waka: A replacement for HTTP Fielding: Designing a new protocol for the Web December 2002 waka: A replacement for HTTP Roy T. Fielding, Ph.D. Chief Scientist, Day Software Director, The Apache Software Foundation Co-founder, Apache HTTP Server Project Elected member, W3C Technical Architecture Group http://www.apache.org/~fielding/
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 waka: A replacement for HTTP November 2002
But what about Web Services? Fielding: Designing a new protocol for the Web December 2002 But what about Web Services? ONC / DCE RPC COM / DCOM CORBA J2EE Web Services .NET They have all tried to solve the same problem … waka: A replacement for HTTP November 2002
EAI - the hard way app5 app1 app2 app3 app4 4 applications = 6 integrations 5 applications = 10 integrations 6 applications = 15 integrations waka: A replacement for HTTP November 2002
EAI - the Web way waka: A replacement for HTTP November 2002
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 waka: A replacement for HTTP November 2002
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, … waka: A replacement for HTTP November 2002
REST Style Derivation Graph Virtual Machine Code-On-Demand LCS Layered System Stateless Client Server Cache Replicated Repository REST Uniform Interface waka: A replacement for HTTP November 2002
REST Process View Layered Client-Server $ Layered Client-Server Uniform Interface (like Pipe and Filter) Stateless, Cacheable Communication Optional Code-on-Demand waka: A replacement for HTTP November 2002
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 waka: A replacement for HTTP November 2002
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] waka: A replacement for HTTP November 2002
HTTP Message Syntax GET /Test/hello.html HTTP/1.1\r\n Host: kiwi.ics.uci.edu:8080\r\n Accept: text/html, text/*, */*\r\n User-Agent: GET/7 libwww-perl/5.40\r\n \r\n HTTP/1.1 200 OK\r\n Date: Thu, 09 Mar 2000 15:40:09 GMT\r\n Server: Apache/1.3.12\r\n Content-Type: text/html\r\n Content-Language: en\r\n Transfer-Encoding: chunked\r\n Etag: “a797cd-465af”\r\n Cache-control: max-age=3600\r\n Vary: Accept-Language\r\n 4090\r\n <HTML><HEAD> … waka: A replacement for HTTP November 2002
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 waka: A replacement for HTTP November 2002
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 replacement for HTTP November 2002
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 waka: A replacement for HTTP November 2002
New Request Semantics RENDER method MONITOR 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 waka: A replacement for HTTP November 2002
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: A replacement for HTTP November 2002
Waka Syntax Uniform syntax Self-descriptive Efficient and Extensible 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 waka: A replacement for HTTP November 2002
A work just beginning waka http://www.apache.org/~fielding/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/ waka: A replacement for HTTP November 2002
Web Architecture Understanding the Web’s Success Goals 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 waka: A replacement for HTTP November 2002
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 waka: A replacement for HTTP November 2002
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 waka: A replacement for HTTP November 2002
Example Principles Information hiding Separation of concerns 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 waka: A replacement for HTTP November 2002
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 waka: A replacement for HTTP November 2002