high performance web server

High Performance Web Server

NTUIM

R89725018 Chen Pei-wen

R89725013 Cheng Pei-chun

Outline

Introduction Load balancing Content-based Switching Implementation Architecture Conclusion Reference

Introduction

Performance and high availability are critical at web sites which receive large number of requests.

The QoS a web server provides to end users depends on– Network-transfer speed– Server-response time

Introduction(con’d)

Network-transfer speed is mainly a matter of Internet-link bandwidth.

Server-response time depends upon available resources:– Single server– Multiple servers


Single server

Hardware

Operation System

Web-server Software

Specific operation systemAdjust operation-system parameter

Well-designed server processAdjust web-server software

Install more RAM Replace the CPU with a faster oneUse faster SCSI controllers and disks


Multiple Servers– Improve performance by increasing the

number of Web servers.– This involves an attempt to distribute the

traffic onto a cluster of back-end Web servers.

– Load balancing is needed.

Load Balancing

Goal– To balance the traffic onto available server– The technical distribution is totally

transparent to the end user

Load Balancing(con’d)

Benefits– Improve reliability (fault tolerance)

• If you are using a single server and it fails, the site goes down with it.

• This is especially bad for e-commerce and financial sites which lose money if they are out of service.

• With a load balanced group of servers, loss of a single server will only slightly affect overall site performance and the site will not go down.


– Improve performance• Load balancing allows multiple servers to be available to

handle larger number of incoming client requests.

– Lower cost• With load balancing providing fault tolerance to the entire

site, the reliability of each individual server is less critical. • We can use lower-cost servers without compromising

overall reliability.


– Improve scalability and flexibility• With one server, all you can do if your traffic increases is

upgrade that server or buy a bigger one.• With load balanced groups of servers, you can simply

add more servers gracefully to server farm.

– Improve maintainability• The flexibility of a load balanced group of servers allows

you to remove individual servers from service for repair or upgrade without affecting the overall availability of the site.


Load balancing algorithm (RFC 2391)

1. Round-Robin• This is the simplest scheme, where a host is selected

simply on a round robin basis, without regard to load on the host.

2. Least Load first (session count)• the host with least number of sessions bound to it is

selected to service a new session.• Each session is assumed to be as resource consuming

as any other session


3. Least traffic first (bytes or packet count)• measure system load by tracking packet count or byte

count directed from or to each of the member hosts over a period of time.

4. Least weighted load • Weights to sessions, based on likely resource

consumption estimates of session types• Weights to hosts based on resource availability.

5. Fastest response • periodically ping member hosts and measure the

response time to determine how busy the hosts really are

Load Balancing - RR DNS

Round-Robin DNS Approach– Allows a single domain name to be associated wit

h several IP addresses • Example : using CNAME (canonical name) resource records

www.foo.dom. IN CNAME www1.foo.dom

IN CNAME www2.foo.dom




Operation– A browser has to take to retrieve the URL is to resolve the c

orresponding IP address

– A name resolver that calls a nearby DNS server, which then actively iterates over the distributed DNS server hierarchy on the Internet until it reaches the Round-Robin DNS server, which finally gives the IP address

– A browser takes the IP address and create a connection with the assigned server


Attractiveness of Round-Robin DNS– The concept is simple– It requires no additional hardware

Drawbacks of Round-Robin DNS– DNS is unaware of the status of web servers– All servers are assumed to have equal capability

to offer all services


– The caching of DNS data can cause load imbalances

• In practice, DNS servers cache the resolved data at any point in the DNS hierarchy both to decrease the resolver traffic and to speed up resolving.

User process

Nameresolver

NameServer

query

response

query

response

RR DNSServer

query

response

cache cache cache

Database Database

Load Balancing - L4 Switch

Layer-4 Switch Approach– These switches sit between the connection to the

Internet and the server farm


Operation– The switch recognizes when a client is requesting a new

session by identifying the TCP SYN packet

– The request is forwarded to the best available server based on the configured load balancing algorithm

– The switch maintains a session-server binding table that associates each active session with the real server to which it is assigned


– It performs address substitution so that the real server will transparently receive packets for that session

– Likewise, the switch intercepts packets traveling from the real server to the client and performs the reverse address substitution

– The switch recognizes when the session is terminated by identifying the TCP FIN packet

– Then it removes the session-server binding from its binding table


Attractiveness of Layer-4 Switch– Good load balancing can be achieved– No problem of the caching of DNS data– Sophisticated algorithm can be used– Aware of the failures of web servers

Limitation of Layer-4 Switch– It has no concept of what content is being

requested


– All content should be replicated– Cache hit rate may be low

Load Balancing

Benefits of Content Awareness

Content-based Switching

Content-based Switching – Intelligently load balances traffic across delivery

nodes, dynamically directing specific content requests to the best site and server at that moment.

– Based on content availability, application availability and server load.

– Adds protection against flash crowds and ensures transaction continuity for e-commerce applications.

– Enables advanced personalization and prioritization for important content and customers.

