DYNAMIC LOAD BALANCING ON WEB-SERVER SYSTEMS by Valeria Cardellini Michele Colajanni Philip S. Yu.
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DYNAMIC LOAD BALANCINGON WEB-SERVER SYSTEMS
byValeria CardelliniMichele Colajanni
Philip S. Yu
Contents INTRODUCTION CLIENT-BASED APPROACH DNS-BASED APPROACH DISPATCHER-BASED APPROACH SERVER-BASED APPROACH COMPARING THE APPROACH CONCLUSIONS
INTRODUCTION disproportionate increase in client requests to
popular Web sites solution
mirroring replicate information across a mirrored-server system not user-transparent
distributed Web-server system Information can be distributed among server nodes:
1st : content tree replication – LAN, WAN 2nd : information sharing – LAN
Successful load-balancing transparent users appear as a single host to outside world
four distributed Web-server architectures Client-based DNS-based , dispatcher-based , server-based
CLIENT-BASED APPROCH Web Clients
Web client selects a node of the cluster and submits the request to the selected node Netscape Navigator – random select Limited practical applicability and is not scalable
Smart Clients Migrates server functionality to the client through a Java
applet Increase network traffic and network delay
Client-Side Proxies Web Cluster standpoint, proxy servers are similar to
clients not universally applicable
DNS-BASED APPROACH
The cluster DNS translates URL to the IP address User transparent ex) www.yahoo.com www.cnn.com
DNS-BASED APPROACH (con’t)
Drawbacks The DNS a limited control on the request reaching the Web clust
er Between the client and the web server DNS, many intermediate na
me servers can cache the logical name to IP address mapping to reduce network traffic and every web browser typically caches some address resolution
scheduling algorithm that the cluster DNS uses to balance the Web-server node’s load constant TTL algorims
System-stateless Server-state-based Client-state-based
adaptive TTL algorims DNS-Based Architecture Comparison
DISPATCHER-BASED APPROACH Centralize request scheduling and completely control clie
nt-request routing Request routing among server is transparent-unlike DNS
-based DNS deals address at the URL level, the dispatcher has a single, virtua
l IP address(IP-SVA) Dispatcher uniquely identifies each server in the system
through a private address Dispatcher typically use simple algorithms to select the
Web server difference by routing mechanism
Packet Single-Rewriting Packet Double-Rewriting Pachet Forwarding (network dispatcher, One-ip address) HTTP redirection
Packet Single-Rewriting
TCP router acts as an IP address dispatcher High System availability
Packet Double-Rewriting
Two solution using this approach Magicrouter
round-robin, random, incremental load algrithm Cisco System’s Local Director
least number of active connections
Packet Forwarding
Network Dispatcher Dispatcher forward packets to the selected server using its
physical address without IP modification Level 1 : single-rewriting mechanisim Level 2 : LAN network dispatcher
Packet Forwarding (con’t)
ONE-IP address uses the ifconfig alias option to configure a Web-serv
er system with multiple machines implemented with two techniques
Routing-based dispatching Broadcast-based dispatching
HTTP Redirection
among the Web-server nodes through the HTTP’s redirection mechanism
no IP address modification two techniques
Server-state-based dispatching used by Distributed Server Groups architecture
Location-based dispatching used by Cisco System’s DistributedDirector appliance
Dispatcher-Based Architecture Comparison
SERVER-BASED APPROACH Use two level dispatching mechanism
Integrating the DNS based approach with redirection techniques executed by Web server
Solves most DNS scheduling problem Two Solution
HTTP redirection
Packet redirection
COMPARING THE APPROACHES Approach Trade-off Summary
COMPARING THE APPROACHES Performance
Evaluation
CONCLUSIONS network bandwidth can constrain load-
balancing performance LAN-distributed Web-server cluster are
thus a limited solution to increased client requests
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