Added backpressure handling to prevent memory buildup and system degredation under high load

spring-boot-admin-server/src/main/java/de/codecentric/boot/admin/server/services/IntervalCheck.java

@@ -18,12 +18,15 @@
 
 import java.time.Duration;
 import java.time.Instant;
+import java.util.Collections;
 import java.util.Map;
 import java.util.concurrent.ConcurrentHashMap;
+import java.util.function.Consumer;
 import java.util.function.Function;
 import java.util.logging.Level;
 
 import lombok.Getter;
+import lombok.NonNull;
 import lombok.Setter;
 import lombok.extern.slf4j.Slf4j;
 import org.springframework.lang.Nullable;
@@ -68,9 +71,14 @@ public class IntervalCheck {
 	@Nullable
 	private Scheduler scheduler;
 
+	@Setter
+	@NonNull
+	private Consumer<Throwable> retryConsumer;
+
 	public IntervalCheck(String name, Function<InstanceId, Mono<Void>> checkFn, Duration interval,
 			Duration minRetention, Duration maxBackoff) {
 		this.name = name;
+		this.retryConsumer = (Throwable throwable) -> log.warn("Unexpected error in {}-check", this.name, throwable);
 		this.checkFn = checkFn;
 		this.interval = interval;
 		this.minRetention = minRetention;
@@ -80,14 +88,22 @@ public IntervalCheck(String name, Function<InstanceId, Mono<Void>> checkFn, Dura
 	public void start() {
 		this.scheduler = Schedulers.newSingle(this.name + "-check");
 		this.subscription = Flux.interval(this.interval)
+			// ensure the most recent interval tick is always processed, preventing
+			// lost checks under overload.
+			.onBackpressureLatest()
 			.doOnSubscribe((s) -> log.debug("Scheduled {}-check every {}", this.name, this.interval))
-			.log(log.getName(), Level.FINEST)
-			.subscribeOn(this.scheduler)
-			.concatMap((i) -> this.checkAllInstances())
-			.retryWhen(Retry.backoff(Long.MAX_VALUE, Duration.ofSeconds(1))
-				.maxBackoff(maxBackoff)
-				.doBeforeRetry((s) -> log.warn("Unexpected error in {}-check", this.name, s.failure())))
-			.subscribe(null, (error) -> log.error("Unexpected error in {}-check", name, error));
+			.log(log.getName(), Level.FINEST) //
+			.subscribeOn(this.scheduler) //
+			// Allow concurrent check cycles if previous is slow
+			.flatMap((i) -> this.checkAllInstances(), Math.max(1, Runtime.getRuntime().availableProcessors() / 2))
+			.retryWhen(createRetrySpec())
+			.subscribe(null, (Throwable error) -> log.error("Unexpected error in {}-check", this.name, error));
+	}
+
+	private Retry createRetrySpec() {
+		return Retry.backoff(Long.MAX_VALUE, Duration.ofSeconds(1))
+			.maxBackoff(maxBackoff)
+			.doBeforeRetry((s) -> this.retryConsumer.accept(s.failure()));
 	}
 
 	public void markAsChecked(InstanceId instanceId) {
@@ -104,6 +120,10 @@ protected Mono<Void> checkAllInstances() {
 			.then();
 	}
 
+	protected Map<InstanceId, Instant> getLastChecked() {
+		return Collections.unmodifiableMap(lastChecked);
+	}
+
 	public void stop() {
 		if (this.subscription != null) {
 			this.subscription.dispose();

spring-boot-admin-server/src/test/java/de/codecentric/boot/admin/server/services/IntervalCheckTest.java

@@ -17,18 +17,28 @@
 package de.codecentric.boot.admin.server.services;
 
 import java.time.Duration;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.CopyOnWriteArrayList;
 import java.util.function.Function;
+import java.util.stream.Collectors;
+import java.util.stream.IntStream;
 
 import org.junit.jupiter.api.AfterEach;
 import org.junit.jupiter.api.Test;
+import org.mockito.invocation.InvocationOnMock;
 import reactor.core.publisher.Mono;
 
 import de.codecentric.boot.admin.server.domain.values.InstanceId;
 
+import static org.assertj.core.api.Assertions.assertThat;
 import static org.awaitility.Awaitility.await;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.atLeast;
 import static org.mockito.Mockito.atLeastOnce;
+import static org.mockito.Mockito.doAnswer;
 import static org.mockito.Mockito.mock;
 import static org.mockito.Mockito.never;
 import static org.mockito.Mockito.verify;
@@ -106,9 +116,133 @@ void should_check_after_error() {
 			.untilAsserted(() -> verify(this.checkFn, atLeast(2)).apply(InstanceId.of("Test")));
 	}
 
