Add Remote UDP extension #515 (#538)

Co-authored-by: Stefaan Lippens <soxofaan@users.noreply.github.com>

CHANGELOG.md

@@ -6,6 +6,11 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased / Draft
 
+### Added
+
+- **New extensions:**
+    - [Remote UDP Extension](./extensions/remote-udp/README.md)
+
 ### Fixed
 
 - `GET /file_formats`: Base paramater on top of normal JSON Schema, not Process JSON Schema

README.md

@@ -33,6 +33,7 @@ See also the [changelog](CHANGELOG.md) and the [milestones](https://github.com/O
 | ---------------------------------------------- | ------- | ------------ | ----------- |
 | [Commercial Data](extensions/commercial-data/) | 0.1.0   | experimental | Provides an interface for discovering, ordering and using commercial data. |
 | [Federation](extensions/federation/)           | 0.1.0   | experimental | Covers federation aspects, i.e. where multiple back-ends are exposed as a single API. |
+| [Remote UDP](extensions/remote-udp/)           | 0.1.0   | experimental | Load user-defined processes that are hosted externally through the process namespace into process graphs. |
 
 ## Repository
 

extensions/remote-udp/README.md

@@ -0,0 +1,69 @@
+# Remote UDP Extension
+
+The openEO API is a specification for interoperable cloud-based processing of large Earth observation datasets.
+
+This extension enables user to load user-defined processes that are hosted external to the openEO API (e.g., GitHub or cloud storage) through the process namespace into process graphs.
+
+- Version: **0.1.0**
+- Stability: **experimental**
+- Conformance class: `https://api.openeo.org/extensions/remote-udp/0.1.0`
+
+## Justification
+
+The openEO API defines the `namespace` property in a process node of a process graph as follows:
+>  The following options are predefined by the openEO API, but additional namespaces may be introduced by back-ends or in a future version of the API.
+> * `null` [...]
+> * `backend` [...]
+> * `user` [...]
+
+This makes it possible for this extension to add additional allowed values to the `namespace` property.
+
+## Specification
+
+This extension extends the `namespace` property of process graph nodes so that it accepts **absolute** URL with the protocols `https` (**recommended**) and `http` (discouraged). The URLs specified MUST return one of the following two options:
+
+1. A single process, compatible\* to the endpoint `GET /process_graphs/{process_graph_id}`.
+   In this case, the `id` property of the process graph node MUST be equal to the `id` of the process, 
+   otherwise a `ProcessNamespaceInvalid` error is thrown
+2. A list of processes, compatible\* to the endpoint `GET /process_graphs`.
+   In this case, the `id` property of the process graph node is used to identify the process from the list.
+   If not found a `ProcessNamespaceInvalid` error is thrown
+
+\* Compatible means in this context that the requests and responses must comply to the openEO API specification with the following exceptions:
+
+- User credentials / tokens that are obtained through the openEO API MUST NOT not be sent to URIs that are external to the openEO API.
+  The requirement to provide an `Authorization` header for the respective endpoints doesn't apply.
+- For a list of processes, the full process description MUST be provided for the process with the given ID within the first request.
+  This means that the recommendation to omit large roperties such as `process_graph` doesn't apply.
+  It also requires that the requester doesn't need to paginate through additional pages to find the process with the given ID.
+  Ideally, the list of processes is not paginated as otherwise the process with the given ID may move to other pages over time.
+
+### Client Considerations
+
+If a client is conncected to a specific backend, the client MUST only offer this functionality to users if the conformance class of this extension is listed in the `conformsTo` property of the `GET /` endpoint.
+
+The protocol `http` is discouraged for URLs as web-based clients may not be able to retrieve HTTP URLs from a HTTPS context.
+For the same reason it is also RECOMMENDED to enable CORS for all URLs.
+
+### Error Handling
+
+The following error SHOULD be reported if the namespace can't be resolved:
+
+- Code: `ProcessNamespaceInvalid`
+- Message: `The value passed for namespace '{namespace}' in process '{process}' is invalid: {reason}`
+- HTTP Status Code: 400
+
+## Example
+
+An exemplary process graph node:
+
+```json
+{
+  "process_id": "echo",
+  "namespace": "https://openeo.example/processes/echo",
+  "arguments": {
+    "message": "Hello World"
+  },
+  "result": true
+}
+```