[net2-wg] Proposal for dissemination structure and interfaces

[Philip Levis]

On Dec 17, 2005, at 6:03 PM, Kyle Jamieson wrote:

> This looks like a very well thought-out design.  I have a few small
> questions as follows.
>
>> interface DisseminationValue <t> {
>>   command const t* get();
>>   event void changed();
>> }
>
> How about simplifying this down to
>
> interface DisseminationValue <t> {
>   event void changed(const t*);
> }

The one issue here is that it requires the component above to store  
the pointer in its frame. So let's say there's some configuration  
parameter. Every component that wants to be able to access the  
configuration parameter needs to store a pointer to it, which wastes  
RAM.

> This is a nit-pick, but why not two configs, say DisseminatorC
> and DisseminationC, which each provide interface DisseminationUpdate
> and interface DisseminationValue, respectively?  I guess the
> motivation behind this is clarity for users.  Or, is there perhaps a
> situation where an app might use both DisseminationValue and
> DisseminationUpdate? (I can't think of one..)

This is an interesting point. There is a little trickiness in  
managing the keyspace, but I think it could be possible. I don't have  
a good reason why the two should be coupled, except it might be less  
bug-prone in terms of keys. But it could also be less clear. I'm on  
the fence.

> Question: is the reason for this ease of implementation?  Or is there
> something in the interface that precludes large objects?  (I can't see
> anything).
>

More so than ease of implementation, I think the goal is simplicity  
of implementation. There are existing dissemination systems for many  
different data object sizes, and they could all be put into a single  
dissemination stack, but that would involve a lot of code. That is,  
the reasoning isn't that implementing larger objects would be hard,  
it might just waste code space and require more RAM. nesC's dead code  
elimination would have trouble at this granularity, as it's only  
based on call graphs and is flow/context insensitive.


> Just to clarify, you're saying that a dissemination version number is
> an (app version number, AM addr) pair?

I'm not sure what you mean by app version number.

Phil


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