NLGov REST API Design Rules

/core/naming-resources: Use nouns to name resources

Statement

Resources are referred to using nouns (instead of verbs) that are relevant from the perspective of the user of the API.

A few correct examples of nouns as part of a URI:

• Gebouw
• Vergunning

This is different than RPC-style APIs, where verbs are often used to perform certain actions:

• Opvragen
• Registreren

Rationale

Resources describe objects not actions.

Implications

Adherence to this rule needs to be manually verified.

/core/naming-collections: Use plural nouns to name collection resources

Statement

A collection resource represents multiple things.

Rationale

The path segment describing the name of the collection resource MUST be written in the plural form.

Example collection resources, describing a list of things:

https://api.example.org/v1/gebouwen
https://api.example.org/v1/vergunningen

Singular resources contained within a collection resource are generally named by appending a path segment for the identification of each individual resource.

Example singular resource, contained within a collection resource:

https://api.example.org/v1/gebouwen/3b9710c4-6614-467a-ab82-36822cf48db1
https://api.example.org/v1/vergunningen/d285e05c-6b01-45c3-92d8-5e19a946b66f

Singular resources that stand on their own, i.e. which are not contained within a collection resource, MUST be named with a path segment that is written in the singular form.

Example singular resource describing the profile of the currently authenticated user:

https://api.example.org/v1/gebruikersprofiel

Implications

Adherence to this rule needs to be manually verified.

/core/interface-language: Define interfaces in Dutch unless there is an official English glossary available

Statement

Resources and the underlying attributes SHOULD be defined in the Dutch language unless there is an official English glossary available.

Rationale

The exact meaning of concepts is often lost in translation. Publishing an API for an international audience might also be a reason to define interfaces in English. Note that glossaries exist that define useful sets of attributes which SHOULD preferably be reused. Examples can be found at schema.org.

Implications

Adherence to this rule needs to be manually verified.

/core/no-trailing-slash: Leave off trailing slashes from URIs

Statement

A URI MUST never contain a trailing slash. When requesting a resource including a trailing slash, this MUST result in a 404 (not found) error response and not a redirect. This enforces API consumers to use the correct URI.

Rationale

Leaving off trailing slashes, and not implementing a redirect, enforces API consumers to use the correct URI. This avoids confusion and ambiguity.

URI without a trailing slash (correct):

https://api.example.org/v1/gebouwen

URI with a trailing slash (incorrect):

https://api.example.org/v1/gebouwen/

Implications

This rule can be tested automatically and an example of the test is included in the automatic tests on developer.overheid.nl. The specific tests are published in the [ADR-Validator] repository.

How to test

Loop all resource paths in the OpenAPI Description and check that no resources paths end with a forward slash (/).

/core/hide-implementation: Hide irrelevant implementation details

Statement

An API SHOULD not expose implementation details of the underlying application, development platforms/frameworks or database systems/persistence models.

Rationale

• The primary motivation behind this design rule is that an API design MUST focus on usability for the client, regardless of the implementation details under the hood.
• The API, application and infrastructure need to be able to evolve independently to ease the task of maintaining backwards compatibility for APIs during an agile development process.
• The API design of Convenience,- and Process API types (as described in Aanbeveling 2 of the NL API Strategie) SHOULD not be a 1-on-1 mapping of the underlying domain- or persistence model.
• The API design of a System API type (as described in Aanbeveling 2 of the NL API Strategie) MAY be a mapping of the underlying persistence model.

Implications

• The API SHOULD not expose information about the technical components being used, such as development platforms/frameworks or database systems.
• The API SHOULD offer client-friendly attribute names and values, while persisted data may contain abbreviated terms or serializations which might be cumbersome for consumption.

/core/http-methods: Only apply standard HTTP methods

Statement

Resources MUST be retrieved or manipulated using standard HTTP methods (GET/POST/PUT/PATCH/DELETE).

Rationale

The HTTP specifications offer a set of standard methods, where every method is designed with explicit semantics. Adhering to the HTTP specification is crucial, since HTTP clients and middleware applications rely on standardized characteristics.

Method	Operation	Description
GET	Read	Retrieve a resource representation for the given URI. Data is only retrieved and never modified.
POST	Create	Create a subresource as part of a collection resource. This operation is not relevant for singular resources. This method can also be used for exceptional cases.
PUT	Create/update	Create a resource with the given URI or replace (full update) a resource when the resource already exists.
PATCH	Update	Partially updates an existing resource. The request only contains the resource modifications instead of the full resource representation.
DELETE	Delete	Remove a resource with the given URI.

