What is the Web? - CIS Personal Web Pages

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Transcript What is the Web? - CIS Personal Web Pages 

Web-Based Information Systems
What is (was?) the Web?
• A set of cross-linked hypertext documents
spanning the internet
– (Notionally)
• In reality
– this is what the Web was in its infancy (c1993)
– it now links transparently through most internet protocols and
applications
– many “documents” are virtual
• created according to dynamic context and not stored
Richard Connor
Slide 1
Web-Based Information Systems
What is its importance?
• probably the largest collection of high-quality
data in almost any field known to mankind
– used by experts in many fields (other than computing!) as the
only viable way to communicate volumes of data
• enormous potential for fine-grained
communications
– can potentially replace huge numbers of human “drones”
• information is dynamically immediately available
and mechanically searchable
– barely started to happen yet
Richard Connor
Slide 2
Web-Based Information Systems
How does it work?
• the internet
– mechanically connected machines
– unique IP addresses (eg 130.209.240.50)
– local networks, routers
• DNS (Dynamic Name Service)
– maps logical name (eg hawaii.dcs.gla.ac.uk) to IP address
• the http protocol
– sends hypertext files on request from an IP address
• the HTML standard
– a document format which includes a way of describing files
using DNS and the http protocol
Richard Connor
Slide 3
Web-Based Information Systems
Slide 4
The Internet
• all the physically interconnected machines in the
world
• each connected machine has a unique 32-bit
identifier
• this is split into a network id and a machine id
within the network (lan)
– depends on “subnet mask”
• each lan contains a designated “router” which
knows how to connect to different networks
– (on a similar hierarchal basis)
• allows point-to-point packet-based
communication between arbitrary machines
Richard Connor
Web-Based Information Systems
Dynamic Name Service
• maps a logical name based on sub-domains to
an IP address
– (is not responsible for data transmission)
• each top-level domain (eg .edu, .uk, .com) maps
to a known machine location
• the service at each domain responds with the IP
address of any registered sub-domain
– also gives contact info for registering or un-registering
subdomains (this is a human-based activity often requiring
money!)
Richard Connor
Slide 5
Web-Based Information Systems
Slide 6
The http protocol
• a listening service which accepts requests and
returns hypertext files
• implemented by http servers
– listen on a dedicated port no (part of the IP protocol)
• maps URLs to local files and sends them to
requesting machine
– (URL - Uniform Resource Locator)
– with a simple header attached
• rather a complex protocol, most of which is
commonly unused
Richard Connor
Web-Based Information Systems
Slide 7
URLs
a three-part resource locator:
• protocol
– eg http, telnet, mailto
• resource address
– domain name + port
• resource details
– typcially directory and file in http
http://www.cs.strath.ac.uk:80/~richard/default.html
Richard Connor
Web-Based Information Systems
Slide 8
HTML
• a markup language
– contains additions (“tags”) to ordinary text files
– the additions are optionally interpreted by programs that
display the data
• most tags are in pairs which imply an attribute of
the enclosed text
– <HTML>...blah...</HTML> asserts the text is in HTML format
– <EM>...blah...</EM> means the text should be displayed as
emphatic
• anchor - the hypertext tag
– <A HREF=“a URL”>...blah...</A>
– means the text should be displayed as a hyper-link, if pressed
should attempt to go to the indicated resource
Richard Connor