Re: [Xen-devel] Is continuous replication of state possible?

[Harry Butterworth <harry@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>]

On Mon, 2005-01-10 at 01:21 +0900, aq wrote:
> Hello,
> 
> I am sorry for a possible stupid question, but may anybody please
> explain to me what this "deterministic" means?

Deterministic means that the outcome is a function of nothing more than
the starting state and the input so if you start with the same starting
state and apply the same input you are guaranteed to end up with the
same outcome every time.

So, deterministic code works with the replicated state machine approach
because the replicas start off in the same state and have the same input
applied and the same execution happens on all the replicas and they end
up in the same state and so remain replicas of one another.

If you try to execute non-deterministic code in the context of the
replicated state machine approach then the replicas might start off in
the same state and receive the same input but the different results from
executing the non-deterministic code would cause them to diverge.

Once the replicas have diverged they are no longer good as backups of
one another.

Examples of operations that would ordinarily be non-deterministic:

Asking the CPU for a random number---All of the replicas start off in
the same state with the program counter pointing to an instruction that
asks for a random number; all of the replicas receive the same input: a
command instructing them to execute one instruction; all of the replicas
ask the CPU for a random number. All the replicas get a different random
number from their CPU and the subsequent execution diverges.

Taking an interrupt from a peripheral device---typically this will only
happen on one of the nodes. If it was fed directly to a replica then
that replica would immediately diverge from the others.

-- 
Harry Butterworth <harry@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx>



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