Copyright © NaN the Linux Foundation
The Overlay Specification is an extension or companion to the [OpenAPI] specification. An Overlay describes a set of changes to be applied or “overlaid” onto an existing OpenAPI description.
The main purpose of the Overlay Specification is to provide a way to repeatably apply transformations to one or many OpenAPI descriptions. Use cases include updating descriptions, adding metadata to be consumed by another tool, or removing certain elements from an API description before sharing it with partners. An Overlay may be specific to a single OpenAPI description, or be designed to apply the same transform to any OpenAPI description.
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.
This document is licensed under The Apache License, Version 2.0.
The Overlay Specification is an extension or companion to the [OpenAPI] specification. An Overlay describes a set of changes to be applied or “overlaid” onto an existing OpenAPI description.
The main purpose of the Overlay Specification is to provide a way to repeatably apply transformations to one or many OpenAPI descriptions. Use cases include updating descriptions, adding metadata to be consumed by another tool, or removing certain elements from an API description before sharing it with partners. An Overlay may be specific to a single OpenAPI description, or be designed to apply the same transform to any OpenAPI description.
An Overlay is a JSON or YAML structure containing an ordered list of Action Objects that are to be applied to the target document. Each Action Object has a target property and a modifier type (update or remove). The target property is a JSONPath query expression that identifies the elements of the target document to be updated and the modifier determines the change.
The Overlay Specification is versioned using a major.minor.patch versioning scheme. The major.minor portion of the version string (for example 1.0) SHALL designate the Overlay feature set. .patch versions address errors in, or provide clarifications to, this document, not the feature set. The patch version SHOULD NOT be considered by tooling, making no distinction between 1.0.0 and 1.0.1 for example.
Note: Version 1.0.0 of the Overlay Specification is being released after spending some time in draft, and after being adopted by tool providers. Check with your tool provider for the details of what is supported in each tool.
An Overlay document that conforms to the Overlay Specification is itself a JSON object, which may be represented either in JSON or YAML format.
All field names in the specification are case sensitive. This includes all fields that are used as keys in a map, except where explicitly noted that keys are case insensitive.
In order to preserve the ability to round-trip between YAML and JSON formats, YAML version 1.2 is RECOMMENDED along with some additional constraints:
It is RECOMMENDED that the root Overlay document be named: overlay.json or overlay.yaml.
Unless specified otherwise, all properties that are URLs MAY be relative references as defined by [RFC3986] Section 4.2. Unless specified otherwise, relative references are resolved using the URL of the referring document.
In the following description, if a field is not explicitly REQUIRED or described with a MUST or SHALL, it can be considered OPTIONAL.
This is the root object of the Overlay.
| Field Name | Type | Description |
|---|---|---|
| overlay | string |
REQUIRED. This string MUST be the version number of the Overlay Specification that the Overlay document uses. The overlay field SHOULD be used by tooling to interpret the Overlay document. |
| info | Info Object | REQUIRED. Provides metadata about the Overlay. The metadata MAY be used by tooling as required. |
| extends | string |
URL to the target document (such as an [OpenAPI] document) this overlay applies to. This MUST be in the form of a URL. |
| actions | [Action Object] | REQUIRED An ordered list of actions to be applied to the target document. The array MUST contain at least one value. |
This object MAY be extended with Specification Extensions.
The list of actions MUST be applied in sequential order to ensure a consistent outcome. Actions 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, for example.
The extends property can be used to indicate that the Overlay was designed to update a specific [OpenAPI] document. Where no extends is provided it is the responsibility of tooling to apply the Overlay documents to the appropriate OpenAPI document(s).
The object provides metadata about the Overlay. The metadata MAY be used by the clients if needed.
| Field Name | Type | Description |
|---|---|---|
| title | string |
REQUIRED. A human readable description of the purpose of the overlay. |
| version | string |
REQUIRED. A version identifer for indicating changes to the Overlay document. |
This object MAY be extended with Specification Extensions.
This object represents one or more changes to be applied to the target document at the location defined by the target JSONPath expression.
| Field Name | Type | Description |
|---|---|---|
| target | string |
REQUIRED A JSONPath expression selecting nodes in the target document. |
| description | string |
A description of the action. [CommonMark] syntax MAY be used for rich text representation. |
| update | Any | If the target selects an object node, the value of this field should be an object with the properties and values to merge with the node. If the target selects an array, the value of this field should be an entry to append to the array. This field has no impact if the remove field of this action object is true. |
| 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 result of the target JSONPath expression must be zero or more objects or arrays (not primitive types or null values).
To update a primitive property value such as a string, the target expression should select the containing object in the target document and update should contain an object with the property and its new primitive value.
Primitive-valued items of an array cannot be replaced or removed individually, only the complete array can be replaced.
The properties of the update object MUST be compatible with the target object referenced by the JSONPath 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.
This object MAY be extended with Specification Extensions.
When updating properties throughout the target document it may be more efficient to create a single Action Object that mirrors the structure of the target document. e.g.
overlay: 1.0.0
info:
title: Structured Overlay
version: 1.0.0
actions:
- target: '$' # Root of document
update:
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 Action Object MAY be more targeted.
overlay: 1.0.0
info:
title: Targeted Overlays
version: 1.0.0
actions:
- target: $.paths['/foo'].get
update:
description: This is the new description
- target: $.paths['/bar'].get
update:
description: This is the updated description
- target: $.paths['/bar']
update:
post:
description: This is an updated description of a child object
x-safe: false
One significant advantage of using the JSONPath syntax is 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 and other multi-value selectors.
overlay: 1.0.0
info:
title: Update many objects at once
version: 1.0.0
actions:
- target: $.paths.*.get
update:
x-safe: true
- target: $.paths.*.get.parameters[?@.name=='filter' && @.in=='query']
update:
schema:
$ref: '/components/schemas/filterSchema'
Array elements MAY be deleted using the remove property. Use of array indexes to remove array items should be avoided where possible as indexes will change when items are removed.
overlay: 1.0.0
info:
title: Add an array element
version: 1.0.0
actions:
- target: $.paths.*.get.parameters
update:
name: newParam
in: query
overlay: 1.0.0
info:
title: Remove a array element
version: 1.0.0
actions:
- target: $.paths.*.get.parameters[?@.name == 'dummy']
remove: true
By annotating a target document (such as an [OpenAPI] document) using specification extensions such as x-oai-traits, the author of the target document MAY identify where overlay updates should be applied.
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 document, the following Overlay document will apply the necessary updates to describe how paging is implemented, where that trait has been applied.
overlay: 1.0.0
info:
title: Apply Traits
version: 1.0.0
actions:
- target: $.paths.*.get[?@.x-oai-traits.paged]
update:
parameters:
- name: top
in: query
# ...
- name: skip
in: query
# ...
This approach allows inversion of control as to where the Overlay updates apply to the target document itself.
While the Overlay Specification tries to accommodate most use cases, additional data can be added to extend the specification at certain points.
The extension properties are implemented as patterned fields that are always prefixed by "x-".
| Field Pattern | Type | Description |
|---|---|---|
| ^x- | Any | Allows extensions to the Overlay Specification. The field name MUST begin with x-, for example, x-internal-id. Field names beginning x-oai- and x-oas- are reserved for uses defined by the OpenAPI Initiative. The value MAY be null, a primitive, an array or an object. |
The extensions may or may not be supported by the available tooling, but those may be extended as well to add requested support (if tools are internal or open-sourced).
| Version | Date | Notes |
|---|---|---|
| 1.0.0 | TBD | First release of the Overlay Specification |