Define common components for oneof parity across protobuf and OAS.

json_schema/oneof.yaml

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+$schema: https://json-schema.org/draft/2020-12/schema
+$id: https://aep.dev/component.oneof.json
+title: x-aep-oneof
+description: |
+  Represents a mutual exclusion constraint over multiple properties.
+
+  It is an error for any properties specified by a oneof to be required.
+type: object
+required: [properties]
+additionalProperties: false
+properties:
+  properties:
+    description: |
+      A set of properties that are mutually exclusive.
+    type: array
+    items:
+      type: string
+  required:
+    description: |
+      If set, exactly one property from the `properties` array must be present on the object.
+    type: string

proto/aep-api/aep/api/oneof_behavior.proto

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+syntax = "proto3";
+
+package aep.api;
+
+option cc_enable_arenas = true;
+option go_package = "aep.dev/api";
+option java_multiple_files = true;
+option java_outer_classname = "OneOfBehaviorProto";
+option java_package = "dev.aep.api";
+option objc_class_prefix = "AEP";
+
+import "google/protobuf/descriptor.proto";
+
+// Used to specify the behavior of a `oneof`.
+message OneOfBehavior {
+  // If set, one field within the `oneof` must be set.
+  //
+  // When unset, the `oneof` is implicitly optional (the default behavior in
+  // protobuf).
+  bool required = 1;
+}

schemas/oneof.md

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+# Oneof
+
+As used by the [AEPs](http://aep.dev), a `oneof` represents a set of mutually
+exclusive fields.
+
+## Schema
+
+Rather than a generic schema for `oneof`, we provide common components
+supplementing the language features of specific IDLs. Using these enables
+uniform behavior (and JSON serialization) across IDLs.
+
+### protobuf
+
+Protobuf's built-in `oneof` supports mutual exclusion of fields. However, it
+does not have a way for an API indicate that at least one of the fields must be
+set. For this purpose, a protobuf `oneof` may be annotated with
+[`aep.api.OneOfBehavior`](../proto/aep-api/aep/api/oneof_behavior.proto):
+
+```proto
+message Document {
+  // ...
+
+  // The owner of the document.
+  oneof owner {
+    // The user who owns the document.
+    string user = 1 [(google.api.resource_reference) = {
+      type: "apis.example.com/User",
+    }];
+
+    // The group that owns the document.
+    string group = 2 [(google.api.resource_reference) = {
+      type: "apis.example.com/Group",
+    }];
+  } [(aep.api.OneOfBehavior) = {required: true}];
+}
+```
+
+### OAS
+
+OAS 3 has a concept called `oneOf`, but it has different semantics than the
+`oneof` defined by this document; the OAS concept says that a single field must
+match exactly one of multiple possible schemas, rather than specifying mutual
+exclusion over multiple properties.
+
+In order to represent a `oneof` in OAS, in a way that produces JSON equivalent
+to that of protobuf, we define an extension `x-aep-oneof`:
+
+```yaml
+components:
+  schemas:
+    Document:
+      type: object
+      properties:
+        user: { type: string }
+        group: { type: string }
+        x-aep-oneof:
+          name: owner
+          properties: [user, group]
+          required: true
+```
+
+The `x-aep-oneof` extension is used to indicate that at least one of the fields
+in the `oneOf` array must be set.
+
+**Note:** OAS extension not yet defined.