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

3/20/2001 Network Computing Laboratory EE. KAIST

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Web Servers & Load Web Servers & Load Balancing TechniquesBalancing Techniques

3/20/2001송준화김영호


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Part I : Web ServersPart I : Web Servers


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OverviewOverview

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


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What is a Web Server?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 Inte

rnet or an Intranet Examples of Web Servers

– Apache– MS Internet Information Server for Windows NT – AOLServer


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Market ShareMarket Share


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How does a Web Server How does a Web Server Work?Work? Static Contents

1. Web server receives a request for a Web page such as 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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Dynamic contents– Dynamic means Web pages created in response to

a user’s input (eg : CGI)– Web server should run programs locally and trans

mit 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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How does a web server serve How does a web server serve contents?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.


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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


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Reception

Request

Analysis

Access

Control

Resource

Handler

Record

Transaction

UtilityOperating System Abstraction Layer

Browser

Operating System

Web Server


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Architecture of Web Architecture of Web ServerServer 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)


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Request Analyzer– Translates the location of the resource from netwo

rk location to local file name • eg) ~/index.html could be transformed to local file /usr/h

ttpd/pub/index.html

Access Control– Enforces the access rules employed by the server– Authenticate the browser and authorizes their acce

ss to the requested resources

February 19, 2001 PC Data Online


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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


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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


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Example (1): ApacheExample (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 (i

n 1995) Over 60% of Internet Web servers run Apache

or an Apache derivative (in the December 2000 survey)


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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


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Translatio

n

core

ResponseMime typeAuthorizatio

n

Authentication

Logging

Util OS Layer

Res. HandlerAccess

Ctrl

Req. analysis

Record

Trans.

Recep.


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ApacheApache

Core: maintains multiple processesRequest_rec: internal representation


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Example(2): AOLServerExample(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.


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Communication

Driver

Daemon

Core

NSLog NSPerm

URL Handle

Timer Util

Database

Interface

TCL

Interpreter

NSthread

Recep.

Req. analysis

Access

Ctrl

Res. Handler

Record

Trans.

Util OSAL

*(NS: NaviSoft)

platform independent

Thread Lib.


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AOLServerAOLServer 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


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JigsawJigsaw

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


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Daemon

Protocol

Frame

In Filter

Resource

In Filter

Protocol

Frame

Out Filter

Resource

Out Filter

Resource

Util

Access

Ctrl

Record

Trans.

Res. Handler

Recep.


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JigsawJigsaw

Daemon: maintains a thread pool for concurrency

Filters: for different experiments???


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Issues on Web ServerIssues 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


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Part II : Part II : Load Balancing Techniques Load Balancing Techniques

Junehwa SongYoung Ho Kim


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Load Balancing TechniqueLoad 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


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MirrorMirror

Replicate information across a mirrored server architecture

User manually select alternative URL

Not user transparentDon’t allow the Web-server

system to control request distribution


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Client Based ApproachClient Based Approach

Web Client– Web client selects a node of the cluster and submi

ts the request to the selected node– Netscape home(http://www.netscape.com) use thi

s technique• When user access this site, Navigator selects a random n

umber 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


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Smart Client– Migrates server functionality to

the client through a Java applet– Increase network traffic and

network delayClient side Proxies

– Web Cluster standpoint, proxy servers are similar to clients


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DNS Based ApproachDNS 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 transparentSimple and easy to implement


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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


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•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


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Dispatcher based Dispatcher based approachapproach


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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


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Packet Single RewritingPacket Single Rewriting


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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 remov

ed from the router’s table – Can be combined with a DNS based solution


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Packet Double RewritingPacket Double Rewriting


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Two solution using this approach– Magicrouter – Cisco System’s Local Director

Because outgoing packets typically outnumber incoming request packets, dispatcher becomes bottleneck


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Network DispatcherNetwork 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


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Core and Sore Lab NRL project– http://core.kaist.ac.kr/nrlintro2.htm


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Server based approachServer based approach

Use two level dispatching mechanism– Integrating the DNS based approach

with redirection techniques executed by Web server

– Solves most DNS scheduling problemTwo Solution

– HTTP redirection– Packet redirection


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HTTP RedirectionHTTP Redirection


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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


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Packet RedirectionPacket 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


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ReferenceReference

[1] A reference architecture for Web Server Reverse Engineering, 2000. Proceedings. Seventh Working Conference on , 2

000 , 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


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