Crosswalk with OGC API - Processes

crosswalks/oap-echo-example.json

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+{
+  "id": "EchoProcess",
+  "title": "Echo Process",
+  "description": "This process accepts and number of input and simple echoes each input as an output.",
+  "version": "1.0.0",
+  "jobControlOptions": [
+    "async-execute",
+    "sync-execute"
+  ],
+  "outputTransmission": [
+    "value",
+    "reference"
+  ],
+  "inputs": {
+    "stringInput": {
+      "title": "String Literal Input Example",
+      "description": "This is an example of a STRING literal input.",
+      "schema": {
+        "type": "string",
+        "enum": [
+          "Value1",
+          "Value2",
+          "Value3"
+        ]
+      },
+      "doubleInput": {
+        "title": "Bounded Double Literal Input Example",
+        "description": "This is an example of a DOUBLE literal input that is bounded between a value greater than 0 and 10.  The default value is 5.",
+        "schema": {
+          "type": "number",
+          "format": "double",
+          "minimum": 0,
+          "maximum": 10,
+          "default": 5,
+          "exclusiveMinimum": true
+        }
+      }
+    }
+  },
+  "outputs": {
+    "stringOutput": {
+      "schema": {
+        "type": "string",
+        "enum": [
+          "Value1",
+          "Value2",
+          "Value3"
+        ]
+      }
+    },
+    "doubleOutput": {
+      "schema": {
+        "type": "number",
+        "format": "double",
+        "minimum": 0,
+        "maximum": 10,
+        "default": 5,
+        "exclusiveMinimum": true
+      }
+    }
+  },
+  "links": [
+    {
+      "href": "https://processing.example.org/oapi-p/processes/EchoProcess/execution",
+      "rel": "http://www.opengis.net/def/rel/ogc/1.0/execute",
+      "title": "Execute endpoint"
+    }
+  ]
+}

