Representational State Transfer:An Architectural Style for

Distributed Hypermedia Interaction

Roy T. FieldingUniversity of California, Irvine

http://www.ics.uci.edu/~fielding/

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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

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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

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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

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User-perceived Performance user-perceived latency impacted by

– setup overhead– network distance xx round trips– blocking/multithreading– collisions

user-perceived throughput impacted by– available network bandwidth– message control overhead– message buffering, layer mismatches– loss of synchronization

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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

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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

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Client/Server

most hyped, least meaningful style emphasizes separation of concerns initiators (clients) and listeners (servers)

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Remote Session

each client initiates a session on server application state kept on server commands are used to exchange data or

change session state flexible, interactive, easy to extend services scalability is a problem

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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

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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

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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

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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

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Distributed Process Paradigms

[Gregory Andrews, 1991] Heartbeat Probe/Echo Broadcast Token-passing Replicated/shared server, blackboard Replicated workers, bag of tasks

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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

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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!

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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

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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

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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

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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

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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

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Proxy Model

translate multiple services into HTTP transform data streams according to client

limitations (e.g., image translation) enforce security policies enable shared caching

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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

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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

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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

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HTTP/0.9 (pre-1993)

Absolute SimplicityGET /url-path<TITLE>Hello World</TITLE>Hello World

No Extensibility

– only one method (GET)– no request modifiers– no response metadata

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HTTP/1.0 (1993-present)

Simple and (mostly) ExtensibleGET /Test/hello.html HTTP/1.0Accept: text/htmlUser-Agent: GET/5 libwww-perl/0.40

HTTP/1.0 200 OKDate: Fri, 12 Jan 1996 01:02:49 GMTServer: Apache/1.0.5Content-type: text/htmlContent-length: 38Last-modified: Wed, 10 Jan 1996 01:

<TITLE>Hello</TITLE>Hello out there!

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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

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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)

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HTTP/1.1 (1997-????)

Less Simple, More Extensible, but CompatibleGET /Test/hello.html HTTP/1.1Host: kiwi.ics.uci.edu:8080User-Agent: GET/7 libwww-perl/5.40

HTTP/1.1 200 OKDate: Fri, 07 Jan 1997 15:40:09 GMTServer: Apache/1.2b6Content-type: text/htmlTransfer-Encoding: chunkedEtag: “a797cd-465af”Cache-control: max-age=3600Vary: Accept-Language …

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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

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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...

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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

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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

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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

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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)

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Questions?

Slides available late next week:– http://www.ics.uci.edu/~fielding/talks/

webarch_9805/