Implications

This rule can be tested automatically and an example of the test is included in the automatic tests on developer.overheid.nl. The specific testscripts are published in the [ADR-Validator] repository.

The following table shows some examples of the use of standard HTTP methods:

Request	Description
GET /rijksmonumenten	Retrieves a list of national monuments.
GET /rijksmonumenten/12	Retrieves an individual national monument.
POST /rijksmonumenten	Creates a new national monument.
PUT /rijksmonumenten/12	Modifies national monument #12 completely.
PATCH /rijksmonumenten/12	Modifies national monument #12 partially.
DELETE /rijksmonumenten/12	Deletes national monument #12.

Note

The HTTP specification [rfc9110] offers a set of standard methods, where every method is designed with explicit semantics. HTTP also defines other methods, e.g. HEAD, OPTIONS, TRACE, and CONNECT.
The OpenAPI Specification 3.0 Path Item Object also supports these methods, except for CONNECT.
According to RFC 9110 9.1 the GET and HEAD HTTP methods MUST be supported by the server, all other methods are optional.
In addition to the standard HTTP methods, a server may support other optional methods as well, e.g. PROPFIND, COPY, PURGE, VIEW, LINK, UNLINK, LOCK, UNLOCK, etc.
If an optional HTTP request method is sent to a server and the server does not support that HTTP method for the target resource, an HTTP status code 405 Method Not Allowed shall be returned and a list of allowed methods for the target resource shall be provided in the Allow header in the response as stated in RFC 9110 15.5.6.

How to test

The OpenAPI Description MUST NOT include non standard HTTP methods for retrieving or manipulating resources.

/core/http-safety: Adhere to HTTP safety and idempotency semantics for operations

Statement

The following table describes which HTTP methods MUST behave as safe and/or idempotent:

Method	Safe	Idempotent
GET	Yes	Yes
HEAD	Yes	Yes
OPTIONS	Yes	Yes
POST	No	No
PUT	No	Yes
PATCH	No	No
DELETE	No	Yes

Rationale

The HTTP protocol [rfc9110] specifies whether an HTTP method SHOULD be considered safe and/or idempotent. These characteristics are important for clients and middleware applications, because they SHOULD be taken into account when implementing caching and fault tolerance strategies.

Implications

Request methods are considered safe if their defined semantics are essentially read-only; i.e., the client does not request, and does not expect, any state change on the origin server as a result of applying a safe method to a target resource. A request method is considered idempotent if the intended effect on the server of multiple identical requests with that method is the same as the effect for a single such request.

/core/http-response-code: Adhere to HTTP status codes to convey appropriate errors

Statement

Always use the semantically appropriate HTTP status code ([rfc9110]) for the response.

Rationale

The server SHOULD NOT only use 200 for success and 404 for error states. Use the semantically appropriate status code for success or failure.

In case of an error, the server SHOULD NOT pass technical details (e.g. call stacks or other internal hints) to the client. The error message SHOULD be generic to avoid revealing additional details and expose internal information which can be used with malicious intent.

Implications

Adherence to this rule needs to be manually verified.

/core/stateless: Do not maintain session state on the server

Statement

In the context of REST APIs, the server MUST not maintain or require any notion of the functionality of the client application and the corresponding end user interactions.

Rationale

To achieve full decoupling between client and server, and to benefit from the advantages mentioned above, no session state MUST reside on the server. Session state MUST therefore reside entirely on the client.

Implications

Adherence to this rule needs to be manually verified.

/core/nested-child: Use nested URIs for child resources

Statement

When having a child resource which can only exist in the context of a parent resource, the URI SHOULD be nested.

Rationale

In this use case, the child resource does not necessarily have a top-level collection resource. The best way to explain this design rule is by example.

When modelling resources for a news platform including the ability for users to write comments, it might be a good strategy to model the collection resources hierarchically:

https://api.example.org/v1/articles/123/comments

The platform might also offer a photo section, where the same commenting functionality is offered. In the same way as for articles, the corresponding sub-collection resource might be published at:

https://api.example.org/v1/photos/456/comments

These nested sub-collection resources can be used to post a new comment (POST method) and to retrieve a list of comments (GET method) belonging to the parent resource, i.e. the article or photo. An important consideration is that these comments could never have existed without the existence of the parent resource.

