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• be applied per KafkaPrincipal as it's a smaller subset than which is known to the administrator of the cluster. It will not be applied to ClientId (which is not enforced by the client config) or nor a combination of KafkaPrincipal and ClientId. 
• keep a cache of user (KafkaPrincipal) to unique active PIDs to track active PIDs. The cache will be implemented using a simple bloom filter controlled by time to avoid any unwanted growth that might cause OOM. (More details on this is explained in the next section) 
• add rating metrics which will increment if caching layer doesn't contain the PID. And user will be throttled once we reach the allowed quota. 

Cache Active PIDs per KafkaPrincipal

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