I'm experimenting with an event notification service based on a publish-subscribe model for some projects I'm working on. When a subscription comes in, a 'Subscription' object is made, that basically looks like this: class Subscription(Base): def __init__(self, subscriber, eventType, filter=None): self.subscriber = subscriber self.eventType = eventType self.filter = filter def __hash__(self): return hash(self.subscriber) & \ hash(self.eventType) & \ hash(self.filter) 'subscriber' is a reference to the subscribing object, and it's very likely to be to an object in the ZODB. Is it wise to have more than one persistent reference to a single persistent object? I swear that I had once heard Jim say (vocally) that you could do references like this in the ZODB now. I'm trying to avoid using Paths because objects have a tendency to move around[*], and I have performance concerns for a single event service object to have to call 'unrestrictedTraverse' to every subscriber. [*] (it's due to the annoyances with manage_beforeDelete() and friends that I'm writing this tool.) Second question: If I use the hash of the Subscription as a key, is there any advantages/disadvantages with using an IOBTree to hold Subscription objects instead of a PersistentMapping? Jeffrey P Shell, jeffrey@cuemedia.com