COMP 655:Distributed/Operating

SystemsSummer 2011

Dr. Chunbo ChuWeek 10: Web

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The Web• Origin and overview of the web• Drill-down on distributed system aspects

– Communication– Processes– Naming– Synchronization– Replication (especially caching)– Fault tolerance– Security

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Origin of the web• CERN (European particle physics lab)• Purpose: facilitate document sharing

– Large user community– Geographically dispersed

• Founder: Tim Berners-Lee• Use exploded in late 90’s

– Graphical user interfaces (Mosaic and descendants)

– Huge amounts of content– Search engines– Interactive pages

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Definition of the Web• Many standards

– HTML– HTTP– DNS– URL, URI, URN– XML– DOM

• W3C• IETF

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A word about RFCs• Standards track

– Proposed standard– Draft standard (at least two independent

and interoperable implementations)– Internet standard (also has STD number,

for example IP is STD-005 and RFC-0791)• “Off-track”

– Experimental– Informational– Historic(al)

See RFC 2026 for details

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Yet more words about RFCs

Before using an RFC,• check the Obsolete RFC list• or find it on the Active RFC list

I use the RFC index at faqs.org because I find it a bit easier to use than the IETF’s list. Remember, if there’s a conflict, IETF is the authority.

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Overall structure

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What’s in a web page?

Client-side script

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Some web pages are XML

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XML document type definition

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Other document types

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CGI – early Web interaction

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Problems with CGI• Process per request• Wide variety in server-side runtime

environments

• Solutions– Server-side scripting (JSP, ASP, PHP)– Servlets

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Problems with browsers• Browser-based user interfaces tend

to be clunky and limited

• Solutions:– Client-side scripting– Applets– More recently, AJAX

• An example: http://www.javarss.com/ajax/j2ee-ajax.html

• See http://en.wikipedia.org/wiki/AJAX for more information

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Server-side scripts and servlets

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Nothing’s perfect• What Web technology has big

problems with server-side page generation?

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Communcation on the web: HTTP

• TCP-based client/server protocol– Create connection– Send request– Send response– Close connection

• HTTP 1.1 reduces connection overhead with persistent connections

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HTTP connections

non-persistent persistent

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HTTP request types

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HTTP request example

GET /xyzzy HTTP/1.1Connection: Keep-AliveAccept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-powerpoint, application/vnd.ms-excel, application/msword, application/x-shockwave-flash, */*Accept-Language: en-usHost: laptop:1215If-Modified-Since: Sun, 27 Jun 2004 00:58:28 GMTUser-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)

type path protocol

headers

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HTTP header types

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Processes• Browsers• Proxies• Apache web server framework

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Browser with plug-in

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Web proxy

Most browsers today support ftp. However, proxies are still used for shared caching.

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Apache

www.apache.org

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Server cluster – simple minded

Web Server Clusters

A scalable content-aware cluster of Web servers

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Web naming

URI

URL

URN

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URI examples from RFC 2396

ftp://ftp.is.co.za/rfc/rfc1808.txt -- ftp scheme for File Transfer Protocol services

gopher://spinaltap.micro.umn.edu/00/Weather/California/Los%20Angeles -- gopher scheme for Gopher and Gopher+ Protocol services

http://www.math.uio.no/faq/compression-faq/part1.html -- http scheme for Hypertext Transfer Protocol services

mailto:mduerst@ifi.unizh.ch -- mailto scheme for electronic mail addresses

news:comp.infosystems.www.servers.unix -- news scheme for USENET news groups and articles

telnet://melvyl.ucop.edu/ -- telnet scheme for interactive services via the TELNET Protocol

More examples on page 670

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Naming – URL – how to access

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Naming – URN – true resource identifier

RFC 2648 defines a URN namespace for IETF documents. RFC 2141 defines URN syntax. RFC 3406 is a BCP (Best Current Practice) for defining URN namespaces.

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Activity – hitting a web page

• Check your understanding: draw a UML sequence diagram showing the interaction of key software elements when a browser hits a web page containing graphics

• Assume the web page and the images are on different servers

• “Classes” in the diagram should include– Browser– DNS resolver– DNS server– Server for the page– Server for the images

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Not much to synchronize …

• Generally, web clients don’t exchange information with other clients, and servers don’t exchange with other servers

• Most documents have a single author – few write/write conflicts

• However, WebDAV is a simple locking and versioning scheme– Locks are connection-independent– Handling abandoned locks is left to

implementation

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Replication – client and proxy

Virtually all browsers can cache

Many organizations run proxy serversSome proxies

can cooperate

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Security on the Web

If using client authentication

NOTE: using both public and private key encryption, for performance reasons

NOTE: client has to use same server for entire session