From the consumer's perspective, this approach makes logical sense, because the most obvious use case is to show comments below the parent article or photo (e.g. on the same web page) including the possibility to paginate through the comments. The process of posting a comment is separate from the process of publishing a new article. Another client use case might also be to show a global latest comments section in the sidebar. For this use case, an additional resource could be provided:

https://api.example.org/v1/comments

If this would have not been a meaningful use case, this resource should not exist at all. Because it doesn't make sense to post a new comment from a global context, this resource would be read-only (only GET method is supported) and may possibly provide a more compact representation than the parent-specific sub-collections.

The singular resources for comments, referenced from all 3 collections, could still be modelled on a higher level to avoid deep nesting of URIs (which might increase complexity or problems due to the URI length):

https://api.example.org/v1/comments/123
https://api.example.org/v1/comments/456

Although this approach might seem counterintuitive from a technical perspective (we simply could have modelled a single /comments resource with optional filters for article and photo) and might introduce partially redundant functionality, it makes perfect sense from the perspective of the consumer, which increases developer experience.

Implications

Adherence to this rule needs to be manually verified.

/core/resource-operations: Model resource operations as a sub-resource or dedicated resource

Statement

Model resource operations as a sub-resource or dedicated resource.

Rationale

There are resource operations which might not seem to fit well in the CRUD interaction model. For example, approving of a submission or notifying a customer. Depending on the type of the operation, there are three possible approaches:

1. Re-model the resource to incorporate extra fields supporting the particular operation. For example, an approval operation can be modelled in a boolean attribute goedgekeurd that can be modified by issuing a PATCH request against the resource. Drawback of this approach is that the resource does not contain any metadata about the operation (when and by whom was the approval given? Was the submission declined in an earlier stage?). Furthermore, this requires a fine-grained authorization model, since approval might require a specific role.
2. Treat the operation as a sub-resource. For example, model a sub-collection resource /inzendingen/12/beoordelingen and add an approval or declination by issuing a POST request. To be able to retrieve the review history (and to consistently adhere to the REST principles), also support the GET method for this resource. The /inzendingen/12 resource might still provide a goedgekeurd boolean attribute (same as approach 1) which gets automatically updated on the background after adding a review. This attribute SHOULD however be read-only.
3. In exceptional cases, the approaches above still don't offer an appropriate solution. An example of such an operation is a global search across multiple resources. In this case, the creation of a dedicated resource, possibly nested under an existing resource, is the most obvious solution. Use the imperative mood of a verb, maybe even prefix it with a underscore to distinguish these resources from regular resources. For example: /search or /_search. Depending on the operation characteristics, GET and/or POST method MAY be supported for such a resource.

Implications

Adherence to this rule needs to be manually verified.

/core/doc-openapi: Use OpenAPI Specification for documentation

Statement

API documentation MUST be provided in the form of an OpenAPI definition document which conforms to the OpenAPI Specification (from v3 onwards).

Rationale

The OpenAPI Specification (OAS) [OPENAPIS] defines a standard, language-agnostic interface to RESTful APIs which allows both humans and computers to discover and understand the capabilities of the service without access to source code, documentation, or through network traffic inspection. When properly defined, a consumer can understand and interact with the remote service with a minimal amount of implementation logic. API documentation MUST be provided in the form of an OpenAPI definition document which conforms to the OpenAPI Specification (from v3 onwards). As a result, a variety of tools can be used to render the documentation (e.g. Swagger UI or ReDoc) or automate tasks such as testing or code generation. The OAS document SHOULD provide clear descriptions and examples.

Implications

This rule can be tested automatically and an example of the test is included in the automatic tests on developer.overheid.nl. The specific tests are published in the [ADR-Validator] repository.

How to test

• Step 1: The API MUST meet the prerequisets to be tested. These include that an OAS file is publicly available, parsable, all $refs are resolvable and paths are defined.
• Step 2: Check the specification type.
• Step 3: All references MUST be publicly resolvable, including the external references.

/core/doc-openapi-contact: Document contact information for publicly available APIs

Statement

OpenAPI definition document SHOULD include the info.contact object for publicly available APIs. Contact information SHOULD NOT be a generic contact address for the whole organisation.

