Re: [MirageOS-devel] Lifecycle

[Richard Mortier <richard.mortier@xxxxxxxxxxxx>]

On 19 July 2016 at 21:05, Daniel Bünzli <daniel.buenzli@xxxxxxxxxxxx> wrote:
> Le mardi, 19 juillet 2016 à 21:13, Richard Mortier a écrit :
>> Sure, in principle -- I was wondering about a concrete example,
>> particularly given what I understand intended / common uses of
>> unikernels to be, ie., in micro-service / serverless environments.
>
> Personally I'm more interested in embedded systems which seems to be missing 
> from this list.

Oh, that's true, I forgot about them :)

> But in any case, a simple, concrete example, would be to simply log the 
> information. In case of problems this can give you more information about 
> what happened to the unikernel: stopped vs stopped in order to be rebooted.
>

Hm-- I can certainly imagine passing a log message in with the `Stop
might be useful (though even then, maybe for those that cared,
supporting a -- presumably authenticated -- "log this" message might
be better?). I guess I interpreted "hint" as something richer than
that, to be used to trigger some other behaviour.

> A more hypothetical case would be to try to persist some of the operating 
> unikernel state; e.g. related to network but Mindy who just sent a message 
> seems to say NO. Nevertheless, depending on how fast you are able too boot it 
> could be used for pseudo-hot swapping a unikernel with a new implementation — 
> you don't reboot with the same unikernel but one that is able to reuse the 
> persisted state and act as if nothing happened — outsiders will only see that 
> as the network being slow; but meanwhile you will have upgraded you 
> unikernel. High availability bla bla, a buzzword there must be or I can 
> invent one if needed.
>
> So if you have the hint you can have unikernels that provide both Mindy reset 
> semantics on stop/start and hot swap/keep state through stop with restart 
> hint/start. Though it can be argued there could be other means to achieve 
> that like sending a command to the unikernel before poweroff.
>

Yeah-- rather than mandate something like this as an API that *all*
unikernels (Mirage and potentially others) must support, I think I'd
rather see those unikernels that care implementing their own
mechanisms to explicitly do things like persist state. It seems likely
to be more pertinent to whatever the particular case being handled is.
I can imagine that most unikernels might usefully respond to `Start
and `Stop, but I'm not sure what else though.

(On the basis that only two things in life are certain -- three if you
could the implicit `Birth -- then perhaps there's a `Taxes equivalent.
Some kind of "gimme some statistics about your operation" message
perhaps? :)


-- 
Richard Mortier
richard.mortier@xxxxxxxxxxxx

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