Enhance cluster management features in S4

In S4 the number of partition is fixed for all streams and is dependent on the number of nodes in the cluster. Adding new nodes to S4 cluster causes the number of partitions to change. This results in lot of data movement. For example if there are 4 nodes and you add another node then nearly all keys will be remapped which result is huge data movement where as ideally only 20% of the data should move.

By using Helix, every stream can be partitioned differently and independent of the number of nodes. Helix distributes the partitions evenly among the nodes. When new nodes are added, partitions can be migrated to new nodes without changing the number of partitions and minimizes the data movement.

In S4 handles failures by having stand by nodes that are idle most of the time and become active when a node fails. Even though this works, its not ideal in terms of efficient hardware usage since the stand by nodes are idle most of the time. This also increases the fail over time since the PE state has to be transfered to only one node.

Helix allows S4 to have Active and Standby nodes at a partition level so that all nodes can be active but some partitions will be Active and some in stand by mode. When a node fails, the partitions that were Active on that node will be evenly distributed among the remaining nodes. This provides automatic load balancing and also improves fail over time, since PE state can be transfered to multiple nodes in parallel.

I have a prototype implementation here https://github.com/kishoreg/incubator-s4

Instructions to build it and try it out are in the Readme.