package com.ankurm.vthreadswebflux;

import org.junit.jupiter.api.Test;
import reactor.test.StepVerifier;

import java.time.Duration;

/**
 * Proves the backpressure claim in docs/01-webflux-benchmark-methodology.md is real rather
 * than asserted: a subscriber that requests only 3 items at a time never receives a 4th until
 * it asks. StepVerifier.create(flux, 3) starts the subscription with an initial request of 3
 * (not unbounded, which is StepVerifier's default) -- if ReactiveDemoController.streamResults()
 * ignored backpressure and pushed everything immediately, this test would see events beyond
 * the first 3 before the additional .thenRequest(...) calls run, and StepVerifier would fail
 * the sequence.
 *
 * Output: docs/output/04-stream-backpressure.txt.
 */
class StreamBackpressureTest {

    @Test
    void subscriberControlsEmissionRate() {
        ReactiveDemoController controller = new ReactiveDemoController();

        StepVerifier.create(controller.streamResults(), 3)
                .expectNext("event-0", "event-1", "event-2")
                .expectNoEvent(Duration.ofMillis(80)) // no 4th item until we ask for one
                .thenRequest(2)
                .expectNext("event-3", "event-4")
                .thenRequest(45)
                .expectNextCount(45)
                .expectComplete()
                .verify(Duration.ofSeconds(10));

        Transcript t = Transcript.start("04-stream-backpressure.txt",
                "Flux backpressure proof: /stream, requested in batches of 3, 2, then 45");
        t.line("StepVerifier.create(controller.streamResults(), 3)");
        t.line("  .expectNext(\"event-0\", \"event-1\", \"event-2\")");
        t.line("  .expectNoEvent(Duration.ofMillis(80))   -- no 4th item arrives without a request");
        t.line("  .thenRequest(2).expectNext(\"event-3\", \"event-4\")");
        t.line("  .thenRequest(45).expectNextCount(45)");
        t.line("  .expectComplete()");
        t.blank();
        t.line("RESULT: verified -- the flux emitted exactly as many items as were requested, in");
        t.line("the order requested, with no items arriving ahead of a pending request. This is");
        t.line("what \"backpressure is part of the Flux contract\" means concretely: the subscriber,");
        t.line("not the producer, controls the emission rate.");
        t.save();
    }
}