Experiment work stealing pools

reactor-pool/build.gradle

@@ -127,6 +127,7 @@ task japicmp(type: JapicmpTask) {
 	classExcludes = [
 	]
 	methodExcludes = [
+			"reactor.pool.decorators.InstrumentedPoolDecorators#concurrentPools(int, org.reactivestreams.Publisher, java.util.function.Function)"
 	]
 }
 check.dependsOn japicmp

reactor-pool/src/main/java/reactor/pool/AbstractPool.java

@@ -22,6 +22,7 @@
 import java.time.Duration;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicIntegerFieldUpdater;
+import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Function;
 
 import org.reactivestreams.Publisher;
@@ -390,7 +391,7 @@ static final class Borrower<POOLABLE> extends AtomicBoolean implements Scannable
 		static final Disposable TIMEOUT_DISPOSED = Disposables.disposed();
 
 		final CoreSubscriber<? super AbstractPooledRef<POOLABLE>> actual;
-		final AbstractPool<POOLABLE> pool;
+		final AtomicReference<AbstractPool<POOLABLE>> pool;
 		final Duration pendingAcquireTimeout;
 
 		long pendingAcquireStart;
@@ -400,7 +401,7 @@ static final class Borrower<POOLABLE> extends AtomicBoolean implements Scannable
 				AbstractPool<POOLABLE> pool,
 				Duration pendingAcquireTimeout) {
 			this.actual = actual;
-			this.pool = pool;
+			this.pool = new AtomicReference<>(pool);
 			this.pendingAcquireTimeout = pendingAcquireTimeout;
 			this.timeoutTask = TIMEOUT_DISPOSED;
 		}
@@ -414,7 +415,7 @@ public void run() {
 			if (Borrower.this.compareAndSet(false, true)) {
 				// this is failure, a timeout was observed
 				stopPendingCountdown(false);
-				pool.cancelAcquire(Borrower.this);
+				pool().cancelAcquire(Borrower.this);
 				actual.onError(new PoolAcquireTimeoutException(pendingAcquireTimeout));
 			}
 		}
@@ -423,7 +424,7 @@ public void run() {
 		public void request(long n) {
 			if (Operators.validate(n)) {
 				// doAcquire will check for acquire timeout
-				pool.doAcquire(this);
+				pool().doAcquire(this);
 			}
 		}
 
@@ -432,6 +433,7 @@ public void request(long n) {
 		 */
 		void stopPendingCountdown(boolean success) {
 			if (pendingAcquireStart > 0) {
+				AbstractPool<POOLABLE> pool = pool();
 				if (success) {
 					pool.metricsRecorder.recordPendingSuccessAndLatency(pool.clock.millis() - pendingAcquireStart);
 				} else {
@@ -446,7 +448,7 @@ void stopPendingCountdown(boolean success) {
 		@Override
 		public void cancel() {
 			set(true);
-			pool.cancelAcquire(this);
+			pool().cancelAcquire(this);
 			stopPendingCountdown(true); // this is not failure, the subscription was canceled
 		}
 
@@ -485,6 +487,14 @@ void fail(Throwable error) {
 		public String toString() {
 			return get() ? "Borrower(cancelled)" : "Borrower";
 		}
+
+		AbstractPool<POOLABLE> pool() {
+			return pool.get();
+		}
+
+		void setPool(AbstractPool<POOLABLE> replace) {
+			pool.set(replace);
+		}
 	}
 
 }

reactor-pool/src/main/java/reactor/pool/DefaultPoolConfig.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2022 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2019-2024 VMware Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,7 +27,6 @@
 import reactor.core.Disposable;
 import reactor.core.publisher.Mono;
 import reactor.core.scheduler.Scheduler;
-import reactor.core.scheduler.Schedulers;
 
