Using the Pub/Sub model with NSB, the following two scenarios seemingly cause the dead-letter queue to fill up, eventually resulting in a "Insufficient resources" error.
1) Publishing an event type that has no subscribers 2) Subscriber is offline
For our purposes we are not interested in historical events when the subscriber starts up, so the incoming queue is purged on startup. Events published while the subscriber is offline fill up the dead-letter queue, however.
Have i misunderstood the command vs. event? This is the behaviour i was expecting from Commands, but expected events to disappear if not subscribed to.
When using NServiceBus, events are considered just as important as commands, and thus are subject to the same guarantees regarding durability, delivery, etc.
So, if your subscriber does not care about events when it is offline, it could unsubscribe before shutting down - this way, it's an explicit decision made by your subscriber that it does not care about what happens when it's not around to hear it... just make sure that it doesn't get confused or chokes somehow if there's a few (old) events lying in its input queue when it comes back online later on, because stuff might get published in the time between the unsubscribe message is sent and it gets to the publisher.
Another option is to supply the [TimeToBeReceived(...)] attribute on your event messages, but that should only be used if it can be safely determined that the event contents lose their relevance after a fixed time for all subscribers.