[Openais] cpg join/leave lists

[Steven Dake]

On Mon, 2009-07-13 at 11:13 -0500, David Teigland wrote:
> I've been told a number of times recently that the join and left lists
> provided by cpg confchg callbacks cannot really be believed.  Steve says some
> objections prevented him from removing them altogether.
> 

Hmm I don't recall saying they are unreliable or cannot be believed.
The problem with the join and left list is they don't follow the
semantics of virtual synchrony, meaning there is no consistent view of
these join/left lists on each node.  The calculations that generate them
assume a node-local view instead of a global view.

So they are not reliable for making distributed decisions because they
don't follow VS, not because they don't generate proper output.

If they dont generate node-local proper output, then that is a bug and
we should address it.

> I don't care much because figuring out join/left lists is trivial for apps by
> comparing consecutive member lists. (Except in the case of the initial join
> confchg where there's no previous member list.  That's the one case where a
> reliable, consistent join list would be extremely helpful, even with an
> exception for partition merges, oh well.)
> 
> What I'd like to suggest is that whatever the behavior is, it be clearly
> defined and stated, instead of undefined and misleading.  We can then test and
> verify that behavior.  What we have now might be the worst case, which is: it
> sort of works like you'd expect, most of the time, except in cases where it
> doesn't, which are complicated or unknown or both.
> 

I agree a clear definition of what join/leave should mean to the user of
the api would be helpful in the man pages and for api verification.

> The join/leave lists are one of the more prominent characteristics of the api
> we tell everyone to use.  The fact that it's undefined and unreliable isn't
> good.
> 
> Dave
> 
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