- create hash table to save ric_req_id (key) and array of ran_param_id(s) (values), per each subscription
  - create RB tree to store list of ric_req_id(s) for each ran_param_id
    => when the async event occurs, it is easier and faster to search per ran_param_id and send the indication message to all xApps (ric_req_id(s)) subscribed to the same ran_param_id
  - it is important to mention that both data structures need to be maintained, especially when unsubscription occurs (free_aperiodic_subscription)