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If the leader of the stateful processors group generates an optimal, host-aware task assignment to processors within the JobModel, each follower will pick up their appropriate assignments from the JobModel and begin the processing after the rebalance phase(similar to non-stateful jobs). The goal is to guarantee that the task assignment to processors is optimal and minimizing the tasks movement between the processors. 

A new abstraction LocationIdProvider is introduced as a part of this change to generate locationId for a physical execution environment. Here’re few reasons for introducing a new abstraction to generate locationId rather than using processorID as locationId:

• LocationId denotes the physical execution environment required to run a stream processor. LocationId is used to uniquely identify a environment amongst all available physical execution environments. ProcessorId is used to uniquely identify a stream processor in a processors group. ProcessorId and localityId are two different, logically orthogonal concepts which cannot be unified.
• Standalone model supports running multiple stream processors from a single JVM on a physical host. If a stream processor(P1) running a physical host(H) dies, it’s optimal to redistribute the tasks of the dead processor(P1) to the other processors running on the host(H). If processorId is used as localityId, this optimal generation cannot be achieved(since task to localityId association is not maintained).

ZK Data Model to support host affinity:

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