1. 3/20/2001 Network Computing Laboratory EE. KAIST 1 Web Servers & Load Balancing Techniques 3/20/2001 송준화 김영호

2. 3/20/2001 Network Computing Laboratory EE. KAIST 2 Part I : Web Servers

3. 3/20/2001 Network Computing Laboratory EE. KAIST 3 Overview What is a web server? Market share How a web server works? How does a web server serve contents? Architectures of Web Servers –Example : Apache, AOLServer, Jigsaw Issues on Web Servers Load Balancing Techniques (part 2) References

4. 3/20/2001 Network Computing Laboratory EE. KAIST 4 What is a Web Server? An advanced application which runs on a server and does the following –Provides connections to remote computers –Sends web pages to remote computers via the Internet or an Intranet Examples of Web Servers –Apache –MS Internet Information Server for Windows NT –AOLServer

5. 3/20/2001 Network Computing Laboratory EE. KAIST 5 Market Share

6. 3/20/2001 Network Computing Laboratory EE. KAIST 6 How does a Web Server Work? Static Contents 1.Web server receives a request for a Web page such as http://www.kaist.ac.kr/index.html http://www.kaist.ac.kr/index.html 2.Server maps URL to a local file on the host server. 3.The server then loads this file from disk and serves it out across the network to the user's Web browser.

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8. 3/20/2001 Network Computing Laboratory EE. KAIST 8 Dynamic contents –Dynamic means Web pages created in response to a user ’ s input (eg : CGI) –Web server should run programs locally and transmit their output through the Web server to the user's Web browser that is requesting the dynamic content –user's Web browser never really has to know that the content is dynamic because CGI is basically a Web server extension protocol.

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10. 3/20/2001 Network Computing Laboratory EE. KAIST 10 How does a web server serve contents? The primary mechanism for deciding how to display content is the MIME type header. Multipurpose Internet Mail Extension (MIME) types tell a Web browser what sort of document is being sent. Multipurpose Internet Mail Extension (MIME)

11. 3/20/2001 Network Computing Laboratory EE. KAIST 11 More than 370 MIME types are distributed with the Apache Web server by default in the mime.types configuration file. eg) Apache mime.types file. –text/xml xml –video/mpeg mpeg mpg mpe –video/quicktime qt mov

12. 3/20/2001 Network Computing Laboratory EE. KAIST 12 Reception Request Analysis Access Control Resource Handler Record Transaction Utility Operating System Abstraction Layer Browser Operating System Web Server

13. 3/20/2001 Network Computing Laboratory EE. KAIST 13 Architecture of Web Server Reception –Interprets the resource request protocol –Parses the requests, and builds an internal representation of the request –Determines capabilities of the browser (e.g., simple text browser or graphics capable browser)

14. 3/20/2001 Network Computing Laboratory EE. KAIST 14 Request Analyzer –Translates the location of the resource from network location to local file name eg) ~/index.html could be transformed to local file /usr/httpd/pub/index.html Access Control –Enforces the access rules employed by the server –Authenticate the browser and authorizes their access to the requested resources February 19, 2001 PC Data Online

15. 3/20/2001 Network Computing Laboratory EE. KAIST 15 Resource Handler –Determines the type of the resource requested by the browser, executes it and generates the response. Record Transaction –Records all the requests and their result. Support Layer –Utility and Operating System Abstraction Layer –Provide functions used by the above subsystems

16. 3/20/2001 Network Computing Laboratory EE. KAIST 16 Utility subsystem –Contains functions that are used by all other subsystems. –It has functions for manipulating strings or URLs and many commonly used functions Operating System Abstraction Layer –Encapsulates the OS specific functionality to facilitate the porting of the server to different platforms

17. 3/20/2001 Network Computing Laboratory EE. KAIST 17 Example (1): Apache Freely Available –Source code –binaries for many platforms (version 1.3.x includes also the Windows NT) Web server originally based on NCSA server (in 1995) Over 60% of Internet Web servers run Apache or an Apache derivative (in the December 2000 survey)

18. 3/20/2001 Network Computing Laboratory EE. KAIST 18 Process based –2.0 will support multi threads Very configurable, lots of directives... Optional modules provide extra functionality Apache is “ A PAtCHy server ” –Patches on NCSA Httpd 1.3 Powerful performance and Continually upgrade

19. 3/20/2001 Network Computing Laboratory EE. KAIST 19 Translation core Response Mime type Authorization Authentication Logging UtilOS Layer Res. Handler Access Ctrl Req. analysis Record Trans. Recep.