Rationale

The OpenAPI Specification (OAS) [OPENAPIS] can include contact information to make clear how to reach out to API owners in case of issues or questions. This is relevant for publicly available APIs (such as OData) where no pre-existing communication channel exists between provider and consumer of the API. For internal APIs (where communication channels such as chat or issue trackers are likely already known), the info.contact MAY be provided.

Relevant contact information can include an email address and issue tracker.

{
  "name": "Gebouwen API beheerder",
  "url": "https://www.github.com/ministerie/gebouwen/issues",
  "email": "teamgebouwen@ministerie.nl"
}

Implications

This rule can be tested automatically and an example of the test is included in the automatic tests on developer.overheid.nl. The specific tests are published in the [ADR-Validator] repository.

How to test

/core/doc-language: Publish documentation in Dutch unless there is existing documentation in English

Statement

You SHOULD write the OAS document in Dutch.

Rationale

In line with design rule /core/interface-language, the OAS document (e.g. descriptions and examples) SHOULD be written in Dutch. If relevant, you MAY refer to existing documentation written in English.

Implications

Adherence to this rule needs to be manually verified.

/core/publish-openapi: Publish OAS document at a standard location in JSON-format

Statement

To make the OAS document easy to find and to facilitate self-discovering clients, there SHOULD be one standard location where the OAS document is available for download.

Rationale

Clients (such as Swagger UI or ReDoc) MUST be able to retrieve the document without having to authenticate. Furthermore, the CORS policy for this URI MUST allow external domains to read the documentation from a browser environment.

The standard location for the OAS document is a URI called openapi.json or openapi.yaml within the base path of the API. This can be convenient, because OAS document updates can easily become part of the CI/CD process.

At least the JSON format MUST be supported. When having multiple (major) versions of an API, every API SHOULD provide its own OAS document(s).

An API having base path https://api.example.org/v1 MUST publish the OAS document at:

https://api.example.org/v1/openapi.json

Optionally, the same OAS document MAY be provided in YAML format:

https://api.example.org/v1/openapi.yaml

Implications

This rule can be tested automatically and an example of the test is included in the automatic tests on developer.overheid.nl. The specific tests are published in the [ADR-Validator] repository.

How to test

• Step 1: The API MUST meet the prerequisets to be tested. These include that an OAS file (openapi.json) is publicly available, parsable, all $refs are resolvable and paths are defined.
• Step 2: The openapi.yaml MAY be available. If available it MUST contain yaml, be readable and parsable.
• Step 3: The openapi.yaml MUST contain the same OpenAPI Description as the openapi.json.
• Step 4: The CORS header Access-Control-Allow-Origin MUST allow all origins.

/core/deprecation-schedule: Include a deprecation schedule when deprecating features or versions

Statement

Implement well documented and timely communicated deprecation schedules.

Rationale

Managing change is important. In general, well documented and timely communicated deprecation schedules are the most important for API users. When deprecating features or versions, a deprecation schedule MUST be published. This document SHOULD be published on a public web page. Furthermore, active clients SHOULD be informed by e-mail once the schedule has been updated or when versions have reached end-of-life.

Implications

Adherence to this rule needs to be manually verified.

/core/transition-period: Schedule a fixed transition period for a new major API version

Statement

Old versions MUST remain available for a limited and fixed deprecation period.

Rationale

When releasing a new major API version, the old version MUST remain available for a limited and fixed deprecation period. Offering a deprecation period allows clients to carefully plan and execute the migration from the old to the new API version, as long as they do this prior to the end of the deprecation period. A maximum of 2 major API versions MAY be published concurrently.

Implications

Adherence to this rule needs to be manually verified.

/core/uri-version: Include the major version number in the URI

Statement

The URI of an API MUST include the major version number.

Rationale

The URI of an API (base path) MUST include the major version number, prefixed by the letter v. This allows the exploration of multiple versions of an API in the browser. The minor and patch version numbers are not part of the URI and MAY not have any impact on existing client implementations.

An example of a base path for an API with current version 1.0.2:

https://api.example.org/v1

version: '1.0.2'

servers:
   - description: test environment
   url: https://api.test.example.org/v1
   - description: production environment
   url: https://api.example.org/v1

Implications

This rule can be tested automatically and an example of the test is included in the automatic tests on developer.overheid.nl. The specific tests are published in the [ADR-Validator] repository.

