JSON Entities

The Endpoints algebra does not provide support for describing requests and responses containing JSON entities. The reason for this is that there are different algebra modules to use depending on your needs.

The following diagram summarizes which algebra to use in which case:


The first question to ask is “do I want to publish documentation of my endpoints?”. In order to document the JSON entities of your requests and responses you need a JSON schema for them, that’s why you have to use the JsonEntitiesFromSchemas algebra. See below for more details.

In case you don’t need to document the schema of your JSON entities, the second question to ask is “do I implement both a client and a server?”. If this is the case, then you need to be able to both encode and decode each JSON entity (an entity encoded by the server will be decoded by the client, and vice versa). To ensure that encoders and decoders are consistent together both have to be provided at the definition site of your endpoints, that’s why you should use the JsonEntitiesFromCodecs algebra. See below for more details.

Last, if you answered “no” to both questions, it means that your endpoints will only be served by your server and you don’t need an (abstract) algebra to describe them: you can directly use the JsonEntitiesFromEncodersAndDecoders interpreter for your specific server. With this module, you will have to provide an encoder for JSON responses, and a decoder for JSON requests.


Note that in a same service some endpoints might fall in a category, while some other endpoints might fall in a different category. For instance, if you want to publish an OpenAPI descriptor for your service, the endpoints to include in the documentation should be defined with the JsonEntitiesFromSchemas algebra, but the endpoint that serves the OpenAPI document itself will be defined with a JsonEntitiesFromEncodersAndDecoders module. See the OpenAPI interpreter documentation for an example.

The next section introduces general information about the JsonEntities hierarchy, and the remaining sections provide more details on how to use JsonEntitiesFromSchemas and JsonEntitiesFromCodecs.

The JsonEntities algebras

API documentation

The following diagram shows the relations between the three aforementioned algebras, JsonEntities, JsonEntitiesFromCodecs, and JsonEntitiesFromSchemas, and their relations with the other algebras:

The JsonEntities algebra adds to the Endpoints algebra the capability to describe JSON entities in requests and responses. The JsonEntitiesFromCodecs algebra refines the JsonEntities algebra by aligning the request and response entities to the same JsonCodec type. Finally, the JsonEntitiesFromSchemas algebra refines the JsonEntities algebra by aligning the request and response entities to the same JsonSchema type.

The JsonEntities module (and its specializations) enriches the Endpoints algebra with new constructors for request and response entities. For instance, here is how to define an endpoint taking in its request entity a JSON document for creating an user, and returning in its response entity the created user:

  post(path / "user", jsonRequest[CreateUser]),

The jsonRequest[A] constructor defines a JSON request entity containing a value of type A, provided that there exists an implicit JsonRequest[A] instance. Similarly, the jsonResponse[A] constructor defines a JSON response entity containing a value of type A provided that there exists an implicit JsonResponse[A] instance.

The JsonRequest[A] and JsonResponse[A] types are kept abstract in the JsonEntities algebra. They mean that a value of type A can be serialized in a JSON request or a JSON response, respectively.

The various specializations of the JsonEntities algebra refine the JsonRequest[A] and JsonResponse[A] types.

The JsonEntitiesFromSchemas algebra fixes both types to the same JsonSchema[A] type, which comes from the JsonSchemas algebra (see below for more details).

The JsonEntitiesFromCodecs algebra fixes both types to a same JsonCodec[A] type, which can refer to Circe’s codec or Play JSON’s codecs according to the variant of JsonEntitiesFromCodecs that you use (see below for more details).

Documentation interpreters fix both types to be a JSON schema for A.

Last, the JsonEntitiesFromEncodersAndDecoders server interpreters fix the JsonRequest[A] type to a JSON decoder for A, and the JsonResponse[A] type to a JSON encoder for A.


API documentation

This algebra merges the JsonEntities algebra and the JsonSchemas algebra and aligns both the JsonRequest[A] and JsonResponse[A] types to be JsonCodec[A], which is itself defined to the JsonSchema[A] type provided by the JsonSchemas algebra:

sourcetype JsonCodec[A] = JsonSchema[A]

This means that you have to define such a JsonSchema[A] implicit value (as explained in the JsonSchemas documentation) for each type A that you want to carry as a JSON entity.

These schemas can then be interpreted as documentation (by applying the endpoints4s.openapi.JsonEntitiesFromSchemas interpreter), or codecs (by applying a corresponding interpreter for your client or server, e.g. endpoints4s.pekkohttp.server.JsonEntitiesFromSchemas to use an Pekko HTTP server).


API documentation

In case you don’t need to document the JSON schemas of your request and response entities, the JsonEntitiesFromCodecs family of algebras is the preferred approach. These algebras fix both the JsonRequest and JsonResponse types to a same (abstract) JsonCodec type:

sourcetype JsonCodec[A]

By using the same codec type for both types ensures that the encoding and decoding are consistent.

Generally, you want to use a JsonEntitiesFromCodecs algebra that fixes this JsonCodec type to a concrete type. An example is `endpoints4s.algebra.playjson.JsonEntitiesFromCodecs`, which aligns the JsonCodec type with Play’s Format type:

sourcetype JsonCodec[A] = Format[A]

The Circe analogous is `endpoints4s.algebra.circe.JsonEntitiesFromCodecs`.

These algebras are provided by the following artifacts:

To interpret endpoints defined with such algebras, apply any interpreter named JsonEntitiesFromCodecs that matches your family of interpreters. For instance, if you use interpreters from the endpoints4s.xhr package (ie. the Scala.js web interpreters), you should use the endpoints4s.xhr.JsonEntitiesFromCodecs interpreter.