Content-based Switching(con’d)

Benefits– Increased performance due to improved hit rates

in the back-end’s main memory caches.– Increased secondary storage scalability due to the

ability to partition the server’s database over the different back-end nodes

– The ability to back-end nodes that are specialized for certain types of requests


Products– ArrowPoint's

Content Smart™ Web SwitchesWebNS™

– Foundry network’sServerIron™ Traffic Management system Internet IronWare™

– Nortel Networks’sAccelar Load Balancing Server Switch


Switch Architecture

User makes a content request by typing a URL into a Browser.


Idea

Web switch with virtual IP of the requested URL intercepts the request. Web switch spoofs TCP connect back to client and examines packet URLs. Simultaneously, the Web switch examines HTTP header and URL and compares to current content rules to select best server or cache to satisfy request.

A flow is created between the switch and the optimal server and "snaps" together with the flow from the client to the switch.

A flow control block is created in the port ASIC and all subsequent packets are forwarded without intervention by the switch controllers.


Variant Switching Scheme– URL Switching

Directs HTTP requests to a group of servers using information contained in URL string.

• Greater control over the website deployment to place different web content on different servers

• Eliminating unnecessary duplication of all content across all load-balanced servers.

• Ex: Different file typesDifferent request


Variant Switching Scheme– Cookie Switching

Directs HTTP requests to a server group based on information embedded in a cookie in the HTTP header.

• Cookie specifies which server group should handle the request.

• Ensures that a particular server group always handles requests from a particular client even across sessions.

• Guarantee persistent end-user experience.• Ex: Personalized web page

Prioritized service


Variant Switching Scheme– SSL Session ID Switching

All the SSL connections between a client and server must reach the same host.

• Ensures that all the traffic for a SSL transaction with a given SSL-ID always goes to the same server.

• Key feature for commerce, financial web sites• Ex: Prevent shopping cart loss

Access controlPrevent source address overload


Load balancer evaluation criteria – Plans for Layer 3 or Layer 4 switching– Number of servers and planned growth – Type of content to be balanced – Number of server sites to be balanced – Sophistication of balancing algorithms– Degree of fault tolerance required – Interfaces and port density– Support requirements

Implementation Architecture

Design, Implementation and Performance of a

Content-Based Switch

Infocom 2000

{George Apostolopoulos, David Aubespin, Vinod Peris, Prashant Pradhan, Debanjan Saha}

Implementation Architecture(con’d)

– The Layer 5 system consist of a switch core to which a number of custom built intelligent port controller are attached.

– Layer 5 functions, such as the parsing of HTTP protocol messages and URL based routing, are performed by the processor.

Switch Architecture


– Port controller identify the packets that need to be handled by the processor and forward them to the processor.

– Make sure it can achieve very high speed while delivering sophisticated Layer 5 functionality.

Switch Architecture


– Phase 1: it intercepts the TCP connection setup request from the client and response by establishing a connection to the client.

– Phase 2:after routing decision is made, it sets up a second connection to the appropriate server node.

– Phase 3:splicing the two TCP connections

Operation Blueprint


Processing at Port Controllers


– CPU acts as the end-points for the TCP connections to the client and the server until. they are spliced.

– Splices the connection by sending the appropriate control messages to the port controllers.

– Handling of TCP options deserves special attention.

• Reject all TCP options• Enumerate the minimum set of options supported by all

nodes

Processing at CPU


To be able to dispatch HTTP requests based on URLs, the L5 system has to know mapping from the URL to the web server on which the page resides.

Use a hash function and set default size of all hash buckets to 256.

URL look up

Conclusion

The concept of content-based switching is understandable, but efforts are needed to implement it well.

Content based-service differentiation can be used to provide service differentiation based on the user profiles.

Not only load balancing but also persistence pays in E-Commerce.

Reference RFC 2391 LSNAT Webtechniques Load balancing your web sites

http://www.webtechniques.com/archives/1998/05/engelschall/ HydraWEB Load Balancing

http://www.hydraweb.com/load_balancing/index.asp Techniques for Designing High-Performance Web Sites

http://www.research.ibm.com/people/i/iyengar/ieeeic/ieeeic.html Locality-Aware Request Distribution in Cluster-based Network S

ervice In Architectural Support for Programming Languages and Operating System, 1998

TCP/IP & Related Protocols 2 edition Uyless BlackChapter four : The Domain Name System

http://www.webtechniques.com/archives/1998/05/engelschall/





http://www.hydraweb.com/load_balancing/index.asp

http://www.research.ibm.com/people/i/iyengar/ieeeic/ieeeic.html

Reference (con’d) Foundry Products Application Notes

http://www.foundrynet.com/appnotes.html Alteon WebSystems Web Switching White Paper

http://www.alteonwebsystems.com/products/whitepapers/index.asp Design, Implementation and Performance of a Content-Based S

witch Infocom 2000{George Apostolopoulos, David Aubespin, Vinod Peris, Prashant Pradhan, Debanjan Saha}

Cisco CDNshttp://www.cisco.com/warp/public/779/largeent/learn/technologies/content_networking/

http://www.foundrynet.com/appnotes.html





http://www.alteonwebsystems.com/products/whitepapers/index.asp





URL Switching

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Cookie Switching

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SSL Session ID Switching

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