Favoring some transitions when rebalancing in full_auto mode

[Matthieu Morel]

Hi,

In FULL_AUTO mode, helix computes both partitioning and states.

In a leader-replica model, I observed that when rebalancing due to a failure of 
the Leader node, Helix does not promote an existing replica to leader, but 
instead assigns a new leader (I.e. going from offline to replica to leader).

For quick failover, we need to have the replica promoted to leader instead. Is 
there a way to do so in FULL_AUTO mode?

Apparently with SEMI_AUTO that would be possible, but it would imply we control 
the partitioning, and we'd prefer Helix to control that as well.

I tried to play with the priorities in the definition of the state model, with 
no luck so far.

(See the example below for an example of how rebalancing currently takes place)

Thanks!

Matthieu


Here we have a deployment with 3 nodes, 3 partitions and 2 desired states, 
Leader and Replica (and offline).

// initial states

"mapFields":{
    "MY_RESOURCE_0":{
      "instance_1":"REPLICA"
      ,"instance_2":"LEADER"
    }
    ,"MY_RESOURCE_1":{
      "instance_0":"REPLICA"
      ,"instance_1":"LEADER"
    }
    ,"MY_RESOURCE_2":{
      "instance_0":"LEADER"
      ,"instance_2":"REPLICA"  // Instance2 is replica
    }
  }
}


// instance 0 dies

"mapFields":{
    "MY_RESOURCE_0":{
      "instance_1":"REPLICA"
      ,"instance_2":"LEADER"
    }
    ,"MY_RESOURCE_1":{
      "instance_1":"LEADER"
      ,"instance_2":"REPLICA"
    }
    ,"MY_RESOURCE_2":{
      "instance_1":"LEADER" // Helix preferred to assign leadership of resource 
2 to instance 1 rather than promoting instance_2 from replica to leader
      ,"instance_2":"REPLICA" // instance 2 is still replica for resource 2
    }
  }
}