Page History

...

3. Wrapping

The OpenMI standard was designed to allow easy migration of existing model engines. The
standard is implemented in C# running under the .NET framework. Almost all existing model
engines are implemented in other programming languages, such as Fortran, Pascal, C and
C++. In order to bridge the gap between the different technologies and to minimize the
amount of changes needed to be made to the engine core a wrapping pattern will be the most
attractive choice in most cases.
This chapter describes the process of wrapping and the generic wrapper that is provided by
the OpenMI Software Development Kit.

3.1. A general wrapping pattern

Wrapping basically means that you create a C# class that implements the
ILinkableComponent interface. This wrapper will communicate internally with your engine
core. The wrapper will appear to the users as a 'black box', which means that all
communication will take place through the ILinkebleComponent interface (Figure 5).
Image Removed
Fig. 5 OpenMI wrapping pattern
 
One further advantage of using the wrapping pattern is that you can keep the OpenMI specific implementations separated from your engine core. Typically, the engines will also be used as standalone applications where OpenMI is not used and it is naturally an advantage to be able to use the same engine in different contexts. This means that even in situations where new engines are built the wrapping pattern may still be the best choice.
 

3.2 The LinkableEngine

 Model engines that are doing timestep-based computations have many things in common. It is therefore possible to develop a generic wrapper that can be used for these engines. This wrapper is called LinkableEngine and is located in the org.OpenMI.Utilities.Wrapper package. Basically, the LinkableEngine provides a default implementation of the ILinkableComponent interface. Naturally, the LinkableEngine cannot know the specific behaviour of your model engine; this information is obtained though the IEngine interface.
The recommended design pattern for model engine migration when using the LinkableEngine is shown in Figure 6. The design includes the following classes:
 

• The MyEngineDLL class is the compiled core engine code (e.g. Fortran).
• The MyEngineDLLAccess class is responsible for translating the Win32Api from MyEngineDLL to .NET (C#).
• Calling conventions and exception handling are different for .NET and Fortran. The MyEngineDotNetAccess class ensures that these operations follow the .NET conventions.
• The MyEngineWrapper class implements the IEngine interface, which means that it can be accessed by the LinkableEngine class.
• The MyLinkableEngine class is responsible for the creation of the MyEngineWrapper class and for assigning a reference to this class to a protected field variable in the LinkableEngine class, thus enabling this class to access the MyEngineWrapper class.
   
  More details of these classes are provided in the following sections.
  The OpenMI standard puts a lot of responsibilities on the LinkableComponents. The main idea is that when the GetValues method is invoked the providing component must be able to deliver the requested values so that these apply to the requested time and the requested location. To be able to do this the LinkableComponent may have to interpolate, extrapolate or aggregate both in time and space. These and other things are handled by the LinkableEngine.
   
  The LinkableEngine class includes the following features:
   
• Buffering: When a model is running as an OpenMI component it may be queried for values that correspond to a time that is before the current time of the model. Most models will only keep values for the current timestep and the previous timestep in memory. It is therefore necessary to store data associated with the OpenMI links in a buffer. The LinkableEngine handles the buffering for you.
• Temporal interpolation and extrapolation: Most models are only capable of delivering results at times that correspond to their internal timesteps. The LinkableEngine class handles all the temporal operations that are required for LinkableComponents.
• Spatial operations: The LinkableEngine provides a range of spatial data operations.
• Link book-keeping: The LinkableEngine handles book-keeping for links added to your component.
• Event handling: The LinkableEngine sends events that enable an event-listener to monitor the progress of the linked system when running.
   
  More details about how the LinkableEngine works is given in OATC.OpenMI.SDK technical documentation.

4. Migration - step by step

...

The steps needed for migration are described in this chapter.

4.1 Step 1: Changing your engine core core

The aim of the migration is to develop a class that implements the IEngine interface. As shown in Figure 6, the class that implements the IEngine interface is supported by other classes and the engine DLL.

Fig 6. Wrapping classes and engine core DLL
 
Model engines are typically compiled into an executable file (EXE). Such executable files are not accessible by other components and as such are not very suitable as a basis for OpenMI components. It is therefore necessary for your engine to be compiled into a dynamic link library file (DLL).
Ideally you should make modifications to your engines so that the same engine can be used both when running as an OpenMI component and when running as a standalone application. Having two versions of the same engine leads to unnecessary maintenance work. Therefore you could make a new application (EXE) that calls a function in the engine core DLL which, in turn, makes your engine perform a full simulation.
Figure 7 illustrates the software required to run an engine as a standalone application. The SimpleRiverApplication.EXE file is never used when running in an OpenMI setting.
 

Fig. 7. Running an engine as a standalone application
 
The following steps are required in the conversion of the engine core:

...