Resolving implicit connections (3.0.4 port of #3856)

versions/3.0.4.md

@@ -166,14 +166,50 @@ It is RECOMMENDED that the entry OpenAPI document be named: `openapi.json` or `o
 
 When parsing an OAD, JSON or YAML objects are parsed into specific Objects (such as [Operation Objects](#operationObject), [Response Objects](#responseObject), [Reference Objects](#referenceObject), etc.) based on the parsing context.  Depending on how references are arranged, a given JSON or YAML object can be parsed in multiple different contexts:
 
-* As a full OpenAPI Description document (an [OpenAPI Object](#oasObject) taking up an entire document)
+* As a complete OpenAPI Description document
 * As the Object type implied by its parent Object within the document
 * As a reference target, with the Object type matching the reference source's context
 
 If the same JSON/YAML object is parsed multiple times and the respective contexts require it to be parsed as _different_ Object types, the resulting behavior is _implementation defined_, and MAY be treated as an error if detected.  An example would be referencing an empty Schema Object under `#/components/schemas` where a Path Item Object is expected, as an empty object is valid for both types.  For maximum interoperability, it is RECOMMENDED that OpenAPI Description authors avoid such scenarios.
 
 #### <a name="resolvingImplicitConnections"></a>Resolving Implicit Connections
 
+Several features of this specification require resolution of non-URI-based connections to some other part of the OpenAPI Description (OAD).
+
+These connections are unambiguously resolved in single-document OADs, but the resolution process in multi-document OADs is _implementation-defined_, within the constraints described in this section.
+In some cases, an unambiguous URI-based alternative is available, and OAD authors are RECOMMENDED to always use the alternative:
+
+Source | Target | Alternative
+------ | ------ | -----------
+[Security Requirement Object](#securityRequirementObject) `{name}` |  [Security Scheme Object](#securitySchemeObject) name under the [Components Object](#componentsObject) | _n/a_
+[Discriminator Object](#discriminatorObject) `mapping` _(implicit, or explicit name syntax)_ | [Schema Object](#schemaObject) name under the Components Object | `mapping` _(explicit URI syntax)_
+[Operation Object](#operationObject) `tags` | [Tag Object](#tagObject) `name` (in the Components Object) | _n/a_
+[Link Object](#linkObject) `operationId` | [Path Item Object](#pathItemObject) `operationId` | `operationRef`
+
+A fifth implicit connection involves appending the templated URL paths of the [Paths Object](#pathsObject) to the appropriate [Server Object](#serverObject)'s `url` field.
+This is unambiguous because only the entry document's Paths Object contributes URLs to the described API.
+
+It is RECOMMENDED to consider all Operation Objects from all parsed documents when resolving any Link Object `operationId`.
+This requires parsing all referenced documents prior to determining an `operationId` to be unresolvable.
+
+The implicit connections in the Security Requirement Object and Discriminator Object rely on the _component name_, which is the property name holding the component in the appropriately typed sub-object of the Components Object.
+For example, the component name of the Schema Object at `#/components/schemas/Foo` is `Foo`.
+The implicit connection of `tags` in the Operation Object uses the `name` field of Tag Objects, which (like the Components Object) are found under the root OpenAPI Object.
+This means resolving component names and tag names both depend on starting from the correct OpenAPI Object.
+
+For resolving component and tag name connections from a referenced (non-entry) document, it is RECOMMENDED that tools resolve from the entry document, rather than the current document.
+This allows Security Scheme Objects and Tag Objects to be defined with the API's deployment information (the top-level Server Objects), and treated as an interface for referenced documents to access.
+
+The interface approach can also work for Discriminator Objects and Schema Objects, but it is also possible to keep the Discriminator Object's behavior within a single document using the relative URI-reference syntax of `mapping`.
+
+There are no URI-based alternatives for the Security Requirement Object or for the Operation Object's `tags` field.
+These limitations are expected to be addressed in a future release.
+
+See [Security Requirement in a Referenced Document](#security-requirement-in-a-referenced-document) for an example of the possible resolutions, including which one is recommended by this section.
+The behavior for Discrimator Object non-URI mappings and for the Operation Object's `tags` field operate on the same principles.
+
+Note that no aspect of implicit connection resolution changes how [URLs are resolved](#relativeReferences), or restricts their possible targets.
+
 ### <a name="dataTypes"></a>Data Types
 
 Primitive data types in the OAS are based on the types supported by the [JSON Schema Specification Wright Draft 00](https://tools.ietf.org/html/draft-wright-json-schema-00#section-4.2). 
@@ -3762,6 +3798,102 @@ security:
     - read:pets
 ```
 
+###### Security Requirement in a Referenced Document
+
+See [Resolving Implicit Connections](#resolvingImplicitConnections) for more information.
+
+First, our entry document is where parsing begins. It defines the `MySecurity` security scheme to be JWT-based, and it defines a Path Item as a reference to a component in another document:
+
+```HTTP
+GET /api/description/openapi HTTP/1.1
+Host: www.example.com
+Accept: application/openapi+json
+```
+
+```json
+"components": {
+  "securitySchemes": {
+    "MySecurity": {
+      "type": "http",
+      "scheme": "bearer",
+      "bearerFormat": "JWT"
+    }
+  }
+},
+"paths": {
+  "/foo": {
+    "$ref": "other#/components/pathItems/Foo"
+  }
+}
+```
+
+```HTTP
+GET /api/description/openapi HTTP/1.1
+Host: www.example.com
+Accept: application/openapi+yaml
+```
+
+```yaml
+components:
+  securitySchemes:
+    MySecurity:
+      type: http
+      scheme: bearer
+      bearerFormat: JWT
+paths:
+  /foo:
+    $ref: "other#/components/pathItems/Foo"
+```
+
+Next, we have our referenced document, `other`. The fact that we don't use file extensions gives the client the flexibility to choose an acceptable format on a resource-by-resource basis, assuming both representations are available:
+
+```HTTP
+GET /api/description/other HTTP/1.1
+Host: www.example.com
+Accept: application/openapi+json
+```
+
+```json
+"components": {
+  "securitySchemes": {
+    "MySecurity": {
+      "type": "http",
+      "scheme": "basic"
+    }
+  },
+  "pathItems": {
+    "Foo": {
+      "get": {
+        "security": [
+          "MySecurity": []
+        ]
+      }
+    }
+  }
+}
+```
+
+```HTTP
+GET /api/description/other HTTP/1.1
+Host: www.example.com
+Accept: application/openapi+yaml
+```
+
+```yaml
+components:
+  securitySchemes:
+    MySecurity:
+      type: http
+      scheme: basic
+  pathItems:
+    Foo:
+      get:
+        security:
+        - MySecurity: []
+```
+
+In the `other` document, the referenced path item has a Security Requirement for a Security Scheme, `MySecurity`. The same Security Scheme exists in the original entry document. As outlined in [Resolving Implicit Connections](#resolvingImplicitConnections), `MySecurity` is resolved with an [implementation-defined behavior](#undefinedAndImplementationDefinedBehavior). However, documented in that section, it is RECOMMENDED that tools resolve component names from the [entry document](#documentStructure). As with all implementation-defined behavior, it is important to check tool documentation to determine which behavior is supported.
+
 ### <a name="specificationExtensions"></a>Specification Extensions
 
 While the OpenAPI Specification tries to accommodate most use cases, additional data can be added to extend the specification at certain points.