[Pacemaker] ignoring transitional memberships

[Andrew Beekhof]

On Fri, Feb 26, 2010 at 7:55 PM, Alan Jones <falancluster at gmail.com> wrote:
> On Fri, Feb 26, 2010 at 2:57 AM, Andrew Beekhof <andrew at beekhof.net> wrote:
>>
>> Not only safe but essential, otherwise B would kill A for no reason.
>> Transitional memberships are just corosync's way of saying "i;m still
>> working, but this is what I have so far".
>
> The best explanation I've found for the transitional membership is in:
> "Group Communication Specifications: A Comprehensive Study" 1999
> Vitenberg/Chockler/Keidar/Dolev
>
> It relates to support for positionable memberships.  In Pacemaker it might
> be the case that for B to
> become DC and modify the CIB without A, B would have to STONITH A.  If that
> is the case, then when
> A gets a new membership including B he can assume that B has not modified
> the CIB without him.
>
> However, I'm not convinced that he can assume that B has received all the
> same messages during
> the transition.


He doesn't have to.
The CIB contains version information that it uses to resolve conflicts
and ensure everyone is up-to-date.

The only time you'd have trouble is if you made different changes to
both A and B while they couldn't see each other.
And that would be a strange thing to do.

In that case, the cluster picks one and archives the other.


> If Pacemaker clobbers B's CIB after a new membership then
> it doesn't matter.
>>
>> > How is the CIB replicated?
>> Thats not related to the above question is it?
>>
> It is.  I'm trying to understand when and how copies of the CIB are
> synchronized.  To that end
> I've created a simplified graph of crmd's finite state automata using
> graphviz which others might
> find useful (attached).  I'll try to break this question up:
> 1. When the CIB is updated, are the none DC copies updated by coping the
> entire CIB or with a message that represents only the change?
> 2. Is the update sent to each copy separately or using corosync's multicast?
> 3. When a new DC is elected, what steps are taken to assure that it's copy
> of the CIB is up-to-date?
> For example, a new member that has not finished incorporating the CIB should
> not be eligible for the role of DC; how is this accomplished.
>
> Alan
>
>
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