Use case for double opt-in and other mechanisms for preventingunwanted subscriptions

[David Hull <dmh@tibco.com>]

The Use Case:

Because subscriptions may be made by a third party on behalf of the actual consumer, there must be some means of ensuring that the consumer only receives notifications it is interested in.  There are many possible relationships among the subscriber, producer and consumer.  For example

• The subscriber is provably the same entity as the consumer.  The producer should accept any subscription from the consumer on its own behalf.
• The subscriber, producer and consumer are all in the same isolated environment and implicitly trust each other.  Again, there is no need to restrict subscription.
• The consumer has supplied the subscriber with a secure token (which the NP is able to recognize) authorizing it to subscribe on the consumer's behalf.  The NP should reject subscription requests without proper authorization.
• The consumer does not know the subscriber even exists, but might be interested in some unsolicited subscriptions.  It is thus up to the producer to determine interest, generally by sending a test message under the double-opt-in pattern.
• Either of the previous cases may apply.  The producer should look for the appropriate secure token, and if it's missing, ask the consumer via double-opt-in.
• A notification producer may impose a quota on subscriptions directed toward a given consumer (perhaps because the consumer asked it to).  In this case, a given subscription may either succeed or fail depending on what other subscriptions are open.
  

Clearly, many more variants are possible.

Discussion:

In cases where the producer must query the consumer before beginning the subscription, arbitrarily much time may pass between the subscription request and the definitive answer.  This asynchronous reply would best be handled through a callback mechanism, but we would probably rather not build this into the core subscribe exchange.  In the case of secure tokens, it might make sense for the subscriber to be able to submit and verify a token for a particular consumer once (in the context of a secure connection) instead of passing it with every subscribe request.

It would be desirable to push all such message exchanges out of the core Subscribe request/response.  This is one driver behind having the subscriber and producer be able to first negotiate a "destination" cookie and then use that cookie in the actual subscribe request.  Naturally, this is not the only way to cover these use cases.