How to pass data between various parts of the pipeline in Ktor (Kotlin)

Question

Am building an API and using intercept(ApplicationCallPipeline.Call){} to run some logic before each route execution. I need to pass data from the intercept() method to the called route and am setting data by using call.attributes.put() in the intercept() like this:

val userKey= AttributeKey<User>("userK") call.attributes.put(userKey, userData)

And retrieve userData with call.attributes[userKey] . What happens is that call.attributes[userKey] only works in the intercept() method where I have set the attribute. It doesn't work in the route where I need it. It throws me java.lang.IllegalStateException: No instance for key AttributeKey: userK

I wonder if am doing things in the right way

Answer 1

Here is the simplest code reproducing what you describe:

class KtorTest {

    data class User(val name: String)

    private val userKey = AttributeKey<User>("userK")
    private val expected = "expected name"

    private val module = fun Application.() {
        install(Routing) {
            intercept(ApplicationCallPipeline.Call) {
                println("intercept")
                call.attributes.put(userKey, User(expected))
            }

            get {
                println("call")
                val user = call.attributes[userKey]
                call.respond(user.name)
            }

        }
    }

    @Test fun `pass data`() {
        withTestApplication(module) {
            handleRequest {}.response.content.shouldNotBeNull() shouldBeEqualTo expected
        }
    }

}

I intercept the call, put the user in the attributes, and finally respond with the user in the get request. The test passes.

What ktor version are you using and which engine?