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Can you currently use a single node in a 3-node cluster to manage the network? (I am certain the answer is no, but was requested to get confirmation)

asked 2016-02-05 12:53:15 -0700

DeJuan gravatar image

updated 2016-02-17 11:30:13 -0700

I've seen at and many other locations that if you are running 3 node clustering and two nodes go down, clustering operations are suspended and the controller is unusable. In my understanding, this means that if two nodes go down, the remaining single node will not function and is incapable of managing the network. I've been discussing this with a colleague and they would like to know the exact definition of "cluster operations are suspended"; they believe that if two nodes go down, the last node could still potentially manage the network and just write everything to its local copy of the store, but with no ability to replicate that data. Could you please verify if this is in in fact possible, or if not, please explain in a very clear, undeniably obvious way exactly what the suspension means?

The explanation I gave was that when using HA clustering, everything is transaction based and the leader needs to forward data to the replicas and receive confirmation that at least one replica committed the transaction before it can be committed as actually having occurred. This process is in accordance with the three-phase transaction model ODL is stated to follow. With two nodes down, further transactions cannot possibly be committed as having occurred because there are no replicas to communicate with. If we cannot commit transactions, we cannot process state changes or new information. If we can't process incoming information, we can no longer manage the network. The discussion here is on whether or not ODL currently supports in Lithium the ability for that last cluster node to manage the network alone without replication. It would be a similar case to the 2-Node cluster, which I don't believe is supported in Lithium as the page for that explicitly notes it's a design proposal but doesn't actually mention that it's been implemented, but with 3-Node clustering's final functioning node.

If I am correct, an answer simply affirming that my reasoning is correct would be sufficient closure on this issue.

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answered 2016-03-09 05:40:03 -0700

DeJuan gravatar image

This was confirmed to be the case by an ODL team member at an event. In addition, 2-Node clustering, though the proposal is visible on the wiki, was never actually implemented and is not present in Beryllium; it may be included in Boron, but that remains uncertain.

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Asked: 2016-02-05 12:53:15 -0700

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Last updated: Mar 09 '16