Thoughts on Modular WCF RIA Services


I had an interesting question tweeted to me yesterday from Ariel Ben Horesh requesting my opinion on sharing data models between Prism modules using WCF RIA Services. My opinion is that there is a 1:1 relationship between an editable DomainService/DomainContext and a module. My reasoning is that both a module and a DomainContext are a single unit of work. I would further suggest that is you have a module that requires two separate editable DomainContexts that you actually have two separate modules.

Notice that I keep saying “editable DomainContexts”. If you have a DomainContext which has only read only entities then I see no problem with putting that DomainContext in a shared library referenced by multiple modules. You can even put in ExternalReferences from your modules DomainContext to the shared DomainContext if you really want. I usually discourage the use of ExternalReference as I feel that people misuse the functionality to attach two editable DomainContexts to each other but I don’t see a problem with a non-editable DomainContext.

Ariel then asked how events/commands could be used between the modules using entity data when the modules are not sharing data and, after I gave my answers, asked for some examples. In response, I created this Sample Project which creates several pseudo-modules. It is not using MEF or Prism to create actual modules, just standard WCF RIA Services Class Libraries which are representing modules.

One module, ParentModule, has a model which contains only the Parent entity. ParentModule represents the primary module of an application, the one that works with the “root” table of a database. The Parent entity has a related Child entity which has a ChildTypeID. The other two modules (ChildHeightModule and ChildWidthModule) represent modules that modify only one type of Child entity. One of the Child modules has the Parent entity in its model and the other does not.

Communicating by Key

The primary way of communicating between modules is to pass primary keys. For example, if the application has a Parent entity and wants the Child modules to load the Child entities of that Parent then it simply needs to pass the ParentID to the two child modules which can then easily load data based on the ParentID.

Communicating by Interface

Sometimes just passing keys around isn’t good enough and you need access to entire entities between different modules. The solution to that is to implement shared interfaces on your entities. If you check my sample project you will find that I implement several interfaces using generics. Using the shared entities I could pass an entire Parent entity from the ParentModule to either of the Child modules as an IParent. The limitation of the interface versions of the entities is that they cannot interact with the local module’s DomainContext (i.e. you can’t set the Parent property of the Child to the IParent entity).

I use this technique extensively and it really helps when created shared code to do operations on the different entities. For example, both the ChildWidth and ChildHeight entities implement the IChildDimension interface. If I wanted to create an IValueConverter that operated on objects that implement IChildDimension that is a really easy thing to do. I could also pretty easily have a module that gets sent all  of the IChildDimension entities from all currently loaded modules and displays them all in a grid.

Communicating by Cloning

The third option is to use the WCF RIA Services Contrib ApplyState/ExtractState extensions to clone the entity between the DomainContexts of two different modules. Both ParentModule and ChildWidthModule have the Parent entity. They are different types in different namespaces but they both have the same DataMember properties. This means that the IDictionary<string,object> extracted from Parent in the ParentModule can be applied to a Parent entity in the ChildWidthModule. This is not recommended. Extracting the state from one module and applying it in another is inherently brittle and can create a maintenance nightmare. However, I am sure that someone will hit a situation where cloning is the only solution that works for them so I include it here for that person.

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