  Advertisements and subscriptions are tightly coupled to generate publication
routing paths in content--based publish/subscribe systems. Tight coupling
requires instantaneous updates in routing tables to generate alternative paths
which prevents offering scalable and robust dynamic routing in cyclic overlays
when link congestion is detected. We propose, OctopiA, first distributed
publish/subscribe system for content--based inter--cluster dynamic routing
using purpose--built structured cyclic overlays. OctopiA uses a novel concept
of subscription subgrouping, which divides subscriptions into disjoint sets
called subscription subgroups. The purpose--built structured cyclic overlay is
divided into identical clusters where subscriptions in each subgroup are
broadcast to an exclusive cluster. Our advertisement and subscription
forwarding algorithms use subscription subgrouping to eliminate tight coupling
to offer inter--cluster dynamic routing without requiring updates in routing
tables. Experiments on a cluster testbed with real world data show that OctopiA
reduces the number of saved advertisements in routing tables by 93%,
subscription broadcast delay by 33%, static and dynamic publication delivery
delays by 25% and 54%, respectively.
