connectedness & coordination

Larry, John-

I've been having a discussion with Chip and and struggling with a  
topic that I've heard discussed but doesn't seem to be written down  
anywhere (nor can I find a note that indicates someone is writing  
about it).

The question is about how connectivity is represented and how  
connectivity constraints are represented and managed.

Consider, for example, a researcher who desires to use a collection  
of co-located servers and storage devices attached to a common  
switch.  How and by whom is the switch configured?  If the current  
state of the switch, say because of support for other experiments,  
prevents interconnection between some components, how is that  
represented and resolved?

I have pieces of two possible answers: First, the switch may be  
considered a component and its configuration is the responsibility of  
the experimenter.  If so, there needs to be a way to express the  
desired connectivity and constraints within RSpecs or some other  
way.  (I recall discussion of the need for a connectivity graph of  
some sort.  Is that right?  Where does such a thing live?  How is it  
accessed?)

Alternatively, an aggregate controller could be used to configure the  
switch and select connectable components.   This makes the resource  
allocation problem the controller's, which makes sense since the  
controller would presumably also be handling the resource reservations.

But what about the case when there are connectivity constraints that  
cross aggregates? (Or, there are no aggregates?)  For example,  
consider an experiment requiring components are distributed across  
the GENI system.  There may be multiple, diverse networking resources  
available connecting subsets of the facility.  How does a researcher  
discover whether or not "he can get there from here."

Finally, once an experiment has been established (thinking of long-
running, perhaps popular services), how does one add resources into  
an existing, operational experiment?  This seems harder than  
upgrading a production-grade live service: resources like link  
bandwidth may become unmanageably fragmented and few operating  
systems handle graceful addition of, say, network interfaces (even if  
it's only virtual hardware) without interruption.

--aaron

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