[coyotos-dev] Subsystems replaceability (was Re: Impact of checkpointing on mobile systems)

[Pierre THIERRY]

Scribit Jonathan S. Shapiro dies 07/07/2007 hora 14:52:
> "I don't know how to design a general solution here, so I am going to
> put the problem off to the user".

Well, it seemed to me that there are many place where the developer
won't ever be able to design a general solution, and only the user will
have the knowledge needed to design an adequate local solution. One
example is scheduling: IIUC, you can't hope, with our current knowledge,
to design the one scheduler to rule them all.

> The plain fact is that the user is ill-equipped to make most of those
> decisions

I'd think the contrary: the most ill-equiped to anticipate what the user
will want to do with the system is its architect. I think most systems
should be designed so as to let the user replace most of their parts
without much effort.

If the architect wants to formulate guarantees about the system, he is
free, however, to formulate them on instances of the system with
constraints on what is replaced.

For example, it seems sensible not to try to verify that Coyotos is
still secure with replaced scheduling, paging and checkpointing
subsystems... But making them possible to replace is orthogonal to this
issue.

It also opens the possibility to formulate properties on the system that
are conditioned by properties of the replaced subsystem. It's high-order
programming in system design... Like asserting that the SORT function
will terminate if the predicate function implementing the strict
ordering that is given to it terminates.

> it is the job of a good architect or designer to make most of those
> decisions unnecessary in the usual case.

What about the unusual case?

High-orderly,
Pierre
-- 
nowhere.man at levallois.eu.org
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