PAPER 20 Scribe and Siena are both attempting to provide the same
functionality: the ability for nodes to dynamically create a variety
of groups, and receive information sent to those groups.

Scribe is another tree-based multicast scheme, although it allows any
node with the appropriate credentials to broadcast to the multicast
group.  Scribe is built on top of Pastry, and it takes advantage of
the underlying structure Pastry provides.  Scribe creates multicast
groups by assigning each group a groupID in the address space,
designating the node with the address closest to the groupID as the
rendez-vous node for the group, and building a content distribution
tree out from that node.  Because of the nature of Pastry routing, the
content distribution tree may include nodes which are not part of the
multicast group; if node A has to route through node B to reach the
rendez-vous node, then node B becomes part of the distribution tree.
Scribe does not store information about all the nodes in the multicast
group at the rendez-vous node.  Instead, that information is stored
locally.

Scribe scales well, and supports multiple multicast groups easily.
However, it relies on the underlying Pastry network to provide
locality.  It may also route multicast messages through nodes which
are not interested in those messages.  While the number of hops to
route between nodes in the tree is O(log(n)), it will still increase
the latency in delivering messages to the multicast group, and use up
more bandwidth than is strictly necessary.

Siena is a much more general design for a publish/subscribe system.
Within the paper, several possible designs are discussed, and the
underlying topology is left as a fairly general acyclic graph.  Within
this graph, filters are applied as close to the source as possible,
which will help minimize the bandwidth used. Siena could be run on top
of either a peer-to-peer or a hierarchical network, although the
authors argue that a peer-to-peer network is better under most
circumstances, and the experimental results suggest that this is true
for larger systems.

-- Victoria Krafft

