1. Distributed Systems - Comp 655
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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Distributed Systems - Comp 655

2. Distributed Systems - Comp 655
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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Distributed Systems - Comp 655

3. Distributed Systems - Comp 655
Definition of the Web
Many standards
HTML
HTTP
DNS
URL, URI, URN
XML
DOM
W3C
IETF
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Distributed Systems - Comp 655

4. Distributed Systems - Comp 655
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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Distributed Systems - Comp 655

5. 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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Distributed Systems - Comp 655

6. Distributed Systems - Comp 655
Overall structure
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Distributed Systems - Comp 655

7. Distributed Systems - Comp 655
What’s in a web page?
Client-side script
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Distributed Systems - Comp 655

8. Distributed Systems - Comp 655
Some web pages are XML
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Distributed Systems - Comp 655

9. XML document type definition
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Distributed Systems - Comp 655

10. Distributed Systems - Comp 655
Other document types
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Distributed Systems - Comp 655

11. CGI – early Web interaction
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Distributed Systems - Comp 655

12. Distributed Systems - Comp 655
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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Distributed Systems - Comp 655

13. Problems with browsers
Browser-based user interfaces tend to be clunky and limited
Solutions:
Client-side scripting
Applets
More recently, AJAX
An example:
See for more information
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Distributed Systems - Comp 655

14. Server-side scripts and servlets
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Distributed Systems - Comp 655

15. Distributed Systems - Comp 655
Nothing’s perfect
What Web technology has big problems with server-side page generation?
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Distributed Systems - Comp 655

16. 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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Distributed Systems - Comp 655

17. Distributed Systems - Comp 655
HTTP connections
non-persistent
persistent
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Distributed Systems - Comp 655

18. Distributed Systems - Comp 655
HTTP request types
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Distributed Systems - Comp 655

19. Distributed Systems - Comp 655
HTTP request example
type
path
protocol
GET /xyzzy HTTP/1.1
Connection: Keep-Alive
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-powerpoint, applicat
ion/vnd.ms-excel, application/msword, application/x-shockwave-flash, */*
Accept-Language: en-us
Host: laptop:1215
If-Modified-Since: Sun, 27 Jun :58:28 GMT
User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1)
headers
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Distributed Systems - Comp 655

20. Distributed Systems - Comp 655
HTTP header types
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Distributed Systems - Comp 655

21. Distributed Systems - Comp 655
Processes
Browsers
Proxies
Apache web server framework
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Distributed Systems - Comp 655

22. Distributed Systems - Comp 655
Browser with plug-in
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Distributed Systems - Comp 655

23. Distributed Systems - Comp 655
Web proxy
Most browsers today support ftp. However, proxies are still used for shared caching.
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Distributed Systems - Comp 655

24. Distributed Systems - Comp 655
Apache
modules
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Distributed Systems - Comp 655

25. Server cluster – simple minded
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26. Server cluster - clever
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27. Distributed Systems - Comp 655
Web naming
URI
URL
URN
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Distributed Systems - Comp 655

28. Distributed Systems - Comp 655
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 scheme for Hypertext Transfer Protocol services
-- mailto scheme for electronic mail addresses
news:comp.infosystems.
-- 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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Distributed Systems - Comp 655

29. Naming – URL – how to access
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Distributed Systems - Comp 655

30. 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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Distributed Systems - Comp 655

31. 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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Distributed Systems - Comp 655

32. 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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Distributed Systems - Comp 655

33. Replication – client and proxy
Many organizations run proxy servers
Some proxies can cooperate
Virtually all browsers can cache
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Distributed Systems - Comp 655

34. Replication – server side
Server clusters
Mirror sites
Content delivery networks (CDNs)
For example, Akamai
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Distributed Systems - Comp 655

35. Distributed Systems - Comp 655
CDN operation
In Akamai’s CDN, embedded document URLs get resolved to “closest” CDN server
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Distributed Systems - Comp 655

36. Distributed Systems - Comp 655
Security on the Web
NOTE: using both public and private key encryption, for performance reasons
NOTE: client has to use same server for entire session
If using client authentication
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Distributed Systems - Comp 655