 /**
  * A default {@link PoolConfig} that can be extended to bear more configuration options
@@ -50,6 +49,7 @@ public class DefaultPoolConfig<POOLABLE> implements PoolConfig<POOLABLE> {
 	protected final PoolMetricsRecorder                           metricsRecorder;
 	protected final Clock                                         clock;
 	protected final boolean                                       isIdleLRU;
+	protected final ResourceManager resourceManager;
 
 	public DefaultPoolConfig(Mono<POOLABLE> allocator,
 			AllocationStrategy allocationStrategy,
@@ -64,6 +64,26 @@ public DefaultPoolConfig(Mono<POOLABLE> allocator,
 			PoolMetricsRecorder metricsRecorder,
 			Clock clock,
 			boolean isIdleLRU) {
+		this(allocator, allocationStrategy, maxPending, pendingAcquireTimer, releaseHandler, destroyHandler,
+				evictionPredicate, evictInBackgroundInterval, evictInBackgroundScheduler, acquisitionScheduler,
+				metricsRecorder, clock, isIdleLRU,
+				PoolBuilder.DEFAULT_RESOURCE_MANAGER);
+	}
+
+	public DefaultPoolConfig(Mono<POOLABLE> allocator,
+			AllocationStrategy allocationStrategy,
+			int maxPending,
+			BiFunction<Runnable, Duration, Disposable> pendingAcquireTimer,
+			Function<POOLABLE, ? extends Publisher<Void>> releaseHandler,
+			Function<POOLABLE, ? extends Publisher<Void>> destroyHandler,
+			BiPredicate<POOLABLE, PooledRefMetadata> evictionPredicate,
+			Duration evictInBackgroundInterval,
+			Scheduler evictInBackgroundScheduler,
+			Scheduler acquisitionScheduler,
+			PoolMetricsRecorder metricsRecorder,
+			Clock clock,
+			boolean isIdleLRU,
+			ResourceManager resourceManager) {
 		this.pendingAcquireTimer = pendingAcquireTimer;
 		this.allocator = allocator;
 		this.allocationStrategy = allocationStrategy;
@@ -77,6 +97,7 @@ public DefaultPoolConfig(Mono<POOLABLE> allocator,
 		this.metricsRecorder = metricsRecorder;
 		this.clock = clock;
 		this.isIdleLRU = isIdleLRU;
+		this.resourceManager = resourceManager;
 	}
 
 	/**
@@ -101,6 +122,7 @@ protected DefaultPoolConfig(PoolConfig<POOLABLE> toCopy) {
 			this.metricsRecorder = toCopyDpc.metricsRecorder;
 			this.clock = toCopyDpc.clock;
 			this.isIdleLRU = toCopyDpc.isIdleLRU;
+			this.resourceManager = toCopyDpc.resourceManager;
 		}
 		else {
 			this.allocator = toCopy.allocator();
@@ -116,6 +138,7 @@ protected DefaultPoolConfig(PoolConfig<POOLABLE> toCopy) {
 			this.metricsRecorder = toCopy.metricsRecorder();
 			this.clock = toCopy.clock();
 			this.isIdleLRU = toCopy.reuseIdleResourcesInLruOrder();
+			this.resourceManager = toCopy.resourceManager();
 		}
 	}
 
@@ -183,4 +206,9 @@ public Clock clock() {
 	public boolean reuseIdleResourcesInLruOrder() {
 		return isIdleLRU;
 	}
+
+	@Override
+	public ResourceManager resourceManager() {
+		return resourceManager;
+	}
 }

reactor-pool/src/main/java/reactor/pool/InstrumentedPool.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2019-2023 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2019-2024 VMware Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,6 +31,16 @@ public interface InstrumentedPool<POOLABLE> extends Pool<POOLABLE> {
 	 */
 	PoolMetrics metrics();
 
+	/**
+	 * Estimates if the pool can currently either reuse or create some resources
+	 * @return true if the pool can currently either reuse or create some resources, false if no idles resources are
+	 *         currently available and no more resources can be currently created.
+	 */
+	default boolean hasAvailableResources() {
+		PoolMetrics pm = metrics();
+		return (pm.idleSize() + config().allocationStrategy().estimatePermitCount()) - pm.pendingAcquireSize() >= 0;
+	}
+
 	/**
 	 * An object that can be used to get live information about a {@link Pool}, suitable
 	 * for gauge metrics.

reactor-pool/src/main/java/reactor/pool/Pool.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2022 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2018-2024 VMware Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -178,4 +178,14 @@ default void dispose() {
 	 * @return a Mono triggering the shutdown of the pool once subscribed.
 	 */
 	Mono<Void> disposeLater();
+
+	/**
+	 * Transfer some pending borrowers from another pool into this pool.
+	 *
+	 * @param from another pool to steal resources from
+	 * @return true if some borrowers have been moved from <code>from</code> into this pool instance
+	 */
+	default boolean transferBorrowersFrom(InstrumentedPool<POOLABLE> from) {
+		return false;
+	}
 }

reactor-pool/src/main/java/reactor/pool/PoolBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2023 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2018-2024 VMware Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -79,6 +79,8 @@ public static <T> PoolBuilder<T, PoolConfig<T>> from(Publisher<? extends T> allo
 	boolean                                idleLruOrder         = true;
 	BiFunction<Runnable, Duration, Disposable> pendingAcquireTimer = DEFAULT_PENDING_ACQUIRE_TIMER;
 
