The DD-API V3 is an API for water management in the Netherlands. It is based on standards:

• Observations, Measurements and Sampling (in preview) is an OGC standard and soon-to-be-ISO standard for exchanging observation data.
• OData is an ISO/IEC approved OASIS standard for searching and filtering data.
• MQTT is an open OASIS standard and an ISO recommendation ISO/IEC 20922 for subscriptions in changes of data.

Another standard is integrated into this standard: SensorThings API. Management of entities (Thing and Actuator) are meshed with the reference system and it's specific data format is merged with the observation responses. Control of Thing and Actuator are kept in their original endpoints.

The OM&S implementation is a strict subset:

• Conforms to OM&S
• Is a minimal requirement
• Leaves little to no room for interpretation

The API defines three pieces of information to make sure that the experience of the user is the same across all different implementations of the DD-API V3.

• OData definition
• OAS definition
• Semantic definition

Sample code in C# for the Proof of Concept is maintained here.

All the definition specifications have a semantic version system according to semver.org. Patches are not used, only MAJOR and MINOR versions.

The MAJOR version is always the 4-digit year of the specification.

MINOR versions are not allowed to break anything for implementations that use the same MAJOR version. MINOR versions are only allowed to ADD to the specification and cannot modify behavior. The implementor MUST implement a new MAJOR version within three months of the publication here on GitHub. The implementor is encouraged to support a new MINOR version as soon as possible. Older MAJOR version MUST be supported at least for ONE YEAR after a MAJOR* version is released. This means that two MAJOR versions can co-exist.

The client can request a specific MAJOR version of OData by specifying the appropriate header:

• odata

the 'major-version' header with a specific version, i.e. 2023.

The responses will always return the specific versions for the data:

@sem=2023.01
@odata=2023.01

This information is provided once per result page.

Some other items may be given in the responses, depending on the implementation.

In addition, KennisPlaform API's Rule 57 must be followed. This means that the response header must have property API-Version set with the semantic version of the OAS specification.

The OData definition is standard across all DD-API V3 implementations. It's implementation is mandatory and specifies the minimal implementation required for data query and retrieval. This will be known as 'NL Profiel Waterbeheer'.

An OData definition is written in [Conceptual Schema Definition Language] (https://docs.oasis-open.org/odata/odata-csdl-json/v4.01/odata-csdl-json-v4.01.html) (CSDL) and defines an Entity Data Model.

Compliance may be checked by comparing the definition in this GitHub with the generated definition at the /odata/$metadata endpoint.

See CSDL for background information and the relation between EDM and CSDL.

All implementations are required to implement the following endpoints:

The query endpoints for observation and reference are mandatory.

The following functionalities are optional:

• data management
• SensorThings (STA)
• Subscription endpoints

Optional modules must follow the definitions.

Compliance may be checked by comparing the store definition in this GitHub with the generated definition at the /v3/$openapi endpoint and the /v3/odata/$metadata endpoint.

Some frameworks generate the OAS specification automatically from an OData specification. These may produce the OAS at the $openapi endpoint. However, Kennisplatform API's Rule 51 specifies that the OAS definition must be published at openapi.json or openapi.xml. An option is to copy the $openapi to openapi.json on the correct endpoint to satisfy Kennisplatform API's Rule 51.

The semantic definition specifies where references are defined. The implementation is mandatory. For instance: parameters are defined in Aquo, quantities must exist in group 'quantity' within the Aquo Parameter table, biological taxa are defined in TWN, Length classes are defined as 'length class' in Aquo. This ensures that data is the same across the systems and have the same meaning.

Organisations can have there own classifications, next to the standard definition. However, if the semantic standard defines an entity, the use of that entity is mandatory. So: standard always outweighs an organisation-standard. It is the responsibility of the implementor/data maintainer that the correct mapping is performed.

Kennisplatform API allows only authentication using OAUTH2, with two specific flows:

• Authorisation code flow (user authorisation)
• Client credentials flow (machine-to-machine)

In addition, OpenID Connect is allowed to facilitate user authorization. API keys are not allowed for authorization purposes.

GeoJSON does not support Coordinate Reference Systems (CRS) (anymore). To solve CRS issues in GeoJSON, follow https://docs.geostandaarden.nl/crs/crs/, section 3.4.2. In short: all GeoJSON instances in a response have exactly the same CRS. The default CRS is WGS84. The client can request a different CRS by adding a HTTP header named Accept-Crs with the desired CRS. The response will add an HTTP header named Content-Crs to express that the response is in a different CRS than default. The request of the client may not be honored if the server is not able to convert to the requested CRS. The API is required to support at least the following CRS by their urn:

• WGS84 (urn:ogc:def:crs:EPSG::4326)
• ETRS89 (urn:ogc:def:crs:EPSG::4258)
• RD (urn:ogc:def:crs:EPSG::28992)
• Web Mercator (urn:ogc:def:crs:EPSG::3857)

In the /Source/DotNet part of this GitHub repository, a simple implementation of a proof of concept is available in .NET 7/C# 11. It implements a simple yet functional and fast storage model using PostgreSQL 15. The implementor can, of course, implement their own storage model or extend their own platform to provide the functionality.

The management and subscription endpoints are not yet implemented in the Proof of Concept. That part is Work in Progress.

• As long as there is no official TimeseriesML JSON encoding, define our own encoding, based on om-json.
• Convert to the new Kennisplatform API's modular naming strategy.
• Link to Kennisplatform API's OData module, when available.

• 20230226: Move ObservedProperty, observingProcedure, observer and host are moved to 'Parameter', to simplify filtering. Foi stays a separate entity, since it involves geography and it is required. 'Parameter' is used to keep the search interface clean. OMS allows this as Parameter is defined as a multi-name/value property.
• 20230227: Add 'ParameterDetails' to expand the details of the parameter.
• 20230228: Add 'Metadata' to add additional information that is not referable.
• 20230301: Change PoC to new parameter specification. This also applies to the PoC suggested database model.
• 20230410: Rename UniAPI to DD-API V3, in de description and in code.
• 20230315: Revisit the UML. Base for SensorThings API and data management (including Thing and Actuator).
• 20230427: Improved documentation
• 20230510: Clarify OData => Kennisplatform API's rules