Cardinality rules
Cardinality rules for the Legacy Market Data API include the following:
Consumers can only have one active subscription per destination key
This ensures that the reference count the publisher keeps is correct.
Each subscription must be paired with an unsubscribe
Any subscription that is sent must be paired with an unsubscription at some point. Note that often this means that a monitor must normally send unsubscribe events from its on die action, if and only if it had active subscriptions.
