DotSlash:%20An%20Automated%20Web%20Hotspot%20Rescue%20System

1
DotSlash An Automated Web Hotspot Rescue System

• Weibin Zhao
• Henning Schulzrinne
• Department of Computer Science
• Columbia University

2
The Problem

• Web hotspots
• Also known as flash crowds, the Slashdot effect
• Short-term dramatic load spikes at web servers
• Existing mechanisms are not sufficient
• Over-provisioning
• Inefficient for rare events
• Difficult because the peak load is hard to
  predict
• CDNs
• Expensive for small web sites that experience the
  Slashdot effect

3
The Challenges

• Automate hotspot handling
• Eliminate human intervention to react quickly
• Improve availability during critical periods (15
  minutes of fame)
• Allocate resources dynamically
• Static configuration is insufficient for
  unexpected dramatic load spikes
• Address different bottlenecks
• Access network, web server, application server,
  and database server

4
Our Approach

• DotSlash
• An automated web hotspot rescue system by
  building an adaptive distributed web server
  system on the fly
• Advantages
• Self-configuring
• Service discovery, adaptive control, dynamic
  virtual hosting
• Scalable, easy to use, and transparent to clients

5
DotSlash Overview

• Rescue model
• Mutual aid community using spare capacity
• Potential usage by web hosting companies
• DotSlash components
• Workload monitoring
• Rescue server discovery
• Load migration (request redirection)
• Dynamic virtual hosting
• Adaptive rescue and overload control

6
Handling Load Spikes

• Request redirection
• DNS-RR reduce arrival rate
• HTTP redirect increase service rate
• Handle different bottlenecks

Technique Bottleneck Addressed
Cache static content Network, web server
Replicate scripts dynamically Application server
Cache query results on demand Database server
7
Rescue Example

• Cache static content

client1
(2) HTTP redirect
(4)
(3)
(1)
reverse proxy
origin server
rescue server
(3)
(4)
(1)
client2
DNS server
(2) DNS round robin
8
Rescue Example (2)

• Replicate scripts dynamically

Apache
PHP
origin server
database server
(1)
(6) PHP
client
(5) PHP
(2)
(4)
(3)
(7)
rescue server
(8)
MySQL
Apache
9
Rescue Example (3)

• Cache query results on demand

database server
query result cache
origin server
client
data driver
database server
query result cache
rescue server
data driver
10
Server States
Origin server Get help from others
SOS state
Allocate rescue server
Release all rescues
Normal state
Accept SOS request
Shutdown all rescues
Rescue server Provide help to others
Rescue state
11
Adaptive Overload Control

• Objective
• CPU/network in desired load region
• Origin server
• Allocate/release rescue servers
• Adjust redirect probability
• Rescue server
• Accept SOS requests
• Shutdown rescues
• Adjust allowed redirect rate

12
Implementation

• Based on LAMP (Linux, Apache, MySQL, PHP)
• Apache module (mod_dots), DotSlash daemon
  (dotsd), DotSlash rescue protocol (DSRP)
• Dynamic DNS using BIND with dot-slash.net
• Service discovery using enhanced SLP

SHM
other dotsd
Apache
dotsd
mod_dots
DSRP
HTTP
client
SLP
DNS
BIND
mSLP
13
Evaluation

• Workload generation
• httperf for static content
• RUBBoS (bulletin board) for dynamic content
• Testbed
• LAN cluster and WAN (PlanetLab) nodes
• Linux Redhat 9.0, Apache 2.0.49, MySQL 4.0.18,
  PHP 4.3.6
• Metrics
• Max request rate and max data rate supported

14
Performance

• Static content (httperf)
• 10-fold improvement
• Relieve network and web server bottlenecks
• Dynamic content (RUBBoS)
• Completely remove web/application server
  bottleneck
• Relieve database server bottleneck
• Overall improvement 10 times for read-only mix,
  5 times for submission mix

15
Conclusions

• DotSlash prototype
• Applicable to both static and dynamic content
• Promising performance improvement
• On-going work
• Address security issues in deployment
• Release DotSlash as open source software
• For further information
• http//www.cs.columbia.edu/IRT/dotslash
• DotSlash framework WCW 2004
• Dynamic script replication Global Internet 2005
• On-demand query result cache TR CUCS-035-05
  (under submission)