Masstransit message structure...turtles (interfaces) all the way down

[Shane Holder]

It has been firmly ingrained in me that messages should be interfaces, but how about the content of a messages should it only contain interfaces?

For example an Order which implements IOrder has order lines.  Should the order lines be an interface of IOrderLine or is it OK to provide just use a class of OrderLine?

public interface IOrder {

  List<IOrderLines> Lines {get;set;}

}

or

public interface IOrder {

  List<OrderLines> Lines {get;set;}

}

Thanks,

Shane

[Chris Patterson]

It depends, of course, on your goals. In our application, most (not all, we have many developers and not everyone does it the same -- but we try) of the message contracts are entirely interface-only data types.

For instance, List<T> is mutable, so we use T[] (arrays). And we typically drop the IXxx convention since they are behavior-free data bags, it just makes it cleaner and easier to read.

So we have:

public interface Order {

    OrderLine[] Lines {get;}

}

public interface OrderLine {

    string ItemId {get;}

    string Description {get;}

    decimal UnitPrice {get;}

}

Notice that there are no {set;} methods on the properties, that would allow a consumer to modify the message content.

In a large application (easily 100+ decoupled services, coordinated through Courier routing slips, with over 200 consumers for various events and state machines) this has worked very well.

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[Shane Holder]

Turtles all the way down it is then, thank you so much Chris.

Shane

[Travis Smith]

Shane, the major thought to keep in mind is that a message is an immutable contract; behaviour must exist elsewhere & interfaces are behaviour free to enforce that. I've deployed apps where behaviour has snuck into the messages and it always end up biting us somewhere in a really subtle way. 

-Travis

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[Chris Patterson]

Yeah, it's usually right on the ass. At 4am. On a Sunday. Over a holiday weekend.