Refine and clarify operations in asynchronous caching implementation.

Uses more descriptive names for operations, especially Reactive operations, by either calling a local, private method or introducing a (functional) variable with strongly typed parameters.

Edits and refines Javadoc.

Original Pull Request: #2717

Closes #2743

Author: jxblum

Diff for: src/main/java/org/springframework/data/redis/cache/DefaultRedisCacheWriter.java

@@ -15,15 +15,13 @@
  */
 package org.springframework.data.redis.cache;
 
-import reactor.core.publisher.Flux;
-import reactor.core.publisher.Mono;
-
 import java.nio.ByteBuffer;
 import java.nio.charset.StandardCharsets;
 import java.time.Duration;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.atomic.AtomicLong;
+import java.util.function.Consumer;
 import java.util.function.Function;
 
 import org.springframework.dao.PessimisticLockingFailureException;
@@ -38,7 +36,10 @@
 import org.springframework.lang.Nullable;
 import org.springframework.util.Assert;
 import org.springframework.util.ClassUtils;
-import org.springframework.util.ObjectUtils;
+
+import reactor.core.publisher.Flux;
+import reactor.core.publisher.Mono;
+import reactor.core.publisher.SignalType;
 
 /**
  * {@link RedisCacheWriter} implementation capable of reading/writing binary data from/to Redis in {@literal standalone}
@@ -47,11 +48,11 @@
  * <p>
  * {@link DefaultRedisCacheWriter} can be used in
  * {@link RedisCacheWriter#lockingRedisCacheWriter(RedisConnectionFactory) locking} or
- * {@link RedisCacheWriter#nonLockingRedisCacheWriter(RedisConnectionFactory) non-locking} mode. While
- * {@literal non-locking} aims for maximum performance it may result in overlapping, non-atomic, command execution for
- * operations spanning multiple Redis interactions like {@code putIfAbsent}. The {@literal locking} counterpart prevents
- * command overlap by setting an explicit lock key and checking against presence of this key which leads to additional
- * requests and potential command wait times.
+ * {@link RedisCacheWriter#nonLockingRedisCacheWriter(RedisConnectionFactory) non-locking} mode. While {@literal non-locking}
+ * aims for maximum performance it may result in overlapping, non-atomic, command execution for operations spanning
+ * multiple Redis interactions like {@code putIfAbsent}. The {@literal locking} counterpart prevents command overlap
+ * by setting an explicit lock key and checking against presence of this key which leads to additional requests
+ * and potential command wait times.
  *
  * @author Christoph Strobl
  * @author Mark Paluch
@@ -61,8 +62,12 @@
  */
 class DefaultRedisCacheWriter implements RedisCacheWriter {
 
-	private static final boolean REACTIVE_REDIS_CONNECTION_FACTORY_PRESENT = ClassUtils
-			.isPresent("org.springframework.data.redis.connection.ReactiveRedisConnectionFactory", null);
+	public static final boolean FLUX_PRESENT = ClassUtils.isPresent("reactor.core.publisher.Flux", null);
+
+	private static final boolean REACTIVE_REDIS_CONNECTION_FACTORY_PRESENT =
+			ClassUtils.isPresent("org.springframework.data.redis.connection.ReactiveRedisConnectionFactory", null);
+
+	private final AsyncCacheWriter asyncCacheWriter;
 
 	private final BatchStrategy batchStrategy;
 
@@ -74,8 +79,6 @@ class DefaultRedisCacheWriter implements RedisCacheWriter {
 
 	private final TtlFunction lockTtl;
 
-	private final AsyncCacheWriter asyncCacheWriter;
-
 	/**
 	 * @param connectionFactory must not be {@literal null}.
 	 * @param batchStrategy must not be {@literal null}.
@@ -86,8 +89,8 @@ class DefaultRedisCacheWriter implements RedisCacheWriter {
 
 	/**
 	 * @param connectionFactory must not be {@literal null}.
-	 * @param sleepTime sleep time between lock request attempts. Must not be {@literal null}. Use {@link Duration#ZERO}
-	 *          to disable locking.
+	 * @param sleepTime sleep time between lock request attempts. Must not be {@literal null}.
+	 * Use {@link Duration#ZERO} to disable locking.
 	 * @param batchStrategy must not be {@literal null}.
 	 */
 	DefaultRedisCacheWriter(RedisConnectionFactory connectionFactory, Duration sleepTime, BatchStrategy batchStrategy) {
@@ -96,8 +99,8 @@ class DefaultRedisCacheWriter implements RedisCacheWriter {
 
