  This paper analyzes the adoption of unstructured P2P overlay networks to
build publish-subscribe systems. We consider a very simple distributed
communication protocol, based on gossip and on the local knowledge each node
has about subscriptions made by its neighbours. In particular, upon reception
(or generation) of a novel event, a node sends it to those neighbours whose
subscriptions match that event. Moreover, the node gossips the event to its
"non-interested" neighbours, so that the event can be spread through the
overlay. A mathematical analysis is provided to estimate the number of nodes
receiving the event, based on the network topology, the amount of subscribers
and the gossip probability. These outcomes are compared to those obtained via
simulation. Results show even when the amount of subscribers represents a very
small (yet non-negligible) portion of network nodes, by tuning the gossip
probability the event can percolate through the overlay. Hence, the use of
unstructured networks. coupled with simple dissemination protocols, represents
a viable approach to build peer-to-peer publish-subscribe applications.
