Resource-oriented architecture (ROA)

Resource-oriented architecture (ROA) is a style of software architecture that organizes systems around resources – uniquely identifiable entities that are manipulated through a small, fixed set of operations. It was described by Leonard Richardson and Sam Ruby in their 2007 O’Reilly book RESTful Web Services.

ROA is the concrete, web-bound application of Representational State Transfer (REST). Where REST is an abstract set of architectural constraints, ROA is a recipe for building web services that satisfy those constraints using HTTP and URIs. For this reason ROA is sometimes described as "REST applied to the web", and the two terms are often used interchangeably in practice.

The four ingredients

Richardson and Ruby framed ROA around four things that together define a resource-oriented system.

  • Resources. A resource is anything important enough to be named and addressed – a document, a record, a collection, a physical object, or even a concept. Resources are the nouns of the system.
  • Names. Every resource has a unique name, expressed as a URI. The name identifies the resource, not the operation to perform on it. GET /orders/42 names order 42; the verb is carried separately, by the HTTP method.
  • Representations. Clients never touch a resource directly. They exchange representations of it – concrete byte sequences in a format such as JSON, XML, or HTML. A single resource can have many representations, and the client and server negotiate which to use.
  • The uniform interface. A small, fixed set of verbs applies to every resource. On the web these are the HTTP methods – GET, POST, PUT, DELETE, and friends. The same verbs mean the same thing across every resource, which is what lets generic clients, proxies, and caches work without any knowledge of the application.

Relationship to other styles

ROA is a web-centric variation of service-oriented architecture (SOA). Where SOA in general decomposes a system around services that expose operations, ROA decomposes it around resources that expose a uniform interface. The two are not mutually exclusive. A service-oriented system may expose some of its services in a resource-oriented way.

Web-oriented architecture (WOA) is a closely related style, also built on REST and scoped to the web. The two terms overlap heavily. WOA was coined from an analyst’s perspective and emphasises global network effects and mashups, while ROA was framed from a practitioner’s, how-to-build-it perspective.

The natural contrast is Remote Procedure Call (RPC), which organises remote interfaces around named functions and their arguments rather than addressable resources. An RPC-style URL such as /getOrder?id=42 embeds the verb in the path. A ROA-style URL keeps the verb in the HTTP method and the noun in the path. ROA is one style of HTTP API.

See also

References