+	ResourceManager resourceManager = DEFAULT_RESOURCE_MANAGER;
+
 	PoolBuilder(Mono<T> allocator, Function<PoolConfig<T>, CONF> configModifier) {
 		this.allocator = allocator;
 		this.configModifier = configModifier;
@@ -445,6 +447,11 @@ public PoolBuilder<T, CONF> idleResourceReuseOrder(boolean isLru) {
 		return this;
 	}
 
+	public PoolBuilder<T, CONF> resourceManager(ResourceManager resourceManager) {
+		this.resourceManager = resourceManager;
+		return this;
+	}
+
 	/**
 	 * Add implementation-specific configuration, changing the type of {@link PoolConfig}
 	 * passed to the {@link Pool} factory in {@link #build(Function)}.
@@ -508,7 +515,8 @@ CONF buildConfig() {
 				acquisitionScheduler,
 				metricsRecorder,
 				clock,
-				idleLruOrder);
+				idleLruOrder,
+				resourceManager);
 
 		return this.configModifier.apply(baseConfig);
 	}
@@ -531,4 +539,6 @@ static <T> BiPredicate<T, PooledRefMetadata> idlePredicate(Duration maxIdleTime)
 	static final BiPredicate<?, ?>       NEVER_PREDICATE = (ignored1, ignored2) -> false;
 	static final BiFunction<Runnable, Duration, Disposable> DEFAULT_PENDING_ACQUIRE_TIMER = (r, d) -> Schedulers.parallel().schedule(r, d.toNanos(), TimeUnit.NANOSECONDS);
 	static final int DEFAULT_WARMUP_PARALLELISM = 1;
+
+	static final ResourceManager DEFAULT_RESOURCE_MANAGER = () -> {};
 }

reactor-pool/src/main/java/reactor/pool/PoolConfig.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2018-2022 VMware Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2018-2024 VMware Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -141,4 +141,8 @@ default BiFunction<Runnable, Duration, Disposable> pendingAcquireTimer() {
 		return PoolBuilder.DEFAULT_PENDING_ACQUIRE_TIMER;
 	}
 
+	default ResourceManager resourceManager() {
+		return PoolBuilder.DEFAULT_RESOURCE_MANAGER;
+	}
+
 }

reactor-pool/src/main/java/reactor/pool/PoolScheduler.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright (c) 2024 VMware Inc. or its affiliates, All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package reactor.pool;
+
+import java.util.List;
+
+/**
+ * A Pool that can schedule resource acquisition among multiple sub pools,
+ * each managing a portion of resources. Resource acquisitions will
+ * be concurrently distributed across sub pools using sub pool executors, in a work stealing style.
+ */
+public interface PoolScheduler<T> extends InstrumentedPool<T> {
+    /**
+     * Get the number of borrowers steal count (only if the Scheduler supports work stealing).
+     *
+     * @return the number of Pool steal count, or -1
+     */
+    long stealCount();
+
+    /**
+     * Returns the number of sub pools managed this this scheduler.
+     * @return the number of sub pools managed this this scheduler
+     */
+    List<InstrumentedPool<T>> getPools();
+}

reactor-pool/src/main/java/reactor/pool/ResourceManager.java

@@ -0,0 +1,30 @@
+/*
+ * Copyright (c) 2024 VMware Inc. or its affiliates, All Rights Reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package reactor.pool;
+
+/**
+ * A resource manager utilized by concrete Pool implementations. This manager enables Pools to interact
+ * with its pool scheduler, if there is one enabled.
+ * Pools can access their resource manager via the {@link PoolConfig#resourceManager()} method.
+ */
+public interface ResourceManager {
+    /**
+     * Notifies the pool scheduler that some resources can be acquired from the current pool because either certain
+     * resources are currently estimated to be idle or available for allocation.
+     */
+    void resourceAvailable();
+}