Hi, I’m implementing a service providing endpoints...

# ktor

[cpe]

Hi, I’m implementing a service providing endpoints with and without authentication. After repeating the same procedures for some endpoints I wonder if there is a better way in KTor providing repeating context information for services, let me try to give an abstract example:

routing {
  get("/test") {
    call.respondText("test called")
  }
  authenticate("auth-jwt") {
    route("/featureA") {
      get("/") {
        val principalSubject = call.principal<JWTPrincipal>()?.subject
        featureAService.handleGet(call, principalSubject)
      }
      post("/") {
        val principalSubject = call.principal<JWTPrincipal>()?.subject
        featureAService.handlePost(call, principalSubject)
      }
      put("/") {
        val principalSubject = call.principal<JWTPrincipal>()?.subject
        featureAService.handlePut(call, principalSubject)
      }
    }
    route("/featureB") {
      get("/") {
        val principalSubject = call.principal<JWTPrincipal>()?.subject
        featureBService.handleGet(call, principalSubject)
      } 
    }
  }  
}

In this case I’d like to provide the principalSubject to each method (in reality there are a few more context parameters that are shared across those methods like parameters etc.). I don’t want to start each method with retrieving unsers and other context parameters and I also don’t want to repeat those lines in the routing section. Is there any way to reduce the amount of code here by having some preprocessor (could then use context-receivers)?

[August Lilleaas]

you can create a function that returns a lambda that does the repeating tasks

[August Lilleaas]

so, something like this:

fun myWrapper(
    handler: suspend PipelineContext<Unit, ApplicationCall>.(
        principalSubject: String?
    ) -> Unit
): PipelineInterceptor<Unit, ApplicationCall> {
    return {
        val principalSubject = call.principal<JWTPrincipal>()?.subject
        this.handler(principalSubject)
    }
}

[August Lilleaas]

you can invoke it like this:

get("/foo", myWrapper { principal -> 
    featureAService.handleFoo(call, principal)
})

[Rustam Siniukov]

You can also create a simple plugin that will extract auth data and add it to the attributes of the call

[cpe]

Thanks for the solutions. I tried the first one but that will end in repeating the same thing over and over again (in fact I use a class for holding my context values). So I’ll check out how to add it to the attributes of the call.

[Rustam Siniukov]

it should look something like this

val plugin = createRouteScopedPlugin("myPlugin") {

  on(AuthenticationChecked) { call ->
    val data = call.principal.extractData() 
    call.attributes.put(MyKey, data)
  }
}

[Rustam Siniukov]

and you also can create an extension to make it easier on calling side like

val ApplicationCall.myAuthData
  get() = attributes.get(MyKey)

[simon.vergauwen]

I write small typed middleware functions for this, https://github.com/nomisRev/ktor-arrow-example/blob/main/src/main/kotlin/io/github/nomisrev/auth/jwt.kt

And then you can wrap your routes with service.auth { token -> }, and you can easily switch out implementations in testing as well.

[August Lilleaas]

for this particular case you could even just write an extension function on call that calls call.principal<JWTPrincipal>()?.subject

[August Lilleaas]

(call-ception)

[simon.vergauwen]

This still requires you to deal with nullability everywhere 😭

[August Lilleaas]

I’d probably throw in a !! for that one

[August Lilleaas]

I don’t mind a 500 if i try to ask for auth stuff outside of authenticated routes

[simon.vergauwen]

It definitely works. I don't use !! anywhere, the errors it throws are much too vague to debug on Grafana. Additionally, I found it confusing to distinct in this way between required -and optional authentication.

Given the small size in which you can wrap Ktor to provide a more meaningful domain DSL is totally worth it for me.

[simon.vergauwen]

I also had a need for suspend while verifying the JWT tokens, which is not supported in the default authentication validator IIRC.

[cpe]

I’d like to go with the approach of @Rustam Siniukov, but somehow the authentication / principal is null. I installed the plugin in my module section within the embeddedServer. But when calling

call.principal<JWTPrincipal>()

is null (and there is no principal.extractData() function).

When I receive the call within my routing configuration it is set properly and contains a JWTPrincipal.

[cpe]

Okay, my fault. I did not install the plugin within the routeScope but on application level. After installing it within routeScope as the name says it works like a charm. Thanks all for support and the great ideas.