timeoutdigital / docless

A scala DSL to generate JSON schema and swagger documentation for your web services.

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Docless

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A scala DSL to generate JSON schema and swagger documentation for your web services.

Why not just using Swagger-core?

While being to some extent usable for Scala projects, swagger-core suffers from some serious limitations:

  • It heavily relies on Java runtime reflection to generate Json schemas for your data models. This might be fine for plain Java objects, but it does not really play well with key scala idioms such as case classes and sealed trait hierarchies.

  • Swagger is implemented through JAX-RS annotations. These provide way more limited means of abstraction and code reuse than a DSL directly embedded into Scala.

Installation

Add the following to your build.sbt

libraryDependencies += "com.timeout" %% "docless" % doclessVersion

JSON schema derivation

This project uses Shapeless to automatically derive JSON schemas for case classes and ADTs at compile time. By scraping unnecessary boilerplate code, this approach helps keeping documentation in sync with the relevant business entities.

import com.timeout.docless.schema._

case class Pet(id: Int, name: String, tag: Option[String])

val petSchema = JsonSchema.deriveFor[Pet]

Case classes

Given a case class, generating a JSON schema is as easy as calling the deriveFor method and supplying the class as type parameter.

scala> petSchema.asJson
res2: io.circe.Json =
{
  "type" : "object",
  "required" : [
    "id",
    "name"
  ],
  "properties" : {
    "id" : {
      "type" : "integer",
      "format" : "int32"
    },
    "name" : {
      "type" : "string"
    },
    "tag" : {
      "type" : "string"
    }
  }
}

The generated schema can be serialised to JSON by calling the asJson method, which will return a Circe JSON ast.

Algebraic data types

Arguably, the idea of ADT or sum type is best expressed using JsonSchema oneOf keyword. However, as Swagger UI seems to only support the allOf,
this library uses the latter as default. This can be easily overriden by defining an implicit instance of derive.Config in the local scope:

import com.timeout.docless.schema.derive.{Config, Combinator}

sealed trait Contact
case class EmailAndPhoneNum(email: String, phoneNum: String) extends Contact
case class EmailOnly(email: String) extends Contact
case class PhoneOnly(phoneNum: String) extends Contact

object Contact {
  implicit val conf: Config = Config(Combinator.OneOf)
  val schema = JsonSchema.deriveFor[Contact]
}
scala> Contact.schema.asJson
res5: io.circe.Json =
{
  "type" : "object",
  "oneOf" : [
    {
      "$ref" : "#/definitions/EmailAndPhoneNum"
    },
    {
      "$ref" : "#/definitions/EmailOnly"
    },
    {
      "$ref" : "#/definitions/PhoneOnly"
    }
  ]
}

For ADTs, as well as for case classes, the JsonSchema.relatedDefinitions
method can be used to access the child definitions referenced in a schema:

scala> Contact.schema.relatedDefinitions.map(_.id)
res6: scala.collection.immutable.Set[String] = Set(PhoneOnly, EmailOnly, EmailAndPhoneNum)

Enumerable support

Docless can automatically derive a Json schema enum for sum types consisting of case objects only:


sealed trait Diet

case object Herbivore extends Diet
case object Carnivore extends Diet
case object Omnivore extends Diet

Enumeration values can be automatically converted into a string identifier
using one of the pre-defined formats.

import com.timeout.docless.schema.PlainEnum.IdFormat

implicit val format: IdFormat = IdFormat.SnakeCase
val schema = JsonSchema.deriveEnum[Diet]
scala> schema.asJson
res10: io.circe.Json =
{
  "enum" : [
    "herbivore",
    "carnivore",
    "omnivore"
  ]
}

Finally, types that extend enumeratum EnumEntry are also supported through the EnumSchema trait:

import enumeratum._
import com.timeout.docless.schema.EnumSchema

sealed trait RPS extends EnumEntry with EnumEntry.Snakecase 

object RPS extends Enum[RPS] with EnumSchema[RPS] {
  case object Rock extends RPS
  case object Paper extends RPS
  case object Scissors extends RPS
  
  override def values = findValues
}

This trait will define on the companion object an implicit instance of
JsonSchema[RPS].

Swagger DSL

Docless provides a native scala implementation of the Swagger 2.0 model together with a DSL to easily manipulate and transform it.


import com.timeout.docless.swagger._
import com.timeout.docless.schema._

object PetsRoute extends PathGroup {
  val petResp = petSchema.asResponse("The pet")

  val petIdParam = Parameter
    .path(
      name = "id",
      description = Some("The pet id"),
      format = Some(Format.Int32)
    ).as[Int]

  override val definitions = List(petSchema, errSchema).map(_.definition)

  override val paths = List(
    "/pets/{id}"
       .Get(
         Operation(
           summary = Some("info for a specific pet")
         ).withParams(petIdParam)
          .responding(errorResponse)(200 -> petResp)
       )
       .Delete(
         Operation() //...
       )
 )
 
}

This not only provides better means of abstraction that JSON or YAML (i.e. binding, high order functions, implicit conversions, etc.), but it also allows to integrate API documentation more tightly to the application code.

Aggregating documentation from multiple modules

Aside for using Circe for JSON serialisation, Docless is not coupled to any specific Scala web framework. Nevertheless, it does provide a generic facility to enrich separate code modules with Swagger metadata, being these routes, controllers, or whatever else your framework calls them.

import com.timeout.docless.swagger._

case class Dino(name: String, extinctedSinceYears: Long, diet: Diet)

object DinosRoute extends PathGroup {

  val dinoSchema = JsonSchema.deriveFor[Dino]
  val dinoId = Parameter.path("id").as[Int]
  val dinoResp = dinoSchema.asResponse("A dinosaur!")

  override def definitions = Nil //<= this should be instead: `dinoSchema.definitions.toList`

  override def paths = List(
    "/dinos/{id}"
      .Get(
        Operation(
          summary = Some("info for a specific pet")
        ).withParams(dinoId)
         .responding(errorResponse)(200 -> dinoResp)
      )
    )
}

The PathGroup trait allows any Scala class or object to publish a list of endpoint paths and schema definitions. The aggregate method in the PathGroup companion object can then be used to merge the supplied groups into a single Swagger API description.

scala> val apiInfo = Info("Example API")
apiInfo: com.timeout.docless.swagger.Info = Info(Example API,1.0,None,None,None,None)

scala> PathGroup.aggregate(apiInfo, List(PetsRoute, DinosRoute))
res15: cats.data.ValidatedNel[com.timeout.docless.swagger.SchemaError,com.timeout.docless.swagger.APISchema] = Invalid(NonEmptyList(MissingDefinition(RefWithContext(TypeRef(Dino,None),ResponseContext(Get,/dinos/{id})))))

The aggregate method will also verify that the schema definitions referenced either in endpoint responses or in body parameters can be resolved. In the example above, the method returns a non-empty list with a single ResponseRef error, pointing to the missing Dino definition. On correct inputs, the method will return instead the resulting APISchema wrapped into a cats.data.Validated.Valid.

Known issues

Currently Docless does not support recursive types (e.g. trees or linked lists). As a way around, one can always define them manually using the JsonSchema.instance[A] method.