Information Technology Reference
In-Depth Information
O
128
- 1
d
471
f
1
d
467
c
4
d
462b
a
d
46
a
1
c
d
4213
f
Route (
d
46
a
1
c
)
d
13d
a
3
65
a
1
fc
Figure 1.16
Routing a message from node 65
a
1
fc
with key d46a1c. The dots depict live nodes in Pastry's
circular namespace. (From Castro, M., Druschel, P., Kermarrec, A.-M., and Rowstron, A.,
Proceedings of SIGOPS European Workshop, France, September 2002. With permission.)
1.3.1.3 Applications of Pastry
Scribe [34] is a large-scale event notification network based on Pastry. In
Scribe, the publisher publishes a
topicId
to the network. The subscribers route
to the topicId to subscribe for the topic. The Scribe then combines each route
back to the subscribers to get an ALM tree. When the publisher updates topic
related content, the content will be delivered through the multicast tree to
the subscribers. Some of the nodes in the multicast tree do not subscribe to
the topic for themselves—they serve as relays forwarding the content to the
subscribers.
SplitStream employs interior-node-disjoint multicast trees for streaming. In
most tree-based streaming protocols, few interior nodes within the multicast
tree are in charge of forwarding the streaming content. This not only imposes
a disproportionate load on those nodes, but also makes the streaming vulner-
able to node failures. SplitStream divides the streaming into
k
different stripes,
and organizes nodes into
k
or so-called interior-node-disjoint multicast trees.
Participating nodes are interior nodes in one of the
k
trees, and leaf nodes in the
rest of
k
-1 trees. SplitStream uses Scribe and Pastry to build each multicast tree.
For each tree, a stripeId with a different starting digit is assigned so that the
participating nodes are automatically placed into leaf or interior nodes within
each tree. Since there are
k
trees, different error correction coding can be applied
to SplitStream to increase the robustness of the streaming (see Figure 1.17).
The disadvantages of SplitStream are that it assumes that nodes uninter-
ested in the streaming are still willing to be relays to help forward streaming
content to other nodes. Additionally, there will be synchronization problems
when applying error correction coding to different
k
trees.
