Keynote talk at the 2nd International Workshop on Distributed Event-Based Systems (DEBS 2003), 8 June 2003.

1. Some Open Problems in
Publish/Subscribe Networking
David S. Rosenblum
Chief Technology Officer
PreCache Inc.

2. Acknowledgments
 Alexander L. Wolf
 University of Colorado at Boulder
 Antonio Carzaniga
 University of Colorado at Boulder
 PreCache Engineering Team

3. Background

4. Information-Centric
Internet Applications
 Software and Antivirus Updates
 Consumer Alerts
 Location-Based Services for Mobile Wireless
 Multiplayer Online Games
 Web Search Engines
 e-Business (e.g., Supply Chain Mgmt)
 Distributed Sensor Networks
Publish/subscribe is a natural fit!
Publish/subscribe is a natural fit!

5. Publish/Subscribe Networking
 Publish/subscribe is traditionally
implemented by centralized servers
 But server-based realizations do not scale
to Internet-wide applications
 So existing networks require “faking it”
 Request/response interaction
 Continual subscriber polling
 Enormous server farms
 Dumb caching
 And so we must realize publish/subscribe
via a distributed network of routers

6. SIENA Content-Based Routing
Subscription Forwarding
s1:1
s1:1
s1
a s1:a
s1:a 2
1
s1:2
s1:2
s1:2
s1:2 3
5
s1:1
s1:1 4 s1:3
s1:3 6
s1:3
s1:3
7
8
s1:5
s1:5
s1:6
s1:6
9

7. SIENA Content-Based Routing
Subscription Merging
s1 covers s2 s1:1
s1:1
s1:1
s1:1
s2:5
s2:5
s1 covers s2
s1:a
s1:a
a s1:a
s1:a 2
1 s2:2
s2:2
s1:2 s1:2
s1:2
s1:2
s1:2
s1:2
s2:8 3
s2:8
5
s1:1
s1:1 4 s1:3
s1:3 6
s1:3
s1:3
7
s2
s1:5
s1:5 8
b s1:5
s1:5
s2:b
s2:b 9
s1:6
s1:6

8. SIENA Content-Based Routing
Notification Delivery
s1:1
s1:1 n1 matches s1
s2:5
s2:5
s1:a
s1:a n1 matches s2
a 2
1 s2:2
s2:2
s1:2 s1:2
s1:2
s1:2
s2:8
s2:8 3
5
s1:1
s1:1 4 s1:3
s1:3 6
s1:3
s1:3
7
n1
s1:5
s1:5 8
b s2:b
s2:b 9
s1:6
s1:6

9. PreCache
NETINJECTOR Architecture
Internet
Publisher Subscriber
= Event Agent = Routing Engine = Channel Manager

10. PreCache NETINJECTOR
 Routing and forwarding based on SIENA
 Generalize idea of subscription merging
 Compute single subscription covering all received
subscriptions
 Employ approximate matching
 Constant time and space complexity
 Log time and space with additional leakage reduction
 Channel services
 Namespace management
 Resource allocation
 Load balancing, fault tolerance, authentication

11. Open Problems

12. Comments on the Problems
 Problems identified based on
experience with NETINJECTOR and SIENA
 Many of the problems arise because of
a desire for scalability
 Some problems are deeply technical
 Other problems are simply pragmatic

13. Problem
Wireless Mobile Devices (WMDs?)
s1:1
s1:1
s1:a
s1:a
a 2
1
s1:2
s1:2
s1:2
s1:2 3
5
s1:1
s1:1 4 s1:3
s1:3 6
s1:3
s1:3
7
8
a s1:5
s1:5 9
s1:6
s1:6

14. Problem
Issues with Wireless Mobile Devices
 Caching notifications in the network
 Stream reconstruction and duplicate
suppression
 Frequency of movement versus
overhead of reconfiguration
 Gateways for email, SMS, etc.

15. Problem
Security
 Traditional security properties are address-based
 Example: Authentication
 Bob wants to make sure Alice sent the message
 Content-based analog
 Bob wants to make sure a message represents reality
 Pub/sub admits new kinds of vulnerabilities
 Example: Denial of Service
 Highly generic subscription (“Price > 0”) causes flood of
notifications to subscriber
 How do you distinguish a malicious subscriber from a
greedy subscriber?
 How do you do content-based routing when the
content is encrypted???

16. Problem
Client Connections and Firewalls
 Want constant connection between
subscriber and edge router
 Otherwise subscriber polls for notifications
 Connections limits may require multiplexing
 Client must initiate connection to edge
router in order to breach firewall
 And if port 80 is the only open port …
 Need HTTP encapsulation of messages
 May need HTML formatting of messages
 Routers need to multiplex and/or demultiplex
message traffic

17. Problem
Approximate Matching
 Rationale: High-performance routing
 Expect approximate matching to have better
time/space complexity than exact matching
 Approximation must be conservative
 False positives OK, false negatives not
 Must still perform exact match at some point
before delivery to subscriber
 Leakage may increase traffic
 Tradeoff in computational resources
 We need simulation tools to explore this!

18. Problem
Optimizing for Traffic Variations
 Can routers dynamically optimize for traffic
variations?
 Example: The Brittany Spears Effect
 All subscribers want certain notifications N1
 Few subscribers want other notifications N2
 N1 notifications may flood network
 Example: The Google Effect
 Certain subscribers S1 want all notifications
 Other subscribers S2 want few notifications
 S1 subscriptions may dominate routing
 We need simulation tools to explore this!

19. Problem
Service-Provider Deployment
 Difficult to convince network service
providers to enhance their networks with
publish/subscribe
 Application demand not yet critical
 Lack of standards
 Economic barriers govern router design
 Example: 100M users, $10K/router
 1000 users/router: 100K routers, $1G outlay
 100 routers: $1M outlay, 1M users/router

20. Problem
Peer-to-Peer Deployment
 Reasonable alternative to service-provider
deployment
 “Grass roots” generation of demand
 Challenges
 Dynamically aligning peer topology to
underlying network topology
 Dynamically partitioning routing responsibilities
across peers
 Ensuring reliability, privacy and/or integrity of
messages

21. Problem
Unicast Fanout at Edge Routers
 Example: 100M users on 1K routers
 100K users per router
 10Kbyte notification
 >80 milliseconds over OC-192
 >80 seconds over 10Mb Ethernet
 >4 hours over 56K modem
 Idea: Use publish/subscribe for “leveling”
 Partition users into classes
 Example: Last digit of serial number
 Publish once per class
 Tune publication rate to available bandwidth and SLA

22. Conclusion

23. Conclusion
 Many Internet applications naturally
require publish/subscribe messaging
 Scalability can be achieved through
publish/subscribe networking
 SIENA, PreCache, and others have
established many fundamental results
 But many open problems remain to be
solved