 	/**
 	 * @param connectionFactory must not be {@literal null}.
-	 * @param sleepTime sleep time between lock request attempts. Must not be {@literal null}. Use {@link Duration#ZERO}
-	 *          to disable locking.
+	 * @param sleepTime sleep time between lock request attempts. Must not be {@literal null}.
+	 * Use {@link Duration#ZERO} to disable locking.
 	 * @param lockTtl Lock TTL function must not be {@literal null}.
 	 * @param cacheStatisticsCollector must not be {@literal null}.
 	 * @param batchStrategy must not be {@literal null}.
@@ -116,12 +119,13 @@ class DefaultRedisCacheWriter implements RedisCacheWriter {
 		this.lockTtl = lockTtl;
 		this.statistics = cacheStatisticsCollector;
 		this.batchStrategy = batchStrategy;
+		this.asyncCacheWriter = isAsyncCacheSupportEnabled() ? new AsynchronousCacheWriterDelegate()
+				: UnsupportedAsyncCacheWriter.INSTANCE;
+	}
 
-		if (REACTIVE_REDIS_CONNECTION_FACTORY_PRESENT && this.connectionFactory instanceof ReactiveRedisConnectionFactory) {
-			asyncCacheWriter = new AsynchronousCacheWriterDelegate();
-		} else {
-			asyncCacheWriter = UnsupportedAsyncCacheWriter.INSTANCE;
-		}
+	private boolean isAsyncCacheSupportEnabled() {
+		return REACTIVE_REDIS_CONNECTION_FACTORY_PRESENT && FLUX_PRESENT
+				&& this.connectionFactory instanceof ReactiveRedisConnectionFactory;
 	}
 
 	@Override
@@ -168,7 +172,8 @@ public CompletableFuture<byte[]> retrieve(String name, byte[] key, @Nullable Dur
 
 					if (cachedValue != null) {
 						statistics.incHits(name);
-					} else {
+					}
+					else {
 						statistics.incMisses(name);
 					}
 
@@ -186,8 +191,7 @@ public void put(String name, byte[] key, byte[] value, @Nullable Duration ttl) {
 		execute(name, connection -> {
 
 			if (shouldExpireWithin(ttl)) {
-				connection.stringCommands().set(key, value, Expiration.from(ttl.toMillis(), TimeUnit.MILLISECONDS),
-						SetOption.upsert());
+				connection.stringCommands().set(key, value, toExpiration(ttl), SetOption.upsert());
 			} else {
 				connection.stringCommands().set(key, value);
 			}
@@ -224,16 +228,11 @@ public byte[] putIfAbsent(String name, byte[] key, byte[] value, @Nullable Durat
 
 			try {
 
-				boolean put;
-
-				if (shouldExpireWithin(ttl)) {
-					put = ObjectUtils.nullSafeEquals(
-							connection.stringCommands().set(key, value, Expiration.from(ttl), SetOption.ifAbsent()), true);
-				} else {
-					put = ObjectUtils.nullSafeEquals(connection.stringCommands().setNX(key, value), true);
-				}
+				Boolean wasSet = shouldExpireWithin(ttl)
+						? connection.stringCommands().set(key, value, Expiration.from(ttl), SetOption.ifAbsent())
+						: connection.stringCommands().setNX(key, value);
 
-				if (put) {
+				if (Boolean.TRUE.equals(wasSet)) {
 					statistics.incPuts(name);
 					return null;
 				}
@@ -322,9 +321,11 @@ void lock(String name) {
 	private Boolean doLock(String name, Object contextualKey, @Nullable Object contextualValue,
 			RedisConnection connection) {
 
-		Expiration expiration = Expiration.from(this.lockTtl.getTimeToLive(contextualKey, contextualValue));
+		byte[] cacheLockKey = createCacheLockKey(name);
 
-		return connection.stringCommands().set(createCacheLockKey(name), new byte[0], expiration, SetOption.SET_IF_ABSENT);
+		Expiration expiration = toExpiration(contextualKey, contextualValue);
+
+		return connection.stringCommands().set(cacheLockKey, new byte[0], expiration, SetOption.SET_IF_ABSENT);
 	}
 
 	/**
@@ -378,29 +379,40 @@ private void checkAndPotentiallyWaitUntilUnlocked(String name, RedisConnection c
 				Thread.sleep(this.sleepTime.toMillis());
 			}
 		} catch (InterruptedException cause) {
-
 			// Re-interrupt current Thread to allow other participants to react.
 			Thread.currentThread().interrupt();
-
-			throw new PessimisticLockingFailureException(String.format("Interrupted while waiting to unlock cache %s", name),
-					cause);
+			String message = "Interrupted while waiting to unlock cache %s".formatted(name);
+			throw new PessimisticLockingFailureException(message, cause);
 		} finally {
 			this.statistics.incLockTime(name, System.nanoTime() - lockWaitTimeNs);
 		}
 	}
 
 	boolean doCheckLock(String name, RedisConnection connection) {
-		return ObjectUtils.nullSafeEquals(connection.keyCommands().exists(createCacheLockKey(name)), true);
+		Boolean cacheLockExists = connection.keyCommands().exists(createCacheLockKey(name));
+		return Boolean.TRUE.equals(cacheLockExists);
 	}
 
 	byte[] createCacheLockKey(String name) {
 		return (name + "~lock").getBytes(StandardCharsets.UTF_8);
 	}
 
