1. DYNAMIC LOAD BALANCING ON WEB-SERVER SYSTEMS by Valeria Cardellini Michele Colajanni Philip S. Yu

2. Contents  INTRODUCTION  CLIENT-BASED APPROACH  DNS-BASED APPROACH  DISPATCHER-BASED APPROACH  SERVER-BASED APPROACH  COMPARING THE APPROACH  CONCLUSIONS

3. 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:  1 st : content tree replication – LAN, WAN  2 nd : 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

4. 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

5. DNS-BASED APPROACH  The cluster DNS translates URL to the IP address  User transparent  ex) www.yahoo.com www.cnn.comwww.yahoo.comwww.cnn.com

6. DNS-BASED APPROACH (con’t)  Drawbacks  The DNS a limited control on the request reaching the Web cluster  Between the client and the web server DNS, many intermediate name 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

7. DISPATCHER-BASED APPROACH  Centralize request scheduling and completely control client-request routing  Request routing among server is transparent-unlike DNS-based  DNS deals address at the URL level, the dispatcher has a single, virtual 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

8. Packet Single-Rewriting  TCP router acts as an IP address dispatcher  High System availability

9. 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

10. 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

11. Packet Forwarding (con’t)  ONE-IP address  uses the ifconfig alias option to configure a Web- server system with multiple machines  implemented with two techniques  Routing-based dispatching  Broadcast-based dispatching

12. 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

13. 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

14. COMPARING THE APPROACHES  Approach Trade-off Summary

15. COMPARING THE APPROACHES  Performance Evaluation

16. CONCLUSIONS  network bandwidth can constrain load- balancing performance  LAN-distributed Web-server cluster are thus a limited solution to increased client requests