+	@Test
+	void should_not_overflow_when_checks_timeout_randomly() {
+		Duration CHECK_INTERVAL = Duration.ofMillis(500);
+
+		@SuppressWarnings("unchecked")
+		Function<InstanceId, Mono<Void>> timeoutCheckFn = mock(Function.class);
+
+		java.util.concurrent.atomic.AtomicInteger invocationCount = new java.util.concurrent.atomic.AtomicInteger(0);
+		doAnswer((invocation) -> {
+			if (invocationCount.getAndIncrement() % 2 == 0) {
+				// Succeed quickly on even invocations
+				return Mono.empty();
+			}
+			else {
+				// Timeout on odd invocations
+				return Mono.just("slow response").delayElement(CHECK_INTERVAL.plus(Duration.ofSeconds(1))).then();
+			}
+		}).when(timeoutCheckFn).apply(any());
+
+		IntervalCheck timeoutCheck = new IntervalCheck("overflow-test", timeoutCheckFn, CHECK_INTERVAL, CHECK_INTERVAL,
+				Duration.ofSeconds(1));
+
+		List<Throwable> retryErrors = new CopyOnWriteArrayList<>();
+
+		timeoutCheck.setRetryConsumer(retryErrors::add);
+		timeoutCheck.markAsChecked(INSTANCE_ID);
+		timeoutCheck.start();
+		try {
+			await().atMost(Duration.ofSeconds(5))
+				.until(() -> retryErrors.stream()
+					.noneMatch((Throwable er) -> "OverflowException".equalsIgnoreCase(er.getClass().getSimpleName())));
+
+			assertThat(retryErrors).noneMatch((Throwable e) -> e.getCause() != null
+					&& "OverflowException".equalsIgnoreCase(e.getCause().getClass().getSimpleName()));
+		}
+		finally {
+			timeoutCheck.stop();
+		}
+	}
+
+	@Test
+	void should_not_lose_checks_under_backpressure() {
+		Duration CHECK_INTERVAL = Duration.ofMillis(100);
+
+		@SuppressWarnings("unchecked")
+		Function<InstanceId, Mono<Void>> slowCheckFn = mock(Function.class);
+		IntervalCheck slowCheck = new IntervalCheck("backpressure-test", slowCheckFn, CHECK_INTERVAL,
+				Duration.ofMillis(50), Duration.ofSeconds(1));
+
+		List<Long> checkTimes = new CopyOnWriteArrayList<>();
+		doAnswer((invocation) -> {
+			checkTimes.add(System.currentTimeMillis());
+			return Mono.empty();
+		}).when(slowCheckFn).apply(any());
+
+		slowCheck.markAsChecked(INSTANCE_ID);
+		slowCheck.start();
+
+		try {
+			await().atMost(Duration.ofSeconds(2)).until(() -> checkTimes.size() >= 5);
+			// With onBackpressureLatest, we should have processed multiple checks without
+			// drops
+			assertThat(checkTimes).hasSizeGreaterThanOrEqualTo(5);
+		}
+		finally {
+			slowCheck.stop();
+		}
+	}
+
+	@Test
+	void should_not_lose_checks_under_backpressure_latest() {
+		Duration CHECK_INTERVAL = Duration.ofMillis(100);
+
+		@SuppressWarnings("unchecked")
+		Function<InstanceId, Mono<Void>> slowCheckFn = mock(Function.class);
+
+		IntervalCheck slowCheck = new IntervalCheck("backpressure-test", slowCheckFn, CHECK_INTERVAL, CHECK_INTERVAL,
+				Duration.ofSeconds(1));
+
+		// Add multiple instances to increase load and cause drops
+		Set<InstanceId> instanceIds = IntStream.range(0, 50)
+			.mapToObj((i) -> InstanceId.of("Test" + i))
+			.collect(Collectors.toSet());
+
+		instanceIds.forEach((InstanceId instanceId) -> slowCheck.markAsChecked(instanceId));
+
+		List<Long> checkTimes = new CopyOnWriteArrayList<>();
+		Map<String, List<Long>> checkTimesPerInstance = new ConcurrentHashMap<>();
+
+		java.util.concurrent.atomic.AtomicInteger invocationCount = new java.util.concurrent.atomic.AtomicInteger(0);
+		doAnswer((invocation) -> {
+			long checkTime = System.currentTimeMillis();
+			String instanceId = instanceIdString(invocation);
+			List<Long> checkTimesInstance = checkTimesPerInstance.computeIfAbsent(instanceId,
+					(String k) -> new CopyOnWriteArrayList<>());
+			checkTimesInstance.add(checkTime);
+			checkTimes.add(checkTime);
+			if (invocationCount.getAndIncrement() % 2 == 0) {
+				// Sometimes succeed quickly
+				return Mono.empty();
+			}
+			else {
+				// Sometimes slow
+				return Mono.delay(CHECK_INTERVAL.plus(Duration.ofMillis(500))).then();
+			}
+		}).when(slowCheckFn).apply(any());
+
+		slowCheck.start();
+
+		try {
+			await().atMost(Duration.ofSeconds(5)).until(() -> checkTimes.size() >= 500);
+			// With onBackpressureLatest, we should process more checks without drops
+			instanceIds.forEach((InstanceId instanceId) -> assertThat(checkTimesPerInstance.get(instanceId.getValue()))
+				.hasSizeGreaterThanOrEqualTo(10));
+		}
+		finally {
+			slowCheck.stop();
+		}
+	}
+
 	@AfterEach
 	void tearDown() {
 		this.intervalCheck.stop();
 	}
 
+	private static String instanceIdString(InvocationOnMock invocation) {
+		return invocation.getArguments()[0].toString();
+	}
+
 }