crosswalks/ogcapi-processes.md

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+# Crosswalk between openEO API and OGC API - Processes
+
+This document gives a brief overview over similarities and differences between
+- **openEO API, v1.2.0**
+- **OGC API - Processes - Part 1: Core, v1.0.0**
+
+In the following I use **OAP1** as an abbreviation for **OGC API - Processes - Part 1**.
+
+## General
+
+OCG API - Processes defines just processing, while the openEO API has a much broader scope.
+openEO covers many parts that other OGC API specifications define, some are aligned some are not.
+
+The openEO API covers the following "categories" of endpoints:
+- [API discovery](#api-discovery) - partially covered by OGC API - Processes - Part 1
+- [Authentication](#authentication) - not defined by OGC
+- [Data Discovery](#data-discovery) - covered by various other OGC APIs (Coverages, EDR, Features, Records, ...)
+- [Process Discovery](#process-discovery) - covered by OGC API - Processes - Part 1
+  - [Pre-defined processes](#pre-defined-processes) - covered by OGC API - Processes - Part 1
+  - [User-defined processes](#user-defined-processes) - covered by OGC API - Processes - Part 2 and 3
+- [Data Processing](#data-processing) - covered by OGC API - Processes - Part 1 and 3
+  - [Synchronous processing](#synchronous-processing) - covered by OGC API - Processes - Part 1
+  - [Batch Job processing](#batch-job-processing) - covered by OGC API - Processes - Part 1
+  - [On-demand processing](#on-demand-processing) - covered by other OGC APIs (Maps, Tiles, ...)
+- [File Storage](#file-storage) - not covered by OGC APIs
+
+Both APIs use HTTP as the basis and encourage HTTPS.
+HTTP 1.1 is required for OAP1, while openEO doesn't specify a specific HTTP version.
+The APIs follow REST principles and make use of HTTP content negotiation.
+The APIs make broad use of "Web Linking" (compatible between OAP1 and openEO).
+Both specifications recommend the implementation of CORS.
+
+The default encoding for requests and response bodies is JSON. OGC API recommends to also support
+HTML as an response body encoding. openEO uses client software to render HTML output from JSON.
+
+Both specifications make broad use of OpenAPI 3.0 and JSON Schema for specification purposes.
+
+Many API endpoints in OAP1 and openEO support pagination through links with the relation types `next`
+and `prev`. These endpoints have a `limit` parameter to enable pagination for a specific page size.
+**Note:** Pagination is rarely used in openEO implementations and most clients don't support it consistently.
+
+## API discovery
+
+Discovering the API, connecting to it and reading the capabilities is always the first step clients
+need to execute.
+
+### Well-known document
+
+- openEO: `GET /.well-known/openeo`
+- OAP1: n/a
+
+openEO clients usually connect to the well-known document first to discover different versions or 
+implementations of a server. Thus a client can choose a suitable API versions inclusing choosing
+between production and development instances. Then the openEO client connects to the selected
+instance's [landing page](#landing-page), which OGC API - Processes clients always directly connect
+to. openEO clients can usually also directly connect to the landing page though.
+
+This folder structure with the document resides usually outside of the actual API implementations,
+i.e. in the root of the host or domain.
+
+### Landing page
+
+- openEO: `GET /` (required)
+- OAP1: `GET /` (required)
+
+As the landing pages both are based on top of OGC API - Common, they are very similar.
+
+Some differences include:
+- openEO requires the following additional fields:
+  - Defined in OAP1, but not required: `title`, `description`
+  - Not defined in OAP1: `api_version`, `backend_version`, `stac_version`, `id`, `endpoints`, (`type`)
+- The relation type to link to the conformance classes is `conformance` in openEO (originating from
+  OGC API - Features / STAC API) and `http://www.opengis.net/def/rel/ogc/1.0/conformance` in OAP1.
+  Two links with the same target but different relation types will be required.
+- The existance of API endpoints in openEO is primarily checked through `endpoints`, which is
+  not present in OAP1. OAP1 uses links, which openEO primarily uses for non-API-related links.
+  Nevertheless, links such as required in OAP1 can be added easily to an openEO implementation.
+  The following additionl links are relevant for OAP1:
+  - One of the link relations `service-desc` or `service-doc` is required in OAP1. They should link
+    to an API description (e.g. OpenAPI or rendered HTML version).
+  - A link with relation type `http://www.opengis.net/def/rel/ogc/1.0/processes` to `/processes` is 
+    required for OAP1, but not in openEO.
+  - A link with relation type `http://www.opengis.net/def/rel/ogc/1.0/job-list` to `/jobs` can be
+    added in OAP1, but is not present in openEO.
+
+### Conformanc classes
+
+- openEO: `GET /conformance` (optional)
+- OAP1: `GET /conformance` (required)
+
+Both endpoints are 100% equivalent.
+OAP1 requires a separate endpoint that lists conformance classes. 
+openEO additionally allows to list the conformance classes in the landing page (follows STAC API).
+Conformance classes are only fully defined since openEO API v1.2.0.
+
+## Authentication
+
+- openEO: `GET /credentials/basic` and/or `GET /credentials/oidc`
+- OAP1: n/a
+
+OpenEO defines two authentication mechanisms:
+- OpenID Connect (primary)
+- HTTP Basic (secondary, primarily for development/testing purposes)
+
+OAP1 doesn't define any authentication machanisms, but both authentication mechanisms can also be
+implemented for OAP1. The main issue will be that OAP1 clients will likely not support them.
+
+The availability of the authentication mechanisms is detected through the `endpoints` in the 
+[landing page](#landing-page) in openEO, while in OAP1 you need to parse the OpenAPI document for it
+(which implicitly requires a link to an OpenAPI 3.0 file with relation type `service-desc` in the
+landing page).
+
+## Data Discovery
+
+Data Discovery is not covered by OAP1, but it can be "plugged in" through various other OGC APIs
+(e.g. Coverages, EDR, Features, Records, ...). Except for OGC API - Processes - Part 3, it is not
+clear how the processes defined in OAP1 can access resources from the other APIs. The processes 
+probably need to implement data access individually through OGC API clients. As such, OAP1 could
+also be made compliant with STAC API - Collection and STAC API - Features. STAC API - Collections
+is required by openEO, STAC API - Features is optional in openEO, but would likely be required to
+allow data access through OAP1 processes.
+
+## Process Discovery
+
+The OAP1 specification claims that it 
+> does not mandate the use of any specific process description to specify the interface of a
+process.
+
+It defines a "default" encoding in the conformance class "OGC Process Description" though.
+Unfortunately, this is [a somewhat miselading statement](https://github.com/opengeospatial/ogcapi-processes/issues/325)
+and OAP1 still provides a predefined set of fields which conflict with openEO (see below).
+
+The openEO API specifies a single encoding for process descriptions.
+
+An important difference is that in OAP1 you usually execute just one process
+(you can optionally chain them using OGC API - Processes - Part 3).
+This means a process in OAP1 are usually more complex.
+In openEO a process is usually very fine-granular (e.g. addition of two numbers)
+and you usually chain multiple of them to a more complex process (graph).
+
+### Pre-defined processes
+
+- openEO: `GET /processes` (required)
+- OAP1: `GET /processes`, `GET /processes/{processID}` (both required)
+
+In openEO `GET /processes` returns the full process metadata, while in OAP1 it only returns a summary.
+The general structure of the response of `GET /processes` is the same (`processes` and `links`).
+The OGC Process Description and the openEO process description are not compatible though.
+
+openEO doesn't define an endpoint yet to retrieve an indivdual process such as
+`GET /processes/{processID}` in OAP1.
+There is a [proposal](https://github.com/Open-EO/openeo-api/pull/348) available to add such an endpoint.
+openEO has the concept of namespace for processes though and thus defines the endpoint at   
+`GET /processes/{namespace}/{process}` which would conflict with OAP1.
+
+Some notable differences in the process description (only fully delivered via `GET /processes/{processID}`):
+- OAP1 defines `jobControlOptions`, which is
+  [undefined in openEO](https://github.com/Open-EO/openeo-api/issues/429) yet
+  (which implies `["sync-execute","async-execute"]`).
+- OAP1 defines `outputTransmission`, which is not available in openEO.
+  It seems though [this will be removed](https://github.com/opengeospatial/ogcapi-processes/issues/326).
+- OAP1 allows multiple outputs, openEO only allows a single output
+  (potential workaround: wrap in an array or object)
+- OAP1 uses `title` and openEO uses `summary`
+- OAP1 specifies `inputs` as object and the identifier as key,
+  openEO specifies `parameters` as array and the identifier as `name` property
+  (but the content of each of them is otherwise very similar).
+
+Below is a simple process encoded in the two variants discussed here:
+- [OGC Process Description](oap-echo-example.json)
+- [openEO Process](openeo-echo-example.json)
+
+### User-defined processes
+
+  TBD, lower priority as it's not covered by OAP1.
+  Instead it is defined in OGC API - Processes - Part 2.
+
+## Data Processing
+
+### Synchronous processing
+
+- openEO: `POST /result`
+- OAP1: `POST /processes/{processID}/execution`
+
+TBD
+
+### Batch Job processing
+
+#### Job list
+
+- openEO: `GET /jobs`
+- OAP1: `GET /jobs`
+
+TBD
+
+#### Job status
+
+- openEO: `GET /jobs/{jobID}`
+- OAP1: `GET /jobs/{jobID}`
+
+TBD
+
+#### Creating a job
+- openEO: `POST /jobs`
+- OAP1: `POST /processes/{processID}/execution`
+
+In OAP you may provide inputs, outputs, the response type (raw or document) and/or a subscriber.
+
+In openEO you must provide a process (chained processes as process graph with optional additional metadata)
+and you may additionally provide a title, a description, a billing plan and/or a maximum budget.
+
+Both specifications seem to allow providing additional properties.
+
+#### Queueing a job
+
+- openEO: `POST /jobs/{jobID}/results`
+- OAP1: n/a (queued directly after creation)
+
+#### Result Access
+
+- openEO: `GET /jobs/{jobID}/results`
+- OAP1: `GET /jobs/{jobID}/results`
+
+In openEO result access does fully rely on STAC. The response is a valid STAC Item or Collection.
+
+In OAP1 the result access [differs depending on the given parameters](https://docs.ogc.org/is/18-062r2/18-062r2.html#toc34).
+
+### On-demand processing
+
+*On-demand processing in this context means that processing is only executed for extents shown to*
+*the user, e.g. on a web map. Results are processed "on demand" depending on the interaction with*
+*the map (e.g. how Google Earth Engine does it in their Code Editor by default).*
+
+  TBD, lower priority as it's not covered by OAP1. It's also optional and very different in openEO.
+  Instead it is defined as a separate OAP1 API implementation for individual OGC API resources.
+  For example, a combination of OGC API - Maps and OGC API - Processes.
+
+## File Storage
+
+- openEO: `/files` (various methods),  `/files/{path}` (various methods)
+- OAP1: n/a
+
+As it is neither available in OAP1 nor in any of the OGC APIs, we don't make any comparisons here.