20. 3/20/2001 Network Computing Laboratory EE. KAIST 20 Apache Core: maintains multiple processes Request_rec: internal representation

21. 3/20/2001 Network Computing Laboratory EE. KAIST 21 Example(2): AOLServer Commercial Web Server –Developed by AOL –Source opened in 1999 First released in 1995 Powerful support for Database Provide extensibility –By using a maintainable and safe extension language –Using TCL (Tool Command Language) as the extension language.

22. 3/20/2001 Network Computing Laboratory EE. KAIST 22 Communication Driver Daemon Core NSLog NSPerm URL Handle Timer Util Database Interface TCL Interpreter NSthread Recep. Req. analysis Access Ctrl Res. Handler Record Trans. UtilOSAL *(NS: NaviSoft) platform independent Thread Lib.

23. 3/20/2001 Network Computing Laboratory EE. KAIST 23 AOLServer Richer OSAL and Utility subsystems (than Apache) –Portable thread lib. Implementation –Database interface –Timer Event scheduling, time-out of connections, etc –TCL interpreter Support for multiple network protocols Internal structure: Conn

24. 3/20/2001 Network Computing Laboratory EE. KAIST 24 Jigsaw Experimental server developed by W3C –Analyzing Internet protocols and standards Open source, first released in 1996 Written in Java –Platform independent –OSAL does not exist –Extensibility –Object Oriented design

25. 3/20/2001 Network Computing Laboratory EE. KAIST 25 Daemon Protocol Frame In Filter Resource In Filter Protocol Frame Out Filter Resource Out Filter Resource Util Access Ctrl Record Trans. Res. Handler Recep.

26. 3/20/2001 Network Computing Laboratory EE. KAIST 26 Jigsaw Daemon: maintains a thread pool for concurrency Filters: for different experiments???

27. 3/20/2001 Network Computing Laboratory EE. KAIST 27 Issues on Web Server Connections explosion –Due to rapid growth of WWW application on the internet, a web server may encounter the situation that a huge number of connection requests in a very short time Research trend on web server –Load Balancing –Distributed Scalable Web Server

28. 3/20/2001 Network Computing Laboratory EE. KAIST 28 Part II : Load Balancing Techniques Junehwa Song Young Ho Kim

29. 3/20/2001 Network Computing Laboratory EE. KAIST 29 Load Balancing Technique Mirror Client based approach DNS-based approach Dispatcher based approach Packet Single Rewriting Packet Double Rewriting Network Dispatcher Server based approach HTTP redirection Packet redirection

30. 3/20/2001 Network Computing Laboratory EE. KAIST 30 Mirror Replicate information across a mirrored server architecture User manually select alternative URL Not user transparent Don ’ t allow the Web-server system to control request distribution

31. 3/20/2001 Network Computing Laboratory EE. KAIST 31 Client Based Approach Web Client –Web client selects a node of the cluster and submits the request to the selected node –Netscape home(http://www.netscape.com) use this technique When user access this site, Navigator selects a random number i between 1 and the number of servers and directs the request to the node wwwi.netscape.com –Limited practical applicability and is not scalable

32. 3/20/2001 Network Computing Laboratory EE. KAIST 32 Smart Client –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

33. 3/20/2001 Network Computing Laboratory EE. KAIST 33 DNS Based Approach DNS server maps the domain name to multiple IP address Returning more than one IP address for the hostname or returning a different IP address for each DNS request it receives (Round robin) User transparent Simple and easy to implement