How to test

• Step 1: The base path MUST contain a version number.
• Step 2: Each url of the server object of the OpenAPI Description MUST include a version number.
• Step 3: The version in the OAS file MUST be the same as the version in the base path.

/core/changelog: Publish a changelog for API changes between versions

Statement

Publish a changelog.

Rationale

When releasing new (major, minor or patch) versions, all API changes MUST be documented properly in a publicly available changelog.

Implications

Adherence to this rule needs to be manually verified.

/core/semver: Adhere to the Semantic Versioning model when releasing API changes

Statement

Implement Semantic Versioning.

Rationale

Version numbering MUST follow the Semantic Versioning [SemVer] model to prevent breaking changes when releasing new API versions. Release versions are formatted using the major.minor.patch template (examples: 1.0.2, 1.11.0). Pre-release versions MAY be denoted by appending a hyphen and a series of dot separated identifiers (examples: 1.0.2-rc.1, 2.0.0-beta.3). When releasing a new version which contains backwards-incompatible changes, a new major version MUST be released. Minor and patch releases MAY only contain backwards compatible changes (e.g. the addition of an endpoint or an optional attribute).

Implications

This rule can be tested automatically and an example of the test is included in the automatic tests on developer.overheid.nl. The specific tests are published in the [ADR-Validator] repository.

How to test

The "API-Version" response header MUST comply with Semantic Versioning.

/core/version-header: Return the full version number in a response header

Statement

Return the API-Version header.

Rationale

Since the URI only contains the major version, it's useful to provide the full version number in the response headers for every API call. This information could then be used for logging, debugging or auditing purposes. In cases where an intermediate networking component returns an error response (e.g. a reverse proxy enforcing access policies), the version number MAY be omitted.

The version number MUST be returned in an HTTP response header named API-Version (case-insensitive) and SHOULD not be prefixed.

An example of an API version response header:

API-Version: 1.0.2

Implications

This rule can be tested automatically and an example of the test is included in the automatic tests on developer.overheid.nl. The specific tests are published in the [ADR-Validator] repository.

How to test

A response MUST include the header "API-Version".

/core/transport/tls: Secure connections using TLS

Statement

One should secure all APIs assuming they can be accessed from any location on the internet. Information MUST be exchanged over TLS-based secured connections. No exceptions, so everywhere and always. This is required by law.

One MUST follow the latest NCSC guidelines [NCSC 2021].

Rationale

Since the connection is always secured, the access method can be straightforward. This allows the application of basic access tokens instead of encrypted access tokens.

Implications

This rule can be tested automatically and an example of the test is included in the automatic tests on developer.overheid.nl. The specific tests are published in the [ADR-Validator] repository.

How to test

The usage of TLS is machine testable. Follow the latest NCSC guidelines on what is required to test. The serverside is what will be tested, only control over the server is assumed for testing. A testing client will be employed to test adherence of the server. Supporting any protocols, algorithms, key sizes, options or ciphers that are deemed insufficient or phased out by NCSC will lead to failure on the automated test. Both positive and negative scenarios are part of the test: testing that a subset of *Good* and *Sufficient* configurations are supported and configurations deemed *Insufficient* or marked for *Phase out*. A manual exception to the automated test results can be made when configurations designated for *Phase out* are supported; The API provider will have to provide clear documentation regarding the phase out schedule.

/core/transport/no-sensitive-uris: No sensitive information in URIs

Statement

Do not put any sensitive information in URIs

Rationale

Even when using TLS connections, information in URIs is not secured. URIs can be cached and logged outside of the servers controlled by clients and servers. Any information contained in them should therefore be considered readable by anyone with access to the network (in the case of the internet, the whole world) and MUST NOT contain any sensitive information. This includes client secrets used for authentication, privacy sensitive information suchs as BSNs or any other information which should not be shared.

Be aware that queries (anything after the '?' in a URI) are also part of an URI.

Implications

Adherence to this rule needs to be manually verified.

/core/geospatial: Apply the geospatial module for geospatial data

Statement

The API Design Rules Module: Geospatial version 1.0.x MUST be applied when providing geospatial data or functionality.

Rationale

The API Design Rules Module: Geospatial formalizes as set of rules regarding:

1. How to encode geospatial data in request and response payloads.
2. How resource collections can be filtered by a given bounding box.
3. How to deal with different coordinate systems (CRS).

Implications

Adherence to this rule needs to be manually verified.