02-AREAS

unevaluatedProperties: false

This article pulls in lots of aggregated knowledge from earlier articles in the Language of API Design series: Mapping a domain model to OpenAPI Patterns for clean URLs and URL structure Defining the request body for a POST operation Defining API responses with OpenAPI response objects and response body data format with JSON Schema Composing JSON Schemas to keep the OpenAPI definition DRY Understanding the subtleties of JSON schema (notably, unevaluatedProperties) Defining reusable response objects to keep the OpenAPI definition DRY Defining how the API handles problems with client requests as well as server errors

A promise of REST APIs is good decoupling of clients and services. This is achieved in part by reducing business logic as much as possible in the client application. For example, a client application may use a form for collecting information used in a POST operation to create an API resource, or to edit an existing resource before updating it with a PUT or PATCH operations. The client then maps the form fields to the properties of the operation’s request body. Clients can use front end frameworks and libraries to perform lots of low-level validation in the front end corresponding to JSON schema constraints.

However, this only covers “syntactic” or static field-level validation. Often, an API will also have business rules that the client must follow. Secure API services will enforce those business rules in the API operations

Parse the options and (JSON) request body and return a 400 Bad Request if any of the request data is malformed (i.e. does not satisfy the constraints of the operation (such as required body or required parameters) or all the JSON Schemas associated with the operation’s parameters or request body) Verify that the caller passes valid Authorization to the API call, and return 401 Unauthorized if not Verify that the caller is authorized to perform the API operation, and return a 403 Forbidden error if not. Verify the state of the application and return 409 Conflict if the operation would put the application into an inconsistent state Verify the semantics of the request body and return a 422 Unprocessable Content error if the request is incomplete, inconsistent, or otherwise invalid

One pattern is to extend the API operations with a dry run feature. A dry run is a variant of the API operation which performs all (and only) the validation performed by the full operation, but does not execute the associated behavior. As such, it will return the same 400/401/403/409/422 that the full operation would return, allowing the client to highlight invalid form data or otherwise correct the problem. The client can use dry run operation incrementally as the user fills out a form, and disable the “Save” or “Submit” or similar UI controls if there are validation errors.

One way to implement a dry run is to create a separate "validation” operation for each API operation. This has the significant disadvantage of greatly increasing the footprint (size) of the API and adding a lot of duplication.

Rather than duplicate operations to add sibling validation operations, another approach is to add a ?dryRun=true query parameter to the operations. When used, the operation can return 204 No Content if the request contains no problems. The dryRun parameter acts as a “short circuit” in the API operation. The implementation performs the full validation it would normally do before executing the desired behavior, but then stops before actually executing anything other than the validation.

This pattern has a small impact on the API footprint compared to making sibling validation operations. A smaller footprint makes the API easier to read and understand. It is also a good use of the DRY principal, since you do not have to duplicate the definition of all the operation request parameters and request bodies, which opens up the chance for them to become out of sync.

In OAS, a reference object contains a $ref value and may contain summary or description values. However, there is a kernel of usefulness here that can be used outside of defining an API with OpenAPI — we can use a construct inspired by OAS reference objects instead of terse and cryptic Restful JSON URLs or naked resource ID properties.

Such reference objects in API responses (or requests) can include other key identifying data about the referenced resource; these are optional and informative. The id or url of the resource are required to actually identify the referenced resource. This makes the overall JSON payload more self-descriptive and does not send the developer down ratholes trying to understand data they see when exploring and learning the API.

To make these reference objects even more useful, the API service can build them for you instead of making the client construct them. For example, the response from the getUniverse operation can contain a reference or ref property which is the reference object that the client can embed in other requests when citing a universe. The *Item schema in the API’s list responses (see “What am I getting out of this?”) can also include the reference object.