[Maas-devel] RFC: "Serialising" power actions

[Julian Edwards]

On Wednesday 17 Sep 2014 09:38:52 Gavin Panella wrote:
> That's a good point. However, a reboot can be modelled as a single power
> "task". If a power-off is issued subsequently, then I think it should be
> allowed to override the reboot, and transition the machine to a
> powered-off state. If a power-on is issued, we can say that it's not
> permitted to override the in-progress power task. In other words, where
> the final state of an in-progress power task is the same as a subsequent
> power task, the subsequent power task should be discarded.

I agree, but your case above doesn't meet these requirements.  (ie final state 
of a reboot is "power on", so you can't allow a power-off to override it)

> Cancellation is possible. For example, an IPMI power-on command looks
> something like the following:
> 
>     send an IPMI command to power on the node
>     wait a bit
>     is it up? Yes -> we're done
>     send an IPMI command to power on the node
>     wait a bit
>     is it up? Yes -> we're done
>     send an IPMI command to power on the node
>     wait a bit
>     is it up? Yes -> we're done; no -> that's an error
> 
> There are many opportunities to stop in the process above.

You can stop it but you will have no idea what in state the node will end up.  
Did the operation work but you cancelled it before the final ack?  Or did it 
get cancelled early enough to actually stop it?

Cancelling is inherently dangerous because you've no idea if it worked or not.

J