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The Federation SAIL allows multiple datasets to be virtually combined into a single dataset. The Federation SAIL combines multiple RDF stores that may exist on a remote server or are embedded in the same JVM. The Federation uses query optimizations to distribute sections of the query to different members based on the data contained in each of the members. These results are then joined together within the federation to provide the same result as if all the data was co-located within a single repository.

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/*******************************************************************************
 * Copyright (c) 2015 Eclipse RDF4J contributors, Aduna, and others.
 * 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.
 *******************************************************************************/
package org.eclipse.rdf4j.sail.federation;

import java.util.List;
import java.util.Random;

import org.eclipse.rdf4j.model.IRI;
import org.eclipse.rdf4j.model.Resource;
import org.eclipse.rdf4j.model.Value;
import org.eclipse.rdf4j.repository.RepositoryConnection;
import org.eclipse.rdf4j.repository.RepositoryException;
import org.eclipse.rdf4j.sail.SailException;

/**
 * Statements are only written to a single member. Statements that have a {@link IllegalStatementException} throw when
 * added to a member are tried against all other members until it is accepted. If no members accept a statement the
 * original exception is re-thrown.
 *