Messaging vs. RPC: It starts at design-time


Chris Kinsman comments:
I have been looking at both but I have yet to see a compelling treatise that indicates why/how a non-RPC model makes sense. Folks talk about it but I have yet to see .NET code that doesn’t just give lip service to messaging but instead does something with it that can’t be done in the RPC model.


IMO, the difference between messaging and RPC starts at design time. When designing a distributed app using RPC, you design classes, interfaces and object interactions. When designing a messaging app you focus on the message structure (schema). Objects, methods and serialization/deserialization all become implementation conveniences rather than how you design your system. (I’m sure Chris remembers me preaching message-based design in the workshops we did together :-))
I have personally been involved in three large Web services projects that started out with RPC design and quickly ran into problems. In all three projects, the message structure was important (either for interop or because they were implementing a pre-existing contract). All three projects had to go back and start from message schema.
On the second point: Part of the reason there’s little or no .NET code that shows messaging is that asmx exposes an RPC-based programming model and the tools (VS .NET) makes it really easy to start with classes and methods and ignore message schema.

Comments (2)

  1. Chris Kinsman says:

    Your last sentence nails it. You have to go out of your way to go the message route in .NET 1.0. Hence most folks go the RPC route.

  2. I completely agree with you here, Yasser.

    Too many people get locked into RPC-centric designs way too soon in a development project.

    <p>

    That is one of the reason I have recently been doing some presentations on architecture styles for enterprise apps, to start to get people thinking beyond RPC, and consider the interaction style choice up-front.

    See:

    http://www.thearchitect.co.uk/weblog/archives/2003/03/000100.html

    and

    http://www.thearchitect.co.uk/weblog/archives/2003/02/000055.html

    for details of the presentations.

    <p>

    I also agree on the lack of tools support for message-focused design, although we at Cape Clear are trying to do our bit to foster this by offering our WSDL Editor free to the world, and I know many .NET developers use this for many of the reasons you highlight above.

    See

    http://www.capeclear.com/news/pressroom/reports/free_wsdl_editor.htm for some details of the WSDL editor, and http://www.capescience.com/downloads/wsdleditor/index.shtml for the free download.

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