200211 Fielding Apachecon

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waka: A replacement for HTTP

Roy T. Fielding, Ph.D.

Chief Scientist, Day SoftwareDirector, The Apache Software FoundationCo-founder, Apache HTTP Server ProjectElected member, W3C Technical Architecture Group

http://www.apache.org/~fielding/

November 2002 2waka: A replacement for HTTP

How should we design a new application protocol?

Define the architectural style Even a generic protocol must choose one

model for evaluation of efficiency, or choose to be inefficient for all applications

Document desired architectural properties

Identify the architectural elements Components, connectors, data Interfaces, transports, media types

Specify protocol Semantics = component interaction Syntax = efficiency/extensibility


But what about Web Services?

ONC / DCE RPC COM / DCOM CORBA J2EE Web Services .NET

They have all tried to solve the same problem …


EAI - the hard way

4 applications = 6 integrations



1app 2app 3app 4app

5app

€

ii=1

n- 1

å >> n


EAI - the Web way


High-level Web Requirements

Low entry-barrier Hypermedia User Interface Simple protocols for authoring and data transfer Extensibility

Multiple organizational boundaries Anarchic scalability Heterogeneous platforms Gradual and fragmented change (deployment)

Distributed Hypermedia System Efficient for large data transfers Sensitive to user-perceived latency Capable of disconnected operation


REST Architectural Style

My dissertation work at UC Irvine available on Web [Fielding 2000] independent discussion on RESTWiki [Baker et al.]

An architectural style can be used to define the principles behind the Web architecture such that they are visible to future architects A style is a named set of constraints on

architectural elements

REST was used to guide definition and implementation of modern Web architecture modifications to HTTP and URI implementations in Apache, libwww-perl, …


REST Style Derivation Graph

VirtualMachine

Code-On-DemandLCS

LayeredSystem

Stateless

ClientServer

Cache

ReplicatedRepository

REST

UniformInterface


REST Process View

$

$

$

$

$

$

$

$

Layered Client-Server Uniform Interface (like Pipe and Filter) Stateless, Cacheable Communication Optional Code-on-Demand


REST Uniform Interface

Pictures are not sufficient We must define constraints that enforce a

uniform interface

Five primary interface constraints Resource is unit of identification Resource is manipulated through exchange

of representations Resource-generic interaction semantics Self-descriptive messaging Hypermedia is engine of application state


Hypertext Transfer Protocol

The role of HTTP in Web Architecture Extend uniform interface across the net Minimize user-perceived latency Enable layered processing Enable caching Enable extension and evolution

Already survived a decade of evolution 1991-93: HTTP/0.9 [Berners-Lee] 1993-97: HTTP/1.0 [RFC 1945] 1996-now: HTTP/1.1 [RFC 2068/2616]


HTTP Message Syntax

GET /Test/hello.html HTTP/1.1\r\nHost: kiwi.ics.uci.edu:8080\r\nAccept: text/html, text/*, */*\r\nUser-Agent: GET/7 libwww-perl/5.40\r\n

\r\n

HTTP/1.1 200 OK\r\nDate: Thu, 09 Mar 2000 15:40:09 GMT\r\nServer: Apache/1.3.12\r\nContent-Type: text/html\r\nContent-Language: en\r\nTransfer-Encoding: chunked\r\nEtag: “a797cd-465af”\r\nCache-control: max-age=3600\r\nVary: Accept-Language\r\n\r\n 4090\r\n<HTML><HEAD> …


HTTP Current Problems

HTTP/1.1 was limited by pre-existing syntax Overhead of MIME-style message syntax Head-of-line blocking on interactions Metadata unable to come after data Server can’t send unsolicited responses

Messages are not fully self-descriptive Extensions don’t indicate scope, mandatory/optional Response messages don’t indicate request

Low-power and bandwidth-sensitive devices More severely impacted than desktop browsers Typical solution: impose a gateway

device-specific, proprietary protocol loss of fidelity in communication due to evolution fails when devices roam across networks


It’s time for a new standard

A new protocol standard could solve HTTP’s current problems in a generic way

However, building consensus around a new protocol isn't easy How do we differentiate from existing protocols,

particularly those that are supposedly generic? How do we decide among conflicting design

alternatives for a new protocol? How do we design such that the protocol can be

deployed in a heterogeneous environment?

I use the REST architectural style as a guide


Waka

A new protocol designed to match efficiency of REST architectural style

Why “waka”? Mäori word (pronounced “wah-kah”) for the

outrigger canoes used to travel safely on the Pacific Ocean, across hundreds of islands, to Aotearoa (New Zealand)

One of the few four-letter words suitable for a protocol name

Deployable within an HTTP connection via the HTTP/1.1 Upgrade header field defined mapping to HTTP/1.1 for proxies


New Request Semantics

RENDER method display/print/speak this representation

MONITOR method notify me when resource state changes

Authoring methods (DAV simplified) elimination of non-resource identifiers reintroduction of PATCH

Request control data request identifier transaction identifier relative priority


New Response Semantics

Self-descriptive binding to the request Echo of request id, method, target URI Cache key explicitly described

Caches no longer need to save request fields Caches don’t have to guess about Vary info

Enables asynchronous transport Response indicates authoritative or not

Semantics formerly in status code Unsolicited Responses

Cache invalidation messages Multicast event notices


Waka Syntax

Uniform syntax Regardless of message type, direction Padding allowed for 32/64bit alignment

Self-descriptive Explicit typing for message structure, fields Indication of mandate and scope of fields Association of metadata (control, resource, rep.) Premature termination of request or response

Efficient and Extensible Tokens for all standard elements A URI reference can be used in place of any token Macros (client-defined syntax short-hand) Interleaved data and metadata delivery


A work just beginning

waka Has not yet been fully specified Has not yet been implemented Has not yet been deployed

Will eventually be proposed as ASF project Will eventually be submitted to IETF Will have its progress tracked:

http://www.apache.org/~fielding/waka/


Web Architecture

Understanding the Web’s Success Design Notes of Tim Berners-Lee Representational State Transfer (REST) W3C Technical Architecture Group

Goals Document existing design principles Identify methods for evaluating

existing identifiers, formats, protocols proposals for new features proposals for new interaction styles

Define new design principles


Principled Architecture Design

Iterating over: Analyze system

Focus on one phase of operation Choose a level of abstraction Identify components, connectors, data

Establish requirements What must be true across phase of operation

Select design principles Principles motivate architectural constraint

Constrain the architecture Constraints induce architectural properties


Example Requirements

Web as a system must exist at the operational scale of entire societies no "on" or "off" switch system must evolve while in use

Change is inevitable requires planning for evolution

Spans multiple organizations changes cannot be deployed all at once requires gradual and fragmented change


Example Principles

Information hiding a component's visibility into the implementation of

another component should be limited to what is necessary to interoperate with its interface

prevents unintended assumptions about component behavior that may not hold true in the future

applied to improve property of evolvability -- independent evolution of technology over time

Separation of concerns a component that performs two or more separate

activities is better implemented as two or more components if doing so increases cohesion and reduces coupling

applied to improve properties of simplicity and evolvability


Example Constraint

Orthogonal protocols deserve orthogonal specifications If one protocol uses another as data, it must not

restrict the content of that data other than as defined by that protocol, including future compatible revisions of that protocol.

A specification that defines two orthogonal protocols (including data formats) must be split into two specifications, since otherwise the independent evolution of those protocols will be hindered.

Result is simpler protocols able to evolve independently over time enables system to continue operation through

gradual and fragmented change

200211 Fielding Apachecon

Technology

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