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Re: [xmlblaster] Cluster peers
Sounds to be a good idea to base the scenario on the proven OSPF
Michael Lum wrote:
IMO, we would have to look at this using a 'dynamic routing' scenario.
If we explicitly pinned slaves and masters, when one of them dies, we
have the potential for message loss. In other words, if bilbo is only
connected to saroman, and bilbo dies, messages published to bilbo will
be queued until saroman is up, UNLESS you can reconfigure bilbo on the
fly to re-route messages via frodo. It's similar to OSPF routing where
links have a certain cost, and you can 'prefer' links via a metric value.
In a HA scenario, if saroman died, fail-safe subscribers would reconnect
via the VIP to sauron to resubscribe to their topics (or regain their
session if session mirroring is implemented). Next, if frodo died,
publishers would now be publishing to bilbo since the load balancer will
re-route when the health-check fails. However, if bilbo is pinned to
saroman, messages will simply re-queue. Instead, bilbo should have two
route costs, such as 'saroman 10' and 'sauron 100', meaning that bilbo
prefers saroman for cluster delivery, but will route to sauron if the
link is down. This way, messages don't simply queue (and possibly
overflow) on bilbo. Also, if the hardware for saroman was unrecoverable
and would take 3 days to replace, one could run on the degraded cluster
without need for a reconfig, because the route costs would take care of
that for you, until you could purchase new hardware.
Finally, adding a new host 'gandalf' would be pretty simple because you
simply need to express your routing costs for its message domains.
On 03/02/05 00:10, Marcel Ruff wrote:
i have put your drawing online so others can view
it as well (it got bounced by the mailing list):
Probably the 'crossing arrows' between
frodo - saroman
bilbo - sauron
is too complicated. In such a case the messages need to convey
the information that they have reached the other master
already with the direct connection frodo->sauron and
bilbo->saroman and don't need to be mirrored anymore.
And further, it is questionable if bilbo or frodo should
have knowledge about the backend master cluster setup.
This cluster setup could change to 3 or more mirrored master nodes
or sauron could go for maintenance and another mirror
node 'gandalf' pops up.
Conceptually it could be more wise to only mirror on
master level sauron<-->saroman, what do you think?