[Bacnet-developers] Timing Considerations and Message Handling Architecture

[Christoph Z. <cz...@rs...>]

Hi Everybody,

first of all thanks for helping me out on my previous request. Currently I Am designing
a higher-level C++ layer for better abstracting the access to the BACNet network, from
a client's perspective. The goal is to e.g. let the BACNet stacks' 'receive and handle' 
block run assynchronously within a separate thread. All requests and responses will
be stored in two FIFOs. 

Concerning this architecture I would like to ask the following questions:
1) What was the reason for designing the BACNet stack with a callback mechanism 
for handling responses? E.g. why wasn't the npdu_handler(...) designed to 'just' decode
the BACNet protocol and then update some receive message buffers, which can
be simply read directly after npdu_handler(...) had been called successfully?

2) Are there any real-time constraints for the client, server and/or both sides of the
communication?

3) Are there any other not-considered BACNet-related properties which might
render the intended higher-level architecture 'useless'?

To better understand the architecture here is a fictive sample of a user-application:
//Init: Sets up all handlers, data link, address cache etc.
bacnetStackBase::BACNetConnection connection(...)

//Let the connection run asynchronously as a thread.
connection.run()

//Begin the main routine
//E.g. Here a user-application can handle RPCs from another system, GUI, etc.
//NB: The keepRunning flag is a variable, which can be set to false via the
//      appropriate CTRL-C, etc handlers.
while( keepRunning )
{
    //Issue a read property request 
    connection.send( ReadPropertyRequest( ... ), ... )

    //Check the length of the currently enqueued active responses.
    if( connection.activeResponses() )
    {
        //If the invoke IDs match and the response type is the same as
        //the request type the ReadPropertyRequest(...) has been answered.
        //NB: In case of an error an appropriate state flag will be set in the response.
        ...
    }

     //Now here the user-application can interact with a GUI, etc.
}

//After the above loop has been quit the application should be gracefully
//closed.
connection.close()

---------------------------------- END OF PSEUDO-CODE

Thank You in advance for Your help.


Mit freundlichen Grüßen,
Christoph Zach

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