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• be applied per KafkaPrincipal as it's a smaller subset which is known to the administrator of the cluster. It will not be applied to ClientId (which is not enforced by the client config) 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. 

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Caching layer to track active PIDs per KafkaPrincipal

The cache will be represented as a map of KafkaPrincipal to timed controlled bloom filter. The lifecycle of a user's PIDs in the bloom filter in the caching layer will be as the following:

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