Need help understanding why rate limiter doesn't behave like I think it should?

Hi,

I’m using resilience4j and the rate-limiting module together with Spring project reactor. The code I have looks essentially like this:

public class App {


    private final WebClient webClient;
    private final InMemoryRateLimiterRegistry rateLimiterRegistry;

    public App(int port, int rps) {
        webClient = WebClient.builder()
                .baseUrl("http://localhost:" + port)
                .build();

        rateLimiterRegistry = new InMemoryRateLimiterRegistry(RateLimiterConfig.ofDefaults());
        rateLimiterRegistry.rateLimiter("test", RateLimiterConfig.custom()
                .limitRefreshPeriod(Duration.ofSeconds(1))
                .limitForPeriod(rps)
                .timeoutDuration(Duration.ofHours(1))
                .build());
    }

    public Mono<String> makeRequest() {
        RateLimiter rateLimiter = rateLimiterRegistry.rateLimiter("test");
        Mono<String> result = webClient.post()
                .uri("/testing")
                .accept(ALL)
                .contentType(TEXT_PLAIN)
                .syncBody("hello world")
                .retrieve().bodyToMono(String.class);

        return result.transform(RateLimiterOperator.of(rateLimiter));
    }
}

I’ve tried creating a test case in which I’d like to verify that the rate limiter actually limits the requests per second to 10. The test case looks like this:

public class AppTest {

    @Rule
    public WireMockRule wireMockRule = new WireMockRule();

    @Test
    public void whyDoesntThisWorkQuestionMarkQuestionMark() {
        // Given
        int numberOfRequests = 30;
        int rps = 10;
        App app = new App(wireMockRule.port(), rps);

        wireMockRule.addStubMapping(stubFor(post(urlPathEqualTo("/testing"))
                .willReturn(aResponse().withStatus(200).withBody("hello hello!"))));

        // When
        ParallelFlux<String> flux = Flux.range(0, numberOfRequests)
                .parallel()
                .runOn(Schedulers.parallel())
                .flatMap(___ -> app.makeRequest());

        long startTime = new Date().getTime();
        StepVerifier.create(flux).expectNextCount(numberOfRequests).verifyComplete();
        long endTime = new Date().getTime();

        assertThat(endTime - startTime)
                .describedAs("I don't understand why this is not taking longer than this")
                .isGreaterThanOrEqualTo((numberOfRequests / rps) * 1000);
    }
}

But this fails with:

whyDoesntThisWorkQuestionMarkQuestionMark(se.haleby.AppTest)  Time elapsed: 2.376 sec  <<< FAILURE!
java.lang.AssertionError: [I don't understand why this is not taking longer than this] 
Expecting:
 <1466L>
to be greater than or equal to:
 <3000L> 
        at se.haleby.AppTest.whyDoesntThisWorkQuestionMarkQuestionMark(AppTest.java:43)

I don’t really understand why the test is only taking roughly 1,5 seconds instead of the expected 3 seconds (since I’m making 30 requests with an RPS of 10 requests per second). I’m probably missing something quite essential here and I’m ready to be schooled 😃.

My code can be found here: https://github.com/johanhaleby/resilience4j-ratelimiter-question

[RobWin]

I’ve created a PR where I test a delay of a subscription.

@martin-tarjanyi

Please see: https://github.com/resilience4j/resilience4j/blob/0ab113cb7caa4c60a93e13dcc97d2461c0653569/resilience4j-reactor/src/main/java/io/github/resilience4j/reactor/ratelimiter/operator/MonoRateLimiter.java#L37-L47

[johanhaleby]

@RobWin Oh, that probably explains it. I think I get it now and it makes sense. Thanks a lot for taking the time to explain this, really appreciated. And thanks for all the work on resilience4j.