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					               The Web

• Core Ideas and Technologies
   • Resources and Objects (and Services tomorrow)
   • HTTP
   • MIME Types
   • URIs
   • ReST
         Uniform Access - HTTP
• Main access/manipulation protocol of the Web
   • A means of exchanging resource representations across the
     network
   • Request/response based interactions
• Supports a limited number of requests
   • GET – retrieve a resource – no payload
   • POST – send a resource to a server – payload
   • PUT – update or alter a resource on a server - payload
       • If the resource does not exist, the server may allow the resource to be
         created
   • DELETE – remove a resource from a server – no payload
   • HEAD – find out what metadata you would get back if you
     performed a GET – no payload
   • OPTIONS – find out what operations are permitted on a
     resource – no payload
                                 HTTP
• When a server receives a request, it returns a response
  containing a response code, headers and possibly a resource
  representation.
   • Codes are divided into types
       •   100 – 199 – informational
       •   200 – 299 – success
       •   300 – 399 – redirect
       •   400 – 499 – client error
       •   500 – 599 – server error
   • examples:
       •   200 OK
       •   201 Created
       •   403 Forbidden
       •   301 Redirect (with ‘Location’ header)
       •   500 Internal Server Error
                   HTTP Headers
• The HTTP message is an envelope
   • starts with headers
   • Then the body (payload), if any.
   • request:

   GET /weather/ HTTP/1.1
   Host: www.bbc.co.uk
   User-Agent: curl/7.16.3
   Accept: */*

   • response:

   HTTP/1.1 200 OK
   Server: Apache/2.0.59 (Unix)
   Content-Length: 12345
   Content-Type: text/html

   …payload…
                         MIME
• Multipurpose Internet Mail Extensions
   • not used just in email now
• The Content-Type header field points to a MIME Type
• A (slowly growing) list of known media types
   • text/plain
   • text/xml
   • text/html
   • image/gif
   • image/jpeg
• Developers/designers are encouraged to reuse MIME types
   • keeping the list manageable helps interoperability
   • A different approach to Object interfaces
          Common HTTP Headers
•   Host: the target host
•   Content-Length: length (in bytes) of the resource representation
•   Content-Type: MIME type of representation
•   Accept: Accepted MIME type
•   Transfer-Encoding: can be chunked. This means the data is sent in
    chunks, rather than all in one go.
     • Useful for data for which the length is not known when starting
       transmission.
•   Content-Encoding: e.g. gzip, deflate. Allows zipping up content.
•   Etag: a server defined ID for a resource
     • If-Match, If-None-Match: allows conditional requests
•   Date:
     • If-Modified-Since, If-Not-Modified: allows conditional requests
•   Expect: e.g. 100-continue:
     • server responds with a 100 status code if the client should continue
       with the request by sending the payload.
•   WWW-Authenticate: authentication challenge issued by a server
•   Many Caching-related headers
     HTTP Headers are extensible
•   Typically start with „X-‟
•   Allows application specific metadata
•   Examples from Rackspace:
     • X-Auth-User – user name in request
     • X-Auth-Key – user API key in request
     • X-Auth-Token – user token with lifetime in response
•   Mobiles:
     • x-roaming - is the user roaming?
     • x-nokia-msisdn – user‟s mobile number in plain text
     • HTTP_X_BROWSER_HEIGHT
     • HTTP_X_DEVICE_TYPE
•   Even:
     • HTTP_X_ME_CUSTOM_ITEM_1
         Uniform Naming - URIs
• Uniform Resource Identifier
   • An identifier for a resource on the Web.
   • A superset comprising URNs (Uniform Resource Name) and
     URLs (Uniform Resource Location). URLs can be
     dereferenced (i.e. they represent an „address‟)
   • Examples:
       • http://cs.cf.ac.uk/user
       • urn:isbn:0-395-36341-1
       • urn:jxta:uuid:59616261646162614A78746150325033F3BC76
   • The first is a URL - it represents an address - you can GO
     there - PPT spotted that
   • The second is a URN - it identifies a book uniquely
   • The third is a URN, but using the Jxta Protocol, one may be
     able to resolve this to a URL. PPT obviously does not
     support Jxta.
                                URIs
• URIs can be either hierarchical or non-hierarchical.
   • A non hierarchical URI is considered opaque.
   • A hierarchical URI has a number of elements.
• All URIs start with a scheme element
• This example is hierarchical:
                  authority


    http://cs.cf.ac.uk:8080/harrison/resume.html#intro


  scheme           host         port         path        fragment
 • This example is not hierarchical:
     mailto:a.b.harrison@cs.cf.ac.uk


     scheme                   opaque
              Resources and URIs
• Hierarchical URIs can also have a query
  string:
         http://www.google.com/search?q=web

                                        key    value
• This is a simplified URI generated by searching Google for
  the word “web”
   • q is a key to google to say the bit of the query after the = is
     what I typed in.
   • Query strings can contain multiple key/value pairs using an
     ampersand
     ?client=safari&rls=en-us&q=web&ie=UTF-8&oe=UTF-8
•   Experiment - change the value after the q=. Add &hl=ja to the end
    of the URI and see what happens
         Resources and URIs
• Resources have state but not operations over
  that state.
• This is supported through URIs
  • The scheme of the URI determines the operations
    permitted on a resource.
  • By exposing representations of resources at URIs
    with different schemes, you offer different
    operations on that resource.
         Representational State
           Transfer (ReST)
• Term coined by Roy Fielding in his PhD dissertation
  in 2000
      • A PhD thesis that has actually had an impact!!!
• ReST is an architectural style derived from looking at
  the Web and what makes it scalable
   • It is a client/server architectural style in which
     clients retrieve representations of resources.
     When a new resource is retrieved, the state of the
     client, e.g., the browser, is changed.
• The term is now heavily used and misused
   • ReST != HTTP
   • Not all the Web is ReSTful
                               ReST
• Primary constraints
   • Client/server
      • Request/response
   • Statelessness
      • interactions are stateless
   • Caching
      • reduces network load
      • HTTP headers support lots of caching policies
   • Uniform Interface
      • URIs
      • HTTP
      • MIME types
   • Layering
      • Allows servers to act as gateways to areas of the network for
        firewalling, caching
      • Allows evolution of parts of the network to happen incrementally without
        affecting the whole network
   • Code-on-Demand
      • e.g. Applets, SWF
                   Statelessness
• Interactions on the Web are usually stateless
   • Resources have (represent) state, but the interactions do not
   • When I query Google, each query happens in isolation. There is
     no state preserved at the server regarding my query, even if I
     move through a number of results pages
   • Experiment: add &start=20 to the end of your query
   • What happens? You move to a new URI which is independent
     of the previous URI. (Look at the Gooooooogle links at the
     bottom of the page)
• The Cookie is typically used to represent interaction
  state
   • Cookie Header is an opaque pointer to state
   • Stored at the client
   • Understood by the server – the state of interactions is on the
     server – not ReSTful
           Caching and Linkability
• Caching
   • Allows resources to be stored and retrieved locally, or closer to
     the source of the request
        • This saves on bandwidth
        • Is a form of load balancing
   • Is possible because of the resource abstraction.
   • Caching also applies to storage outside of the Web via URIs -
     in desktop applications, on TV, newspapers, billboards, human
     memory
• Linkability
   • Web Architecture (Vol 1): It is a strength of Web Architecture
     that links can be made and shared; a user who has found an
     interesting part of the Web can share this experience just by
     republishing a URI.
   • Makes the Web a web
             Caching and Linkability
•   Linkability and cacheability are achievable through two primary qualities
•   URIs being „addressable‟
     • I can share them, publish them knowing others will get the same
        result as me when they de-reference the URI.
     • A certain longevity to URIs - If I link to a resource today, will it still
        be there tomorrow?
•   Safe and idempotent interactions
     • Safe interactions are those with no side-effects.
          • the user is not to be held accountable for the result of the interaction.
     • I am not entering into a contract or agreeing to terms and
       conditions when I follow a link.
     • If this were the case, how could you be sure that you were not at a
       deeper level within the domain, having unknowingly bypassed the
       terms an conditions page? – Servers use redirects for this.
     • Idempotent interactions. No matter how many times the identical
       request is repeated, it does not change the side-effects of the
       request.
                Uniform Interface
• separation of resource from representation
   • What you see is not the actual thing, but a
     representation. Things may have multiple
     representations.
• manipulation of resources by representations
   • Changing the representation may induce a change in
     the state of the resource itself
• self-descriptive messages
   • Allows each message to be independent
   • No need to track state changes across multiple
     exchanges
• hypermedia as the engine of application state (?)
   • Hypermedia is a medium that allows non-linear progression
     through states.
       • A page with links - its up to you where you go.
       • So state is only perceived and controlled by the client
“Hypermedia as the engine of
     application state”

                 URL
                 Every GET to Google is
                 independent.
                 Google doesn’t perceive
                 your state changes

                 Possible next states
                 The client chooses the next
                 state
                 Only the client percieves the
                 continuum of states
    Conclusion

• The Web core concepts
    • Uniform Access
         •HTTP
    •Naming
         •URIs
    •Representation
         •MIME types
         •primarily HTML
• ReST
    • stateless interactions
    • caching
    • state at the client side

				
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posted:10/14/2011
language:English
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