[Pacemaker] "Follow and then push off"

[Lars Marowsky-Bree]

Hi,

sitting in the Open-HA tutorial, they implemented their support for
replicated resources in an interesting fashion; not via a complex m/s
resource type, but they mapped it to dependencies.

They have a distinction between weak (optional) and strong collocation
dependencies (similar to score="0" and score="infinity" in our world).

Imagine we have two resources, DB_Active and DB_Replica. DB_Active is
assigned higher priority than DB_Replica. One would want to have a)
DB_Active and DB_Replica to be running on different nodes, of course;
and b) if DB_Active fails, it should move to the node where DB_Replica
was running.

To achieve a), they create a strong negative collocation dependency from
DB_Replica to DB_Active (implying that DB_Replica must not run on the
same node as DB_Active).

For b), they create a weak positive collocation dependency from
DB_Active to DB_Replica (implying that DB_Active should be placed on the
node where DB_Replica is running, if it exists).


In case of fail-over, this causes DB_Active to be moved to the node
where DB_Replica is still running, and then DB_Replica is stopped
because it mustn't be running there. Basically a "follow and push off"
strategy.

I sort of like the approach, and wonder if we can do something similar?


Regards,
    Lars

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