+	private ReactiveRedisConnectionFactory getReactiveConnectionFactory() {
+		return (ReactiveRedisConnectionFactory) this.connectionFactory;
+	}
+
 	private static boolean shouldExpireWithin(@Nullable Duration ttl) {
 		return ttl != null && !ttl.isZero() && !ttl.isNegative();
 	}
 
+	private Expiration toExpiration(Duration ttl) {
+		return Expiration.from(ttl.toMillis(), TimeUnit.MILLISECONDS);
+	}
+
+	private Expiration toExpiration(Object key, @Nullable Object value) {
+		return Expiration.from(this.lockTtl.getTimeToLive(key, value));
+	}
+
 	/**
 	 * Interface for asynchronous cache retrieval.
 	 *
@@ -419,8 +431,8 @@ interface AsyncCacheWriter {
 		 * @param name the cache name from which to retrieve the cache entry.
 		 * @param key the cache entry key.
 		 * @param ttl optional TTL to set for Time-to-Idle eviction.
-		 * @return a future that completes either with a value if the value exists or completing with {@code null} if the
-		 *         cache does not contain an entry.
+		 * @return a future that completes either with a value if the value exists or completing with {@code null}
+		 * if the cache does not contain an entry.
 		 */
 		CompletableFuture<byte[]> retrieve(String name, byte[] key, @Nullable Duration ttl);
 
@@ -463,8 +475,8 @@ public CompletableFuture<Void> store(String name, byte[] key, byte[] value, @Nul
 	}
 
 	/**
-	 * Delegate implementing {@link AsyncCacheWriter} to provide asynchronous cache retrieval and storage operations using
-	 * {@link ReactiveRedisConnectionFactory}.
+	 * Delegate implementing {@link AsyncCacheWriter} to provide asynchronous cache retrieval and storage operations
+	 * using {@link ReactiveRedisConnectionFactory}.
 	 *
 	 * @since 3.2
 	 */
@@ -481,11 +493,13 @@ public CompletableFuture<byte[]> retrieve(String name, byte[] key, @Nullable Dur
 			return doWithConnection(connection -> {
 
 				ByteBuffer wrappedKey = ByteBuffer.wrap(key);
+
 				Mono<?> cacheLockCheck = isLockingCacheWriter() ? waitForLock(connection, name) : Mono.empty();
+
 				ReactiveStringCommands stringCommands = connection.stringCommands();
 
 				Mono<ByteBuffer> get = shouldExpireWithin(ttl)
-						? stringCommands.getEx(wrappedKey, Expiration.from(ttl))
+						? stringCommands.getEx(wrappedKey, toExpiration(ttl))
 						: stringCommands.get(wrappedKey);
 
 				return cacheLockCheck.then(get).map(ByteUtils::getBytes).toFuture();
@@ -498,75 +512,97 @@ public CompletableFuture<Void> store(String name, byte[] key, byte[] value, @Nul
 			return doWithConnection(connection -> {
 
 				Mono<?> mono = isLockingCacheWriter()
-						? doStoreWithLocking(name, key, value, ttl, connection)
+						? doLockStoreUnlock(name, key, value, ttl, connection)
 						: doStore(key, value, ttl, connection);
 
 				return mono.then().toFuture();
 			});
 		}
 
-		private Mono<Boolean> doStoreWithLocking(String name, byte[] key, byte[] value, @Nullable Duration ttl,
-				ReactiveRedisConnection connection) {
-
-			return Mono.usingWhen(doLock(name, key, value, connection), unused -> doStore(key, value, ttl, connection),
-					unused -> doUnlock(name, connection));
-		}
-
 		private Mono<Boolean> doStore(byte[] cacheKey, byte[] value, @Nullable Duration ttl,
 				ReactiveRedisConnection connection) {
 
 			ByteBuffer wrappedKey = ByteBuffer.wrap(cacheKey);
 			ByteBuffer wrappedValue = ByteBuffer.wrap(value);
 
-			if (shouldExpireWithin(ttl)) {
-				return connection.stringCommands().set(wrappedKey, wrappedValue,
-						Expiration.from(ttl.toMillis(), TimeUnit.MILLISECONDS), SetOption.upsert());
-			} else {
-				return connection.stringCommands().set(wrappedKey, wrappedValue);
-			}
+			ReactiveStringCommands stringCommands = connection.stringCommands();
+
+			return shouldExpireWithin(ttl)
+					? stringCommands.set(wrappedKey, wrappedValue, toExpiration(ttl), SetOption.upsert())
+					: stringCommands.set(wrappedKey, wrappedValue);
 		}
 
