DotSlashAn Automated Web Hotspot Rescue System

Jonathan BulavaCSC8530 – Distributed SystemsDr. Paul Schragger

What is DotSlash?

An automated web hotspot rescue system.

Implementation is similar to Content Delivery Networks (CDN).

People: Weibin Zhao & Henning Schulzinne at Columbia University

Slashdot Effect

A term used when web servers cannot handle an influx of traffic due to links from more popular websites.

Bandwidth is consumed and/or the large number of requests cannot be processed.

Core Techniques

Detecting and predicting overload conditions in real time.

Discovering rescue servers in wide area networks.

Manage rescue relationships. Designing and evaluating efficient

mechanisms for replicating contents dynamically and redirecting client requests in wide area networks.

A Mutual-aid Community

Cost effective solution compared to CDN systems.

Spare capacity in the community is utilized to handle hotspots for an individual site.

Establishes a peer-to-peer communication model.

Configurations

ConfigurationBottlenecks Addressed

Used By

Dots_Apache Network and web server

Static/dynamic content web sites

Dots_Apache + Dots_PHP

Network, web server, and application server

Dynamic content web sites

Dots_Apache + Dots_PHP + Dots_MySQL

Network, web server, application server, and database server

Dynamic content web sites

The prototype uses a LAMP configuration.Only Dots_Apache is required to use DotSlash.

Dots_Apache

An Apache module that supports basic DotSlash functions. Workload monitoring Rescue server discovery Rescue relationship management Request redirection Dynamic virtual hosting Dynamic DNS update

Dots_PHP An extension for the PHP module

that supports replicating PHP scripts dynamically.

Dots_MySQL A cache-enhanced data driver that

supports caching database query results on demand.

Requires the memcached engine and libmemcache C library.

State Transitions and Restrictions

NormalSOS RescueComplete release

Initial allocation

Additional allocation

Partial release

Accept rescue request

Partial shutdown

Accept rescue request

Complete shutdown

S1 S2

S8

S7

S4

S3

S4

S6

rescuingStates are mutually exclusive

i.e. a server cannot provide a rescue server while receiving a rescue service.

HTTP Request Rescue

Clientorigin.com

DNSrescue.com

DNS

www.origin.com(1.2.3.4)

www.rescue.com(5.6.7.8)

Cache

DNS round-robin and Dynamic Virtual Hosting are implemented to manage rescue servers.

(e.g. www-vh1.rescue.com, www-vh2.rescue.com, etc.)

Rescuing Dynamic Content

Original PHP (or other server-side script) is cached on rescue servers.

File Inclusions (include, require, etc.) Customized error handler for each

script. Security Issues?

Client

www.origin.com(So)

www.rescue.com(Sr)

DB(1)

(2)

(7)

(6)(5)(4)

(3)

(8)

Effectiveness

The Database Bottleneck

Client

DatabaseServer

Query Result CacheWeb/Application

ServerData Driver

Query Result CacheWeb/Application

ServerData Driver

Original Server

Rescue Server

DatabaseServer

References

Weibin Zhao, DotSlash, <http://www1.cs.columbia.edu/~zwb/project/dotslash/>.

Weibin Zhao and Henning Schulzrinne, "Building an Adaptive Distributed Web Server System on the Fly for Handling Web Hotspots," in ACM Symposium on Principles of Distributed Computing (PODC'04) (brief announcement), St. John's, Newfoundland, Canada, July 2004.

Weibin Zhao and Henning Schulzrinne, "DotSlash: Handling Web Hotspots at Dynamic Content Web Sites," in IEEE Global Internet Symposium, Miami, Florida, March 2005.

“Slashdot effect.” Wikipedia.