crosswalks/openeo-echo-example.json

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+{
+  "id": "EchoProcess",
+  "summary": "Echo Process",
+  "description": "This process accepts and number of input and simple echoes each input as an output.",
+  "parameters": [
+    {
+      "name": "stringInput",
+      "description": "This is an example of a STRING literal input.",
+      "schema": {
+        "type": "string",
+        "enum": [
+          "Value1",
+          "Value2",
+          "Value3"
+        ]
+      }
+    },
+    {
+      "name": "doubleInput",
+      "description": "This is an example of a DOUBLE literal input that is bounded between a value greater than 0 and 10. The default value is 5.",
+      "schema": {
+        "type": "number",
+        "minimum": 0,
+        "maximum": 10,
+        "exclusiveMinimum": true
+      },
+      "optional": true,
+      "default": 5
+    }
+  ],
+  "returns": {
+    "schema": {
+      "type": "array",
+      "items": [
+        {
+          "type": "string",
+          "enum": [
+            "Value1",
+            "Value2",
+            "Value3"
+          ]
+        },
+        {
+          "type": "number",
+          "minimum": 0,
+          "maximum": 10,
+          "exclusiveMinimum": true
+        }
+      ]
+    }
+  },
+  "links": [
+    {
+      "href": "https://processing.example.org/openeo/processes/EchoProcess/execution",
+      "rel": "http://www.opengis.net/def/rel/ogc/1.0/execute",
+      "title": "Execute endpoint"
+    }
+  ]
+}