Kompendium

What is Kompendium

Kompendium is intended to be a minimally invasive OpenApi Specification generator for Ktor. Minimally invasive meaning that users will use only Ktor native functions when implementing their API, and will supplement with Kompendium code in order to generate the appropriate spec.

How to install

Kompendium publishes all releases to Maven Central. As such, using the stable version of Kompendium is as simple as declaring it as an implementation dependency in your build.gradle.kts

repositories {
  mavenCentral()
}

dependencies {
  // other (less cool) dependencies
  implementation("io.bkbn:kompendium-core:latest")
  implementation("io.bkbn:kompendium-auth:latest")
  implementation("io.bkbn:kompendium-swagger-ui:latest")
}

The last two dependencies are optional.

If you want to get a little spicy 🤠 every merge of Kompendium is published to the GitHub package registry. Pulling from GitHub is slightly more involved, but such is the price you pay for bleeding edge fake data generation.

// 1 Setup a helper function to import any Github Repository Package
// This step is optional but I have a bunch of stuff stored on github so I find it useful 😄
fun RepositoryHandler.github(packageUrl: String) = maven { 
    name = "GithubPackages"
    url = uri(packageUrl)
    credentials {
      username = java.lang.System.getenv("GITHUB_USER")
      password = java.lang.System.getenv("GITHUB_TOKEN")
    } 
}

// 2 Add the repo in question (in this case Kompendium)
repositories {
    github("https://maven.pkg.github.com/bkbnio/kompendium")
}

// 3 Add the package like any normal dependency
dependencies { 
    implementation("io.bkbn:kompendium-core:1.0.0")
}

In depth

Notarized Routes

Kompendium introduces the concept of notarized HTTP methods. That is, for all your GET, POST, PUT, and DELETE operations, there is a corresponding notarized method. These operations are strongly typed, and use reification for a lot of the class based reflection that powers Kompendium. Generally speaking the three types that a notarized method will consume are

TParam: Used to notarize expected request parameters
TReq: Used to build the object schema for a request body
TResp: Used to build the object schema for a response body

GET and DELETE take TParam and TResp while PUT and POST take all three.

In addition to standard HTTP Methods, Kompendium also introduced the concept of notarizedExceptions. Using the StatusPage extension, users can notarize all handled exceptions, along with their respective HTTP codes and response types. Exceptions that have been notarized require two types as supplemental information

TErr: Used to notarize the exception being handled by this use case. Used for matching responses at the route level.
TResp: Same as above, this dictates the expected return type of the error response.

In keeping with minimal invasion, these extension methods all consume the same code block as a standard Ktor route method, meaning that swapping in a default Ktor route and a Kompendium notarized route is as simple as a single method change.

Supplemental Annotations

In general, Kompendium tries to limit the number of annotations that developers need to use in order to get an app integrated.

Currently, the annotations used by Kompendium are as follows

KompendiumField
KompendiumParam

The intended purpose of KompendiumField is to offer field level overrides such as naming conventions (ie snake instead of camel).

The purpose of KompendiumParam is to provide supplemental information needed to properly assign the type of parameter (cookie, header, query, path) as well as other parameter-level metadata.

Polymorphism

Out of the box, Kompendium has support for sealed classes. At runtime, it will build a mapping of all available sub-classes and build a spec that takes anyOf the implementations. This is currently a weak point of the entire library, and suggestions on better implementations are welcome 🤠

Serialization

Under the hood, Kompendium uses Jackson to serialize the final api spec. However, this implementation detail does not leak to the actual API, meaning that users are free to choose the serialization library of their choice.

Examples

The full source code can be found in the kompendium-playground module. Here is a simple get endpoint example

// Minimal API Example
fun Application.mainModule() {
  install(StatusPages) {
    notarizedException<Exception, ExceptionResponse>(
      info = ResponseInfo(
        KompendiumHttpCodes.BAD_REQUEST,
        "Bad Things Happened"
      )
    ) {
      call.respond(HttpStatusCode.BadRequest, ExceptionResponse("Why you do dis?"))
    }
  }
  routing {
    openApi(oas)
    redoc(oas)
    swaggerUI()
    route("/potato/spud") {
      notarizedGet(simpleGetInfo) {
        call.respond(HttpStatusCode.OK)
      }
    }
  }
}

val simpleGetInfo = GetInfo<Unit, ExampleResponse>(
  summary = "Example Parameters",
  description = "A test for setting parameter examples",
  responseInfo = ResponseInfo(
    status = 200,
    description = "nice",
    examples = mapOf("test" to ExampleResponse(c = "spud"))
  ),
  canThrow = setOf(Exception::class)
)

Kompendium Auth and security schemes

There is a separate library to handle security schemes: kompendium-auth. This needs to be added to your project as dependency.

At the moment, the basic and jwt authentication is only supported.

A minimal example would be:

install(Authentication) {
  notarizedBasic("basic") {
    realm = "Ktor realm 1"
    // configure basic authentication provider..
  }
  notarizedJwt("jwt") {
    realm = "Ktor realm 2"
    // configure jwt authentication provider...
  }
}
routing {
  authenticate("basic") {
    route("/basic_auth") {
      notarizedGet(basicAuthGetInfo) {
        call.respondText { "basic auth" }
      }
    }
  }
  authenticate("jwt") {
    route("/jwt") {
      notarizedGet(jwtAuthGetInfo) {
        call.respondText { "jwt" }
      }
    }
  }
}

val basicAuthGetInfo = MethodInfo<Unit, ExampleResponse>(
  summary = "Another get test", 
  description = "testing more", 
  responseInfo = testGetResponse, 
  securitySchemes = setOf("basic")
)
val jwtAuthGetInfo = basicAuthGetInfo.copy(securitySchemes = setOf("jwt"))

Enabling Swagger ui

To enable Swagger UI, kompendium-swagger-ui needs to be added. This will also add the ktor webjars feature to your classpath as it is required for swagger ui. Minimal Example:

  install(Webjars)
  routing {
    openApi(oas)
    swaggerUI()
  }

Enabling ReDoc

Unlike swagger, redoc is provided (perhaps confusingly, in the core module). This means out of the box with kompendium-core, you can add ReDoc as follows

routing {
  openApi(oas)
  redoc(oas)
}

Limitations

Kompendium as a singleton

Currently, Kompendium exists as a Kotlin object. This comes with a couple perks, but a couple downsides. Primarily, it offers a seriously clean UX where the implementer doesn't need to worry about what instance to send data to. The main drawback, however, is that you are limited to a single API per classpath.

If this is a blocker, please open a GitHub issue, and we can start to think out solutions!

Future Work

Work on V1 of Kompendium has come to a close. This, however, does not mean it has achieved complete parity with the OpenAPI feature spec, nor does it have all-of-the nice to have features that a truly next-gen API spec should have. There are several outstanding features that have been added to the V2 Milestone. Among others, this includes

AsyncAPI Integration
Field Validation
MavenCentral Release

If you have a feature that you would like to see implemented that is not on this list, or discover a 🐞, please open an issue here

josinSbazin/kompendium