DotSlash

1
DotSlash A Web Hotspot Rescue System

• Weibin Zhao
• Henning Schulzrinne
• Department of Computer Science
• Columbia University
• June 11, 2004

2
The Problem

• Web hotspots
• A short-term sudden, dramatic surge of request
  rate
• 15 minutes of fame
• Existing mechanisms
• Capacity planning
• Content delivery networks (CDNs)
• Caching
• Admission control

3
DotSlash

• A hotspot rescue system
• mutual-aid community, use spare capacity
• Self-configuring, adaptive, scalable

server
server
mutual-aid community
server
server
server
server
directory
4
DotSlash Components

• Workload monitoring
• Rescue server discovery
• Load migration HTTP redirect, DNS-RR
• Dynamic replication and caching
• Rescue control

origin
rescue
(2)
(3)
(1)
(2)
(1)
DNS
5
Handling Load Spikes
rescue
origin
rescue
rescue
increasing throughput
reducing arrival rate DNS-RR
origin
increasing service rate HTTP redirect
6
Benefit of Using HTTP Redirect

• Reducing network load
• Caching static content
• Reducing CPU load
• Replicating scripts dynamically

database
PHP
origin server
(1)
Apache
PHP
(2)
MySQL
PHP
rescue server
(3)
web server is the bottleneck, not database server
Apache
7
Server Status
Origin server Getting help from others
SOS
Allocate
Release
Normal
Accept SOS
Shutdown
Rescue
Rescue server Providing help to others
8
Workload Regions
100
heavily loaded region
upper threshold
70
desired load region
60
reference load
50
lower threshold
lightly loaded region
0
9
Rescue Control

• Objective
• CPU/Link in the desired load region
• Origin server
• Rescue serve allocation/release
• Adjusting redirect probability
• Rescue server
• Accept SOS, shutdown rescue
• Adjust allowed redirect (data) rate

10
Origin Server

• Workload monitoring
• HTTP redirect
• Use rescue server aliases
• Dont redirect requests from rescue servers
• Redirect Policies WRR
• DNS round-robin
• Add/remove rescue server IP addresses

11
Rescue Server

• Workload monitoring
• Reverse caching proxy
• Caching concurrency control
• Different requests
• A regular request
• A reverse proxy request to origin server (active
  rescue)
• An HTTP redirect to origin server (expired
  rescue)
• Dynamic DNS update
• Register new aliases

12
Implementation

• LAMP Apache, PHP, MySQL
• Dynamic DNS dot-slash.net
• Service discovery enhanced SLP
• Workload httperf, RUBBoS
• Testing LAN and PlanetLab

SHM
Other Dotsd
Apache
Dotsd
Mod_dots
DSRP
HTTP
Client
SLP
DNS
BIND
mSLP
13
Conclusion

• Preliminary version implemented
• Measurement 10-fold improvement
• Adaptive overload control
• On-going work
• Evaluation
• Dynamic content
• DNS-RR
• Real Slashdot test
• Software distribution