================
 Interface (Java) :             (Unit tests)
======================================|=====
 Logical (Java)   :              Unit tests
======================================|=====
 Low-level access :                   |
......................................|.....
            Java  :                   |
......................................|.....
   JNI (ANSI C++) :                   |
......................................|.....
          C++/CLI :                   |
......................................V.....
             C#   :              Unit tests     
============================================

You can see that the low-level layer contains four sub-layer. One for each technology involved. Each of these sub-layers is fairly simple except for the C# sub-layer. This is because of the need to support the subscription mechanism. The diagram above also shows how the unit tests are distributed. Some tests are restricted to a layer (horizontal) and some tests span all layers (vertically).
Next time we will look more closely at each layers and also at how the unit tests are structured in more details.