+		private Mono<Boolean> doLockStoreUnlock(String name, byte[] key, byte[] value, @Nullable Duration ttl,
+				ReactiveRedisConnection connection) {
+
+			Mono<Object> lock = doLock(name, key, value, connection);
+
+			Function<Object, Mono<Boolean>> store = unused -> doStore(key, value, ttl, connection);
+			Function<Object, Mono<Void>> unlock = unused -> doUnlock(name, connection);
+
+			return Mono.usingWhen(lock, store, unlock);
+		}
 
 		private Mono<Object> doLock(String name, Object contextualKey, @Nullable Object contextualValue,
 				ReactiveRedisConnection connection) {
 
-			ByteBuffer key = ByteBuffer.wrap(createCacheLockKey(name));
+			ByteBuffer key = toCacheLockKey(name);
 			ByteBuffer value = ByteBuffer.wrap(new byte[0]);
-			Expiration expiration = Expiration.from(lockTtl.getTimeToLive(contextualKey, contextualValue));
+
+			Expiration expiration = toExpiration(contextualKey, contextualValue);
 
 			return connection.stringCommands().set(key, value, expiration, SetOption.SET_IF_ABSENT) //
 					// Ensure we emit an object, otherwise, the Mono.usingWhen operator doesn't run the inner resource function.
 					.thenReturn(Boolean.TRUE);
 		}
 
 		private Mono<Void> doUnlock(String name, ReactiveRedisConnection connection) {
-			return connection.keyCommands().del(ByteBuffer.wrap(createCacheLockKey(name))).then();
+			return connection.keyCommands().del(toCacheLockKey(name)).then();
 		}
 
 		private Mono<Void> waitForLock(ReactiveRedisConnection connection, String cacheName) {
 
-			AtomicLong lockWaitTimeNs = new AtomicLong();
-			byte[] cacheLockKey = createCacheLockKey(cacheName);
+			AtomicLong lockWaitNanoTime = new AtomicLong();
 
-			Flux<Long> wait = Flux.interval(Duration.ZERO, sleepTime);
-			Mono<Boolean> exists = connection.keyCommands().exists(ByteBuffer.wrap(cacheLockKey)).filter(it -> !it);
+			Consumer<org.reactivestreams.Subscription> setNanoTimeOnLockWait = subscription ->
+					lockWaitNanoTime.set(System.nanoTime());
 
-			return wait.doOnSubscribe(subscription -> lockWaitTimeNs.set(System.nanoTime())) //
-					.flatMap(it -> exists) //
-					.doFinally(signalType -> statistics.incLockTime(cacheName, System.nanoTime() - lockWaitTimeNs.get())) //
+			Consumer<SignalType> recordStatistics = signalType ->
+					statistics.incLockTime(cacheName, System.nanoTime() - lockWaitNanoTime.get());
+
+			Function<Long, Mono<Boolean>> doWhileCacheLockExists = lockWaitTime -> connection.keyCommands()
+					.exists(toCacheLockKey(cacheName)).filter(cacheLockKeyExists -> !cacheLockKeyExists);
+
+			return waitInterval(sleepTime) //
+					.doOnSubscribe(setNanoTimeOnLockWait) //
+					.flatMap(doWhileCacheLockExists) //
+					.doFinally(recordStatistics) //
 					.next() //
 					.then();
 		}
 
+		private Flux<Long> waitInterval(Duration period) {
+			return Flux.interval(Duration.ZERO, period);
+		}
+
+		private ByteBuffer toCacheLockKey(String cacheName) {
+			return ByteBuffer.wrap(createCacheLockKey(cacheName));
+		}
+
 		private <T> CompletableFuture<T> doWithConnection(
 				Function<ReactiveRedisConnection, CompletableFuture<T>> callback) {
 
-			ReactiveRedisConnectionFactory cf = (ReactiveRedisConnectionFactory) connectionFactory;
+			Mono<ReactiveRedisConnection> reactiveConnection =
+					Mono.fromSupplier(getReactiveConnectionFactory()::getReactiveConnection);
+
+			Function<ReactiveRedisConnection, Mono<T>> commandExecution = connection ->
+					Mono.fromCompletionStage(callback.apply(connection));
+
+			Function<ReactiveRedisConnection, Mono<Void>> connectionClose = ReactiveRedisConnection::closeLater;
+
+			Mono<T> result = Mono.usingWhen(reactiveConnection, commandExecution, connectionClose);
 
-			return Mono.usingWhen(Mono.fromSupplier(cf::getReactiveConnection), //
-					it -> Mono.fromCompletionStage(callback.apply(it)), //
-					ReactiveRedisConnection::closeLater) //
-					.toFuture();
+			return result.toFuture();
 		}
 	}
 }