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36. 3/20/2001 Network Computing Laboratory EE. KAIST 36 Drawbacks –Unable to know the situation of the whole system –Not really fair because DNS uses a simple round robin –DNS may encounter TTL problem in IP-address cache 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

37. 3/20/2001 Network Computing Laboratory EE. KAIST 37 Because of address caching, each address can cause a burst of future requests to the selected server and quickly obsolete the current load information –Many DNS based solutions to this problem System-Stateless algorithms Server-State-based algorithms Client-State-based algorithms Adaptive TTL Algorithms

38. 3/20/2001 Network Computing Laboratory EE. KAIST 38 Dispatcher based approach

39. 3/20/2001 Network Computing Laboratory EE. KAIST 39 To 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

40. 3/20/2001 Network Computing Laboratory EE. KAIST 40 Packet Single Rewriting

41. 3/20/2001 Network Computing Laboratory EE. KAIST 41 TCP router acts as an IP address dispatcher –Router tracks the source IP address for every established TCP connection to route packets regarding the same connection to the same web server node High System availability –When one of server fails, its address can be removed from the router ’ s table –Can be combined with a DNS based solution

42. 3/20/2001 Network Computing Laboratory EE. KAIST 42 Packet Double Rewriting

43. 3/20/2001 Network Computing Laboratory EE. KAIST 43 Two solution using this approach –Magicrouter –Cisco System ’ s Local Director Because outgoing packets typically outnumber incoming request packets, dispatcher becomes bottleneck

44. 3/20/2001 Network Computing Laboratory EE. KAIST 44 Network Dispatcher Extends the basic TCP router mechanism work with both LANs and WANs Dispatcher forward packets to the selected server using its physical address without IP modification

45. 3/20/2001 Network Computing Laboratory EE. KAIST 45 Core and Sore Lab NRL project –http://core.kaist.ac.kr/nrlintro2.htmhttp://core.kaist.ac.kr/nrlintro2.htm

46. 3/20/2001 Network Computing Laboratory EE. KAIST 46 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

47. 3/20/2001 Network Computing Laboratory EE. KAIST 47 HTTP Redirection

48. 3/20/2001 Network Computing Laboratory EE. KAIST 48 Above figure server1 redirect the request to server2. Not client transparent ! Overhead of infra cluster communication –Every server must periodically transmit status information to cluster DNS Increases response time in client side, because of packet redirection

49. 3/20/2001 Network Computing Laboratory EE. KAIST 49 Packet Redirection Use a round robin DNS mechanism to schedule the request among the Web Server Server reached by a request reroutes the connection to another server through a packet rewriting –Transparent to the client! Packet rewriting overhead

50. 3/20/2001 Network Computing Laboratory EE. KAIST 50 Reference [1] A reference architecture for Web Server Reverse Engineering, 2000. Proceedings. Seventh Working Conference on, 2000, Page(s): 150 -159 [2] Dynamic load balancing on Web-server systems Cardellini, V.; Colajanni, M.; Yu, P.S. IEEE Internet Computing Volume: 3 3, May-June 1999, Page(s): 28 -39 [3] Design and practice of a dispatch server architecture Hong, H.C.; Chen, Y.C. Distributed Computing Systems, 1999. Proceedings. 7th IEEE Workshop on Future Trends of, 1999, Page(s): 246 -251 [4] Scalable Web server architectures Mourad, A.; Huiqun Liu Computers and Communications, 1997. Proceedings., Second IEEE Symposium on, 1997, Page(s): 12 -16

51. 3/20/2001 Network Computing Laboratory EE. KAIST 51 [5] TCP/IP Illustrated, Volume1 W. Richard Strevens Addison Wesley [6] TCP/IP Illustrated, Volume3 W. Richard Strevens Addison Wesley [7] Netcraft. The Netcraft WWW server survey Available at http://www.netcraft.co.uk/Survey

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