Demonstration of caching with Project Reactor and Caffeine

CachingUtils.java

import com.github.benmanes.caffeine.cache.AsyncLoadingCache;
import com.github.benmanes.caffeine.cache.Caffeine;
import com.github.benmanes.caffeine.cache.LoadingCache;
import org.jetbrains.annotations.NotNull;
import reactor.core.publisher.Mono;
import reactor.core.scheduler.Schedulers;

import java.time.Duration;
import java.util.function.Function;
import java.util.function.Supplier;

public class CachingUtils {
    private static final String FIXED_KEY = "FIXED_KEY";

    private CachingUtils() {
    }

    public static <T> Function<String, T> of(@NotNull Duration duration, @NotNull Function<String, T> fn) {
        final LoadingCache<String, T> cache = Caffeine.newBuilder()
                .expireAfterWrite(duration)
                .build((String k) -> fn.apply(k));

        return (String key) -> cache.get(key);
    }

    public static <T> Supplier<T> of(@NotNull Duration duration, @NotNull Supplier<T> supplier) {
        Function<String, T> fn = of(duration, k -> supplier.get());
        return () -> fn.apply(FIXED_KEY);
    }

    public static <T> Function<String, Mono<T>> ofMono(@NotNull Duration duration, @NotNull Function<String, Mono<T>> fn) {
        final AsyncLoadingCache<String, T> cache = Caffeine.newBuilder()
                .expireAfterWrite(duration.multipliedBy(2))
                .refreshAfterWrite(duration)
                .buildAsync((k, e) ->
                        fn.apply(k)
                                .subscribeOn(Schedulers.fromExecutor(e))
                                .toFuture());

        return (k) -> Mono.fromFuture(cache.get(k));
    }

    public static <T> Mono<T> ofMonoFixedKey(@NotNull Duration duration, @NotNull Mono<T> mono) {
        Function<String, Mono<T>> monoFn = ofMono(duration, key -> mono);
        return Mono.defer(() -> monoFn.apply(FIXED_KEY));
    }
}

CachingUtilsTest.java

import org.junit.jupiter.api.Test;
import reactor.core.publisher.Mono;
import reactor.test.StepVerifier;

import java.time.Duration;
import java.util.Random;
import java.util.concurrent.ThreadLocalRandom;
import java.util.function.Function;
import java.util.function.Supplier;

import static org.assertj.core.api.Assertions.assertThat;

class CachingUtilsTest {
    @Test
    void testSyncCaching() {
        Random random = new Random();
        Function<String, Integer> fn = (k) -> random.nextInt();
        Function<String, Integer> wrappedFn = CachingUtils.of(Duration.ofSeconds(10), fn);
        int result1 = wrappedFn.apply("key1");

        assertThat(wrappedFn.apply("key1")).isEqualTo(result1);
        assertThat(wrappedFn.apply("key1")).isEqualTo(result1);
        assertThat(wrappedFn.apply("key2")).isNotEqualTo(result1);
    }

    @Test
    void testSupplierCaching() {
        Random random = new Random();
        Supplier<Integer> fn = () -> random.nextInt();
        Supplier<Integer> wrappedSupplier = CachingUtils.of(Duration.ofSeconds(60), fn);
        int result1 = wrappedSupplier.get();

        assertThat(wrappedSupplier.get()).isEqualTo(result1);
        assertThat(wrappedSupplier.get()).isEqualTo(result1);
    }

    private Mono<String> get(String key) {
        Random random = ThreadLocalRandom.current();
        return Mono.fromSupplier(() -> key + random.nextInt());
    }

    @Test
    void testMonoCaching() {
        Function<String, Mono<String>> fn = (k) -> get(k);
        Function<String, Mono<String>> wrappedFn = CachingUtils.ofMono(Duration.ofSeconds(10), fn);
        StepVerifier.create(wrappedFn.apply("key1"))
                .assertNext(result1 -> {
                    StepVerifier.create(wrappedFn.apply("key1"))
                            .assertNext(result2 -> {
                                assertThat(result2).isEqualTo(result1);
                            })
                            .verifyComplete();
                    StepVerifier.create(wrappedFn.apply("key1"))
                            .assertNext(result2 -> {
                                assertThat(result2).isEqualTo(result1);
                            })
                            .verifyComplete();

                    StepVerifier.create(wrappedFn.apply("key2"))
                            .assertNext(result2 -> {
                                assertThat(result2).isNotEqualTo(result1);
                            })
                            .verifyComplete();
                })
                .verifyComplete();
    }

    @Test
    void testMonoCachingFixedKey() {
        Random random = new Random();
        Mono<Integer> mono = Mono.fromCallable(() -> random.nextInt());
        Mono<Integer> wrapped = CachingUtils.ofMonoFixedKey(Duration.ofSeconds(10), mono);
        StepVerifier.create(wrapped)
                .assertNext(result1 -> {
                    StepVerifier.create(wrapped)
                            .assertNext(result2 -> {
                                assertThat(result2).isEqualTo(result1);
                            })
                            .verifyComplete();
                })
                .verifyComplete();
    }
}

You can use AsyncCache so that the in-flight load is available in the cache for other threads to obtain. This avoids cache stampedes. This can be done using the future converters,

public static <T> Function<String, Mono<T>> ofMono(Duration duration, Function<String, Mono<T>> fn) {
    AsyncCache<String, T> cache = Caffeine.newBuilder()
        .expireAfterWrite(duration)
        .buildAsync();
    return key -> Mono.fromFuture(cache.get(key, k -> fn.apply(k).toFuture()));
}

public static <T> Function<String, Mono<T>> ofMono(Duration duration, Function<String, Mono<T>> fn) {
    AsyncCache<String, T> cache = Caffeine.newBuilder()
        .expireAfterWrite(duration)
        .buildAsync();
    return key -> Mono.fromFuture(cache.get(key, (k, executor) -> fn.apply(k).toFuture()));
}