GH-5197: preparation for supporting fair sub-query execution in FedX

This change adds preparational infrastructure for having different
implementations of schedulers. Configuration is here prepared by means
of defining a "SchedulerFactory" interface with a default implementation
aside (which essentially mimics the current behavior).

Note that for ease of development some aspects of
ControlledWorkerScheduler are made accessible to sub-classes. The idea
is that in the end version there is an abstract scheduler class
providing shared functionality and different implementation (e.g. the
current FIFO one and a fair implementation)

tools/federation/src/main/java/org/eclipse/rdf4j/federated/FedX.java

@@ -22,6 +22,8 @@
 import org.eclipse.rdf4j.federated.endpoint.Endpoint;
 import org.eclipse.rdf4j.federated.endpoint.ResolvableEndpoint;
 import org.eclipse.rdf4j.federated.evaluation.FederationEvaluationStrategyFactory;
+import org.eclipse.rdf4j.federated.evaluation.concurrent.DefaultSchedulerFactory;
+import org.eclipse.rdf4j.federated.evaluation.concurrent.SchedulerFactory;
 import org.eclipse.rdf4j.federated.exception.ExceptionUtil;
 import org.eclipse.rdf4j.federated.exception.FedXException;
 import org.eclipse.rdf4j.federated.exception.FedXRuntimeException;
@@ -64,6 +66,8 @@ public class FedX extends AbstractSail implements RepositoryResolverClient {
 
 	private FederationEvaluationStrategyFactory strategyFactory;
 
+	private SchedulerFactory schedulerFactory;
+
 	private WriteStrategyFactory writeStrategyFactory;
 
 	private File dataDir;
@@ -96,6 +100,22 @@ public void setFederationEvaluationStrategy(FederationEvaluationStrategyFactory
 		this.strategyFactory = strategyFactory;
 	}
 
+	/* package */ SchedulerFactory getSchedulerFactory() {
+		if (schedulerFactory == null) {
+			schedulerFactory = DefaultSchedulerFactory.INSTANCE;
+		}
+		return schedulerFactory;
+	}
+
+	/**
+	 * Set the {@link SchedulerFactory}. Can only be done before initialization of the federation
+	 *
+	 * @param schedulerFactory the {@link SchedulerFactory}
+	 */
+	public void setSchedulerFactory(SchedulerFactory schedulerFactory) {
+		this.schedulerFactory = schedulerFactory;
+	}
+
 	/**
 	 *
 	 * @param writeStrategyFactory the {@link WriteStrategyFactory}

tools/federation/src/main/java/org/eclipse/rdf4j/federated/FederationManager.java

@@ -26,6 +26,7 @@
 import org.eclipse.rdf4j.federated.evaluation.concurrent.ControlledWorkerScheduler;
 import org.eclipse.rdf4j.federated.evaluation.concurrent.NamingThreadFactory;
 import org.eclipse.rdf4j.federated.evaluation.concurrent.Scheduler;
+import org.eclipse.rdf4j.federated.evaluation.concurrent.SchedulerFactory;
 import org.eclipse.rdf4j.federated.evaluation.concurrent.TaskWrapper;
 import org.eclipse.rdf4j.federated.evaluation.union.ControlledWorkerUnion;
 import org.eclipse.rdf4j.federated.evaluation.union.SynchronousWorkerUnion;
@@ -118,26 +119,28 @@ public void reset() {
 			log.debug("Scheduler for join and union are reset.");
 		}
 
+		SchedulerFactory schedulerFactory = federation.getSchedulerFactory();
+
 		Optional<TaskWrapper> taskWrapper = federationContext.getConfig().getTaskWrapper();
 		if (joinScheduler != null) {
 			joinScheduler.abort();
 		}
-		joinScheduler = new ControlledWorkerScheduler<>(federationContext.getConfig().getJoinWorkerThreads(),
-				"Join Scheduler");
+		joinScheduler = schedulerFactory.createJoinScheduler(federationContext,
+				federationContext.getConfig().getJoinWorkerThreads());
 		taskWrapper.ifPresent(joinScheduler::setTaskWrapper);
 
 		if (unionScheduler != null) {
 			unionScheduler.abort();
 		}
-		unionScheduler = new ControlledWorkerScheduler<>(federationContext.getConfig().getUnionWorkerThreads(),
-				"Union Scheduler");
+		unionScheduler = schedulerFactory.createUnionScheduler(federationContext,
+				federationContext.getConfig().getUnionWorkerThreads());
 		taskWrapper.ifPresent(unionScheduler::setTaskWrapper);
 
 		if (leftJoinScheduler != null) {
 			leftJoinScheduler.abort();
 		}
-		leftJoinScheduler = new ControlledWorkerScheduler<>(federationContext.getConfig().getLeftJoinWorkerThreads(),
-				"Left Join Scheduler");
+		leftJoinScheduler = schedulerFactory.createLeftJoinScheduler(federationContext,
+				federationContext.getConfig().getLeftJoinWorkerThreads());
 		taskWrapper.ifPresent(leftJoinScheduler::setTaskWrapper);
 
 	}

tools/federation/src/main/java/org/eclipse/rdf4j/federated/evaluation/concurrent/ControlledWorkerScheduler.java

@@ -11,6 +11,7 @@
 package org.eclipse.rdf4j.federated.evaluation.concurrent;
 
 import java.util.List;
+import java.util.concurrent.BlockingQueue;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.Future;
 import java.util.concurrent.LinkedBlockingQueue;
@@ -42,7 +43,8 @@ public class ControlledWorkerScheduler<T> implements Scheduler<T>, TaskWrapperAw
 
 	private final ExecutorService executor;
 
-	private final LinkedBlockingQueue<Runnable> _taskQueue = new LinkedBlockingQueue<>();
+	// Note: initialized in #createExecutorService
+	protected BlockingQueue<Runnable> _taskQueue;
 
 	private final int nWorkers;
 	private final String name;
@@ -57,7 +59,7 @@ public class ControlledWorkerScheduler<T> implements Scheduler<T>, TaskWrapperAw
 	public ControlledWorkerScheduler(int nWorkers, String name) {
 		this.nWorkers = nWorkers;
 		this.name = name;
-		this.executor = createExecutorService();
+		this.executor = createExecutorService(nWorkers, name);
 	}
 
 	/**
@@ -112,13 +114,25 @@ public int getTotalNumberOfWorkers() {
 		return nWorkers;
 	}
 
+	@Deprecated(forRemoval = true) // currently unused and this class is internal
 	public int getNumberOfTasks() {
 		return _taskQueue.size();
 	}
 
-	private ExecutorService createExecutorService() {
+	/**
+	 * Create the {@link ExecutorService} which is managing the individual {@link ParallelTask}s in a thread pool. The
+	 * default implementation creates a thread pool with a {@link LinkedBlockingQueue}.
+	 *
+	 * @param nWorkers the number of workers in the thread pool
+	 * @param name     the base name for threads in the pool
+	 * @return
+	 */
+	protected ExecutorService createExecutorService(int nWorkers, String name) {
+
+		// use a LinkedBlockingQueue by default
+		this._taskQueue = new LinkedBlockingQueue<>();
 
-		ThreadPoolExecutor executor = new ThreadPoolExecutor(nWorkers, nWorkers, 60L, TimeUnit.SECONDS, _taskQueue,
+		ThreadPoolExecutor executor = new ThreadPoolExecutor(nWorkers, nWorkers, 60L, TimeUnit.SECONDS, this._taskQueue,
 				new NamingThreadFactory(name));
 		executor.allowCoreThreadTimeOut(true);
 		return executor;

tools/federation/src/main/java/org/eclipse/rdf4j/federated/evaluation/concurrent/DefaultSchedulerFactory.java

@@ -0,0 +1,44 @@
+/*******************************************************************************
+ * Copyright (c) 2024 Eclipse RDF4J contributors.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Distribution License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *******************************************************************************/
+package org.eclipse.rdf4j.federated.evaluation.concurrent;
+
+import org.eclipse.rdf4j.federated.FederationContext;
+import org.eclipse.rdf4j.query.BindingSet;
+
+/**
+ * The default {@link SchedulerFactory}
+ */
+public class DefaultSchedulerFactory implements SchedulerFactory {
+
+	public static final DefaultSchedulerFactory INSTANCE = new DefaultSchedulerFactory();
+
+	@Override
+	public ControlledWorkerScheduler<BindingSet> createJoinScheduler(FederationContext federationContext,
+			int nWorkers) {
+		return new ControlledWorkerScheduler<>(nWorkers,
+				"Join Scheduler");
+	}
+
+	@Override
+	public ControlledWorkerScheduler<BindingSet> createUnionScheduler(FederationContext federationContext,
+			int nWorkers) {
+		return new ControlledWorkerScheduler<>(nWorkers,
+				"Union Scheduler");
+	}
+
+	@Override
+	public ControlledWorkerScheduler<BindingSet> createLeftJoinScheduler(FederationContext federationContext,
+			int nWorkers) {
+		return new ControlledWorkerScheduler<>(nWorkers,
+				"Left Join Scheduler");
+	}
+
+}

tools/federation/src/main/java/org/eclipse/rdf4j/federated/evaluation/concurrent/SchedulerFactory.java

@@ -0,0 +1,50 @@
+/*******************************************************************************
+ * Copyright (c) 2024 Eclipse RDF4J contributors.
+ *
+ * All rights reserved. This program and the accompanying materials
+ * are made available under the terms of the Eclipse Distribution License v1.0
+ * which accompanies this distribution, and is available at
+ * http://www.eclipse.org/org/documents/edl-v10.php.
+ *
+ * SPDX-License-Identifier: BSD-3-Clause
+ *******************************************************************************/
+package org.eclipse.rdf4j.federated.evaluation.concurrent;
+
+import org.eclipse.rdf4j.federated.FederationContext;
+import org.eclipse.rdf4j.query.BindingSet;
+
+/**
+ * Factory for creating {@link ControlledWorkerScheduler} for executing subqueries (e.g. joins) in the background
+ *
+ * @see DefaultSchedulerFactory
+ * @author Andreas Schwarte
+ */
+public interface SchedulerFactory {
+
+	/**
+	 * Create a {@link ControlledWorkerScheduler} for joins
+	 *
+	 * @param federationContext
+	 * @param nWorkers
+	 * @return
+	 */
+	ControlledWorkerScheduler<BindingSet> createJoinScheduler(FederationContext federationContext, int nWorkers);
+
+	/**
+	 * Create a {@link ControlledWorkerScheduler} for unions
+	 *
+	 * @param federationContext
+	 * @param nWorkers
+	 * @return
+	 */
+	ControlledWorkerScheduler<BindingSet> createUnionScheduler(FederationContext federationContext, int nWorkers);
+
+	/**
+	 * Create a {@link ControlledWorkerScheduler} for left joins
+	 *
+	 * @param federationContext
+	 * @param nWorkers
+	 * @return
+	 */
+	ControlledWorkerScheduler<BindingSet> createLeftJoinScheduler(FederationContext federationContext, int nWorkers);
+}