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Dynamic Replica Placement for
Scalable Content Delivery
Yan Chen, Randy H. Katz,
John D. Kubiatowicz
{yanchen, randy, kubitron}@CS.Berkeley.EDU
EECS Department
UC Berkeley
Motivation Scenario
data
source
replica
data plane
cache
always update
adaptive
coherence
Web content
server
CDN server
client
network plane
Goal and Challenges
Provide content distribution to clients with good Quality of
Service (QoS) while retaining efficient and balanced resource
consumption of the underlying infrastructure
• Dynamic choice of number and location of replicas
– Clients’ QoS constraints
– Servers’ capacity constraints
• Efficient update dissemination
– Delay
– Bandwidth consumption
• Scalability: millions of objects, clients and servers
• No global network topology knowledge
Previous Work (Replica Placement)
• Focused on static replica placement
– Clients’ distributions and access patterns known in
advance
– Assume global IP network topology
• Data Location via DNS-redirection
– Highly inefficient (this is a “hack”)
– Centralized CDN name server cannot record replica
locations
Previous Work (Info Dissemination)
• No inter-domain IP multicast
• Application-level multicast (ALM) unscalable
– Root maintains states for all children (Narada,
Overcast, ALMI, RMX)
– Root handles all “join” requests (Bayeux)
– Root split is common solution, but suffers
consistency overhead
Solutions for Dissemination Tree
• Peer-to-Peer Overlay Location Services
with Good Scalability & Locality
• Simultaneous Replica Placement and Tree
Construction
Peer-to-peer Routing and Location
Services
• Properties Needed by Tree Building Algorithms
– Distributed, scalable location with guaranteed
success
– Search with locality
• P2P Routing and Location Services: Tapestry
– CAN, Chord, Pastry insufficient locality or
flexibility to place objects
• Http://www.cs.berkeley.edu/~ravenben/tapestry
Simultaneous Replica Placement and
Tree Construction
• Static Replica Placement + IP Multicast
– Modeled as a global optimization problem
– Design a greedy algorithm with logN approximation
– Optimal case for comparison
• Dynamic Replica Placement + Application-level Multicast
– Search for qualified local replicas first
– Place new replicas on Tapestry overlay path
– Two approaches: naïve and smart
• Soft-state Tree Maintenance
– Each node only maintains states for its parent and direct children
Dynamic Replica Placement: naïve
data plane
parent candidate
proxy
s
c
Tapestry mesh
Tapestry overlay path
network plane
Dynamic Replica Placement: naïve
data plane
parent candidate
proxy
s
c
Tapestry mesh
Tapestry overlay path
network plane
first placement choice
Dynamic Replica Placement: smart
• Aggressive search
• Greedy load distribution
data plane
parent candidates
client child
proxy
c
s
parent
sibling
server child
network plane
Tapestry overlay path
Dynamic Replica Placement: smart
• Aggressive search
• Lazy placement
• Greedy load distribution
data plane
parent candidates
client child
proxy
c
s
parent
sibling
server child
network plane
Tapestry overlay path
first placement choice
Evaluation Methodology
• Network Topology
– 5000-node network with GT-ITM transit-stub model
– 500 d-tree server nodes, 4500 clients join in random
order
• Dissemination Tree Server Deployment
– Random d-tree
– Backbone d-tree (choose backbone routers and subnet
gateways first)
• Constraints
– 50 ms latency bound and 200 clients/server load bound
Four Approaches for Comparison
• Overlay Dynamic Naïve Placement
(dynamic_naïve)
• Overlay Dynamic Smart Placement
(dynamic_smart)
• Static Placement on Overlay Network
(overlay_static)
• Static Placement on IP Network (IP_static)
Number of Replicas Deployed
and Load Distribution
• Overlay_smart uses much less replicas than
overlay_naïve and very close to IP_static
• Overlay_smart has better load distribution than
od_naïve, overlay_static and very close to IP_static
Multicast
Performance
• 85% of overlay_smart Relative Delay Penalty (RDP)
less than 4
• Bandwidth consumed by overlay_smart is very close
to IP_static and much less than overlay_naive
Tree Construction Traffic
Including “join” requests, “ping” messages, replica
placement and parent/child registration
• Overlay_smart consumes three to four times of traffic
than overlay_naïve, and the traffic of overlay_naïve is
quite close to IP_static
• Far less frequent event than update dissemination
Conclusions
• Peer-to-peer networks can be used to construct CDNs
• Dissemination Tree: dynamic Content Distribution
Network with good QoS, efficiency and load balancing
– P2P location service to improve scalability and locality
– Simultaneous dynamic replica placement and tree construction
• In particular
– Use Tapestry to contact nearby region of tree to select parent
– Lazy placement of new replicas on Tapestry overlay path
– Close to optimal number of replicas, good load distribution, low
multicast delay and bandwidth penalty at the price of reasonable
construction traffic
Future Work
• Evaluate with more diverse topologies and real
workload
• Dynamic replica deletion/migration to adapt to
the shift of users’ interests
• Implementation for OceanStore, a global-scale
persistent data storage system