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+# Resource-oriented design
+
+Resource-oriented design is a pattern for specifying API services, based on
+several high-level design principles (most of which are common to recent public
+HTTP APIs):
+
+- The fundamental building blocks of an API are individually-named _resources_
+  (nouns) and the relationships and hierarchy that exist between them.
+- A small number of standard _operations_ (verbs) provide the semantics for
+  most common operations. However, custom operations are available in
+  situations where the standard operations do not fit.
+- Stateless protocol: Each interaction between the client and the server is
+  independent, and both the client and server have clear roles.
+
+Readers might notice similarities between these principles and some principles
+of [REST][]; resource-oriented design borrows many principles from REST, while
+also defining its own patterns where appropriate.
+
+## Guidance
+
+When designing an API, consider the following (roughly in logical order):
+
+- The resources (nouns) the API will provide
+- The relationships and hierarchies between those resources
+- The schema of each resource
+- The operations (verbs) each resource provides, relying as much as possible on
+  the standard operations.
+
+### Resources
+
+A resource-oriented API **should** generally be modeled as a resource
+hierarchy, where each node is either an individual resource or a collection of
+resources.
+
+A _collection_ contains resources of _the same type_. For example, a publisher
+has the collection of books that it publishes. A resource usually has fields,
+and resources may have any number of sub-resources (usually collections).
+
+**Note:** While there is some conceptual alignment between storage systems and
+APIs, a service with a resource-oriented API is not necessarily a database, and
+has enormous flexibility in how it interprets resources and operations. API
+designers **should not** tightly couple their API surface to their underlying
+database system.
+
+### Operations
+
+Resource-oriented APIs emphasize resources (data model) over the operations
+performed on those resources (functionality). A typical resource-oriented API
+exposes a arbitrary number of resources with a small number of operations on
+each resource. The operations can be either the standard operations or custom
+operations.
+
+**Note:** A custom operation in resource-oriented design does _not_ entail
+defining a new or custom HTTP method. Custom operations use traditional HTTP
+methods (usually `POST`) and define the custom verb in the URI. See AIP-136 for
+more detail.
+
+Resources **should** include all of the standard operations; however, resources
+**may** exclude some standard operations if there is a justification for doing
+so (for example, a read-only resource would exclude all write operations).
+
+Similarly, services **should** prefer standard operations over custom
+operations; the purpose of custom operations is to define functionality that
+does not cleanly map to any of the standard operations. Custom operations offer
+the same design freedom as traditional [RPC][] APIs, which can be used to
+implement common programming patterns, such as database transactions, import
+and export, or data analysis.
+
+### Stateless protocol
+
+As with most public APIs available today, resource-oriented APIs **must**
+operate over a stateless protocol: The fundamental behavior of any individual
+request is independent of other requests made by the caller.
+
+In an API with a stateless protocol, the server has the responsibility for
+persisting data, which may be shared between multiple clients, while clients
+have sole responsibility and authority for maintaining the application state.
+
+[rest]: https://en.wikipedia.org/wiki/Representational_state_transfer
+[rpc]: https://en.wikipedia.org/wiki/Remote_procedure_call
diff --git a/aip/general/0121/aip.yaml b/aip/general/0121/aip.yaml
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+---
+id: 121
+state: approved
+created: 2019-01-26
+placement:
+  category: resource-design
+  order: 10