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An advantage of approach #2 is that it's easier to transactionalize updates to the PdpGroup along with the list of waiting PDPs. One downside is the extra amount of data that must be retrieved when a PdpGroup is extracted from the DB.
One complicating factor of #2 is that the same class, PdpGroup is used as both the data store structure and the PAP REST API structure. If the same structure were to continue to be used, then the extra data fields would have to be handled (e.g., discarded on input from REST). On the other hand, it could provide a mechanism for clients of the REST API to determine the actual PDP deployment status of each policy.