[Infrastructures] state machines

[Wesley Craig]

On 19 Sep 2006, at 17:36, Matt S Trout wrote:
> This is precisely what Steve proposes as the *one* way that you can  
> do a reliable undo.

And if you believe that (speaking practically, I certainly do),  
there's a lot more you can do.  If you're able to efficiently  
snapshot systems, capture changes, etc, then all (or maybe only most)  
the rest is not necessary.  Take, for example, the idea of installing  
Cadence.  If you knew the state of the system before Cadence was  
installed, you could capture the changes that installing Cadence made.

> This does, of course, assume that only the contents of the disk  
> ever varies between machines of the same type, and that everything  
> else (BIOS etc.) is configured identically before deployment and  
> never changed afterwards.

Sure.  In fact, "machines of the same type" is often roulette.  Those  
sorts of problems make any solution less that 100% reliable.  I know  
I've certainly swapped out a machine that was "of the same type" as a  
failed machine, only to find the small underlying differences  
impacted the services the machine was meant to provide.

:wes