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Representational State Transfer: An Architectural Style for Distributed Hypermedia Interaction

Representational State Transfer: An Architectural Style for Distributed Hypermedia Interaction. Roy T. Fielding University of California, Irvine http://www.ics.uci.edu/~fielding/. Network Application Architecture. Software architecture of a network-based app

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Representational State Transfer: An Architectural Style for Distributed Hypermedia Interaction

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  1. Representational State Transfer:An Architectural Style for Distributed Hypermedia Interaction Roy T. Fielding University of California, Irvine http://www.ics.uci.edu/~fielding/

  2. Network Application Architecture • Software architecture of a network-based app • abstract system view and model for comparison • communication restricted to message passing • Defines • how system components are allocated and identified • how the components interact to form a system • the amount and granularity of communication needed for interaction • interface protocols

  3. Network Performance Measures • latency • latent period: time between stimulus and first indication of a response • minimum latency = ping/echo time • throughput • rate of data transfer • round trips • number of interactions per user action

  4. Network Performance Measures • overhead • setup: time to enable application-level interaction • message control • amortization • spreading overhead across many interactions • completion = setup / amortization+ (roundtrips * latency)+ (control + data) / throughput

  5. User-perceived Performance • user-perceived latency impacted by • setup overhead • network distance x round trips • blocking/multithreading • collisions • user-perceived throughput impacted by • available network bandwidth • message control overhead • message buffering, layer mismatches • loss of synchronization

  6. Network Application Performance application  style  architecture  implementation • Network performance is bound by • application requirements • pattern of communication • infrastructure used to communicate • implementation of components • The best network application performanceis obtained by not using the network • disconnected operation

  7. Architectural Styles • Common patterns within system architectures • One system may be composed of multiple styles • Some styles are hybrids of other styles • An architecture is an instantiation of a style • We could equally talk about • computer architecture • network architecture • software architecture [Shaw/Garlan, 1993] • network-based application architecture

  8. Client/Server • most hyped, least meaningful style • emphasizes separation of concerns • initiators (clients) and listeners (servers)

  9. Remote Session • each client initiates a session on server • application state kept on server • commands are used to exchange data orchange session state • flexible, interactive, easy to extend services • scalability is a problem

  10. Remote Data Access (RDA) • Clients send database queries to remote server • Server maintains per-client state for joins/trans. • Client must know enough about data structure to build structure-dependent queries • SQL commonly used to define query • Results transferred by (proprietary) protocol

  11. Pipe-and-Filter • a.k.a., one-way data flow • data stream is filtered through a sequence of components • components do not need to know identity of peers • components are transitive

  12. Event-based Integration • components listen to a message bus or register with a broker • components do not need to know identity of peers • separation of concerns, independent evolution • usually not designed for high-latency networks • poor scalability • collisions or single point of failure

  13. Distributed Objects • Components interact as peers • Emphasizes object management, data hiding, state distribution • Strong typing is usually assumed • identity of peer is required • Uses EBI or brokered client/server • Streaming not supported in general

  14. Distributed Process Paradigms [Gregory Andrews, 1991] • Heartbeat • Probe/Echo • Broadcast • Token-passing • Replicated/shared server, blackboard • Replicated workers, bag of tasks

  15. Web Architectural Style • Web architectural style revolves around five fundamental notions: • resource • representation of a resource • communication to obtain/modify representations • web “page” as an instance of application state • engines to move from one state to the next • browser • spider • any media type handler

  16. What is a Resource? • A resource can be anything that has identity • a document or image • a service, e.g., “today’s weather in Seattle” • a collection of other resources • non-networked objects (e.g., people) • The resource is the conceptual mapping to an entity or set of entities, not necessarily the entity that corresponds to that mapping at any particular point in time!

  17. Representations of a Resource • The Web is designed to manipulate and transfer representations of a resource • A single resource may be associated with multiple representations (content negotiation) • A representation is in the form of a media type • provides information for this resource • Hypermedia-aware media types • provide potential state transitions • Most representations are cachable

  18. Representational State Transfer • optimized for transfer of typed data streams • caching of representations allows application interaction to proceed without using the network • all components can be pipe-and-filter

  19. Origin Server Model • server provides interface to services as a resource hierarchy • implementation details hidden from clients • stateless interaction for scalability • application interaction can be spread across multiple servers • replaceable by a gateway pipe

  20. Gateway Model • appears as a normal origin server to client • provides an interface encapsulation of other services • data flow translation in both directions • also used for high-speed caching

  21. Agent Model • holds all application state • which allows user to manipulate it (history) • or anticipate changes to it (link maps) • application details hidden from server • browser, spider, index robot, personal agent • replaceable by a proxy pipe

  22. Proxy Model • translate multiple services into HTTP • transform data streams according to client limitations (e.g., image translation) • enforce security policies • enable shared caching

  23. Web Architecture Evolution • Uniform Resource Identifiers • http://www.ics.uci.edu/~fielding/talks/ • mailto:fielding@ics.uci.edu • Access protocols • HTTP, FTP, Gopher, ... • Media types • HTML, XML, applet languages • Some architectural misfits • HTTP cookies, HTML frames

  24. Uniform Resource Identifiers • A simple and extensible means of identifying resources • Uniformity allows • different types of resource identification within a single protocol element • uniform semantic interpretation of common syntactic elements • relative syntactic interpretation independent of scheme • Few changes since 1991

  25. Hypertext Transfer Protocol • A protocol (syntax and semantics) for transferring representations of resources • usually across the Internet using TCP • Design goals • speed (stateless, cachable, few round-trips) • simplicity • extensibility • data (payload) independence • A true network-based API

  26. HTTP/0.9 (pre-1993) • Absolute Simplicity GET /url-path <TITLE>Hello World</TITLE> Hello World • No Extensibility • only one method (GET) • no request modifiers • no response metadata

  27. HTTP/1.0 (1993-present) • Simple and (mostly) Extensible GET /Test/hello.html HTTP/1.0 Accept: text/html User-Agent: GET/5 libwww-perl/0.40 HTTP/1.0 200 OK Date: Fri, 12 Jan 1996 01:02:49 GMT Server: Apache/1.0.5 Content-type: text/html Content-length: 38 Last-modified: Wed, 10 Jan 1996 01: <TITLE>Hello</TITLE> Hello out there!

  28. HTTP/1.0 Deficiencies • No complete specification until end of `94 • No minimum standard for compliance • Poor network behavior • one request per connection • no reliable transfer of dynamic content • no control over response caching • failed to anticipate proxies and gateways • created huge demand for vanity addresses • misuse/misunderstanding of MIME

  29. HTTP/1.1 • Culmination of two years work, RFC2068 • with Henrik Frystyk, Jim Gettys, Jeff Mogul • designed at UCI and W3C; expanded in IETF • Improved Reliability • chunked transfer of dynamic content • recognition of proxy and gateway requirements • explicit cachability of responses • Improved Network Behavior • persistent connections • virtual hosts (many names, one address)

  30. HTTP/1.1 (1997-????) • Less Simple, More Extensible, but Compatible GET /Test/hello.html HTTP/1.1 Host: kiwi.ics.uci.edu:8080 User-Agent: GET/7 libwww-perl/5.40 HTTP/1.1 200 OK Date: Fri, 07 Jan 1997 15:40:09 GMT Server: Apache/1.2b6 Content-type: text/html Transfer-Encoding: chunked Etag: “a797cd-465af” Cache-control: max-age=3600 Vary: Accept-Language …

  31. HTTP/1.x Deficiencies • MIME is too verbose (overhead per message) • Control mixed with metadata • Metadata restricted to header or trailer • Meta-metadata requires encapsulation of entire message • Fixed request/response ordering can block progress • Lack of multiplexing prevents getting important part of multiple representations first

  32. HTTP/2.x • Tokenized transfer of common fields • reducing bandwidth usage, latency • removal of MIME syntax limitations • self-descriptive for extensions • Multiplexing control, data, metadata streams • reducing desire for multiple connections • enabling multi-protocol connections • per-stream priority or credit mechanism • Layered streams for meta-metadata, encryption...

  33. Media Types • Web architecture is designed to be media type independent • but we can only use what agents will consume • leading to a chicken-and-egg adoption problem • HTML is still the lingua franca • difficult to extend semantics, rendering • wasteful to extend syntactically • no mechanism for alternatives • ECMAscript, DynamicHTML, applets

  34. XML to the rescue? • “X” for extensible: • self-descriptive syntax • semantics by reference (doctype, namespaces) • rendering by reference (style sheets) • An XML representation is an object turned inside-out, with behavior-by-reference • However, network application performance will demand standards for domain-specific doctypes and style sheets

  35. Conclusions • Web architectural style inherits from • client/server: separation of concerns, scalability • pipe-and-filter: streams, intermediaries, encapsulation • distributed objects: methods, message structure • Advantages of representational state transfer: • application state controlled by the user agent • composed of representations from multiple servers • representations can be cached, shared • matches hypermedia interaction model of combining information and control

  36. Future Work • Dynamic application architectures • Architectural analysis and performance bounds • Impact of future network architectures (ATM) • Balancing secure transfer with firewall visibility • Protocol for manipulating resource mappings • HTTP-NG (W3C/Xerox PARC) • rHTTP (UCI)

  37. Questions? • Slides available late next week: • http://www.ics.uci.edu/~fielding/talks/webarch_9805/

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