Project: OpenAPI (App Definition and Development / Application Definition & Image Build)
File: 2019-12-24-Overlays.md

|Tag |Value | |- | - | |Proposal || |Authors|| |Status |Proposal| |Issues | | |Date |Responsible Party |Description | |- | - | - | | 24th December 2019 | Darrel Miller | Initial draft | | 2nd January 2019 | Darrel Miller | Update to wording around removing items from arrays. Added section on backward compatibility. Clarified process around applying a set of updates. Started to add supported scenarios.| | 29th July 2020 | Darrel Miller | Updated to be explicit about update operations | In recent months we have been discussing various use cases for overlays and various solutions. The following proposal takes a somewhat more radical approach to the problem. It is a more ambitious proposal than the others we have seen before but the additional complexity does allow for supporting many of the scenarios that have been discussed to date. An overlay document contains a list of that are to be applied to the target document. Each has a `target` property and a `value` property. The `target` property is a query that identifies what part of the target document is to be updated and the `value` property contains an object with the properties to be overlaid. This is the root object of the . Field Name | Type | Description |::| <a name="overlayVersion"></a>overlay | `string` | Version of the Overlay specification that this document conforms to. <a name="overlayInfo"></a>info | ] | Identifying information about the overlay. <a name="overlayExtends"></a>extends | `url` | URL to an OpenAPI document this overlay applies to. <a name="overlayUpdates"></a>updates | ] | A list of update objects to be applied to the target document. The list of update objects MUST be applied in sequential order to ensure a consistent outcome. Updates are applied to the result of the previous updates. This enables objects to be deleted in one update and then re-created in a subsequent update. The `extends` property can be used to indicate that the Overlay was designed to update a specific OpenAPI description. This is an optional property. Where no `extends` is provided it is the responsibility of tooling to apply the Overlay documents to the appropriate OpenAPI description. This object contains identifying information about the . Field Name | Type | Description |::| <a name="overlayTitle"></a>title | `string` | A human readable description of the purpose of the overlay. <a name="overlayVersion"></a>version | `string` | A version identifier for indicating changes to an overlay document. This object represents one or more changes to be applied to the target document at the location defined by the target

Field Name | Type | Description |::| <a name="updateTarget"></a>target | `string` | A JMESPath expression referencing the target objects in the target document. <a name="updateAdd"></a>add | | An object to be added as a child of the object(s) referenced by the target. Property has no impact if `remove` property is `true`. <a name="updateMerge"></a>merge | | An object with the properties and values to be merged with the object(s) referenced by the target. Property has no impact if `remove` property is `true`. <a name="updateRemove"></a>remove | `boolean` | A boolean value that indicates that the target object is to be removed from the the map or array it is contained in. The default value is false. The properties of the merge object MUST be compatible with the target object referenced by the JMESPath key. When the Overlay document is applied, the properties in the merge object replace properties in the target object with the same name and new properties are appended to the target object. When updating properties throughout the target document it may be more efficient to create a single `Update Object` that mirrors the structure of the target document. e.g. ```yaml overlay: 1.0.0 info: title: Structured Overlay version: 1.0.0 updates: target: "@" merge: info: x-overlay-applied: structured-overlay paths: "/": summary: "The root resource" get: summary: "Retrieve the root resource" x-rate-limit: 100 "/pets": get: summary: "Retrieve a list of pets" x-rate-limit: 100 components: tags: ``` Alternatively, where only a small number of updates need to be applied to a large document, each can be more targeted. ```yaml overlay: 1.0.0 info: title: Targeted Overlays version: 1.0.0 updates: target: paths."/foo".get merge: description: This is the new description target: paths."/bar".get merge: description: This is the updated description target: paths."/bar" merge: post: description: This is an updated description of a child object x-safe: false ``` One significant advantage of using the JMESPath syntax that it allows referencing multiple nodes in the target document. This would allow a single update object to be applied to multiple target objects using wildcards. ```yaml overlay: 1.0.0 info: title: Update many objects at once version: 1.0.0 updates: target: paths.*.get merge: x-safe: true target: paths.*.get.parameters[?name=='filter' && in=='query'] merge: schema: $ref: "/components/schemas/filterSchema" ``` Due to the fact that we can now reference specific elements of the parameter array, it allows adding parameters. Parameters can be deleted using the `remove` property. Use of indexes to remove array items should be avoided where possible as indexes will change when items are removed. ```yaml overlay: 1.0.0 info: title: Add an array element version: 1.0.0 updates: target:

add: name: newParam in: query ``` ```yaml overlay: 1.0.0 info: title: Remove a array element version: 1.0.0 updates: target: paths[*].get.parameters[? name == 'dummy'] remove: true ``` By annotating an OpenAPI description using extension such as `x-oai-traits` an author of OpenAPI description can identify where overlay updates should be applied. ```yaml openapi: 3.1.0 info: title: Api with a paged collection version: 1.0.0 paths: /items: get: x-oai-traits: ["paged"] responses: 200: description: OK ``` With the above OpenAPI description, following Overlay document will apply the necessary updates to describe how paging is implemented, where that trait has been applied. ```yaml overlay: 1.0.0 info: title: Apply Traits version: 1.0.0 updates: target: $.paths[*].get[?contains(x-traits,'paged')] merge: parameters: name: top in: query name: skip in: query ``` This approach allows flipping control of where Overlays apply updates to the OpenAPI description itself. This approach addresses the two distinct approaches of structured overlay vs targeted overlay which suits distinct but equally valid scenarios. Addresses the problem of modifying the parameters array and removes the need to replace the entire array when a small change is required. Allows sets of related overlays to be stored in a same file. Enables updating a set of objects based on a pattern. This might be an effective way of apply common behaviour across many operations in an API. Tooling will need a JMESPath implementation. Large overlays may be slow to process. Multiple complex pattern based overlays may cause overlapping updates causing confusing outcomes. JMESPath was chosen over JSONPath due to the fact that JMESPath has a and a set of test cases. This will help to ensure compatibility between implementations. Overlays will be described in a new specification that can be used alongside an OpenAPI Description, therefore there will be no compatibility issues for the initial release. Overlay documents can be used against OpenAPI v2 and v3 descriptions. Multi-language support. An Overlay document for each language is used to target a specific OpenAPI description. The Overlay document will likely use a duplicate structure to the original OpenAPI description and replace all `description` properties. Applying API wide standards. An Overlay document contains update objects that describe standard headers, parameters, responses. These documents would use JMESPath queries to target the appropriate objects in the OpenAPI description. Tooling could be used to target the OpenAPI description rather than using extends. Add tool specific OpenAPI metadata. Overlay adds additional metadata such as SLA information, client codegen hints or middleware policies. Using Overlays to manage this data separately is valuable when there is a different audience for the data and/or there the information has different sensitivity levels.