I'm having trouble using `http4k-testing-chaos` to...

# http4k

[Andrew O'Hara]

I'm having trouble using http4k-testing-chaos to test a retry loop. Basically I want to make it so the first request fails and then succeeds every time after. But what seems to happen is the Behaviour (yay for British and Canadian English overlap!) gets called once during the first request, and twice during the second request. So I can't properly test the number of times my server is called. Am I doing this wrong? Or is there a http4k-testing-chaos bug?

@Test
    fun foo() {
        val server = object: ChaoticHttpHandler() {
            override val app: HttpHandler = { Response(Status.OK).body("foo") }
        }

        var requestCount = 0
        server.misbehave(object: Behaviour() {
            override fun invoke(next: HttpHandler): HttpHandler = {
                requestCount += 1
                if (requestCount == 1) {
                    Response(Status.INTERNAL_SERVER_ERROR)
                } else {
                    next(it)
                }
            }
        })

        var attempts = 0
        do {
            attempts += 1
            val response = server(Request(Method.GET, ""))
        } while(!response.status.successful)

        attempts shouldBe 2
        requestCount shouldBe 2
    }

The attempts test passes, but requestCount is actually 3

[dave]

It's been a while but there is Once that you can use maybe? https://github.com/http4k/http4k/blob/2e5d16860fe18826ae93b216d3bf31f389d1b7cd/http4k-testing/chaos/src/test/kotlin/org/http4k/chaos/ChaosTriggerTests.kt#L180

[Andrew O'Hara]

Yes, that would give me the chaos behaviour I want, but not the observability that the custom behaviour gives me

[fredrik.nordin]

@Andrew O'Hara I've had a similar requirement in one of my tests (I needed to assert attempts and total execution time) and this is the approach I used. It might help you out with a possible approach for your usecase.

private inner class TestFixture(
    config: ChaosEngine.() -> ChaosEngine = { this }
) : HttpHandler {

    var callCount = 0
    var totalTimeMs = 0L

    private val handler =
        ResponseFilters.ReportHttpTransaction { totalTimeMs = it.duration.toMillis() }
            .then(retryFilter()) // This is the bit I wanted to test
            .then { next: HttpHandler ->
                {
                    callCount++
                    next(it)
                }
            }
            .then(config(ChaosEngine()))
            .then { Response(Status.OK).body("All is OK!") }

        override fun invoke(request: Request): Response = handler(request)
    }

// In the test
val throwsExceptionForOneRequest: ChaosEngineConfigurator = {
    enable(ThrowException(NoHttpResponseException("ka-boom!")).appliedWhen(Countdown(1).not()))
}
val fixture = TestFixture(config = throwsExceptionForOneRequest)

val response = fixture(Request(Method.GET, "/"))

assertThat(response, hasStatus(Status.OK))
 assertThat(fixture.callCount, equalTo(2))

[fredrik.nordin]

I didn't use the ChaoticHttpHandler directly, but ended up writing my own test fixture that uses the Chaos Engine.

[dave]

Fun fact for people that aren't as ancient as me. The ChaosEngine was christened after this game: https://en.wikipedia.org/wiki/The_Chaos_Engine

[Andrew O'Hara]

Yeah, that game came before my time 🙃

[Andrew O'Hara]

Thanks @fredrik.nordin. I'll see about adapting that