Subscribed clients have a callback queue to deliver all
messages in sequence
(if no message priority is specified), so you have a virtual message
history here
if a consuming client is slow in processing or unreachable
for some time.
What happens if you have a sequence of durable, undelivered messages on the
same oid and a subscriber is unreachable? The oid = persistence key seems to
indicate that there's only one persisted message with a given oid in the
system at one time. What I'm thinking of is the sequence
Subscriber Subscribes
Sensor sends message 1
Sensor sends message 2 [Can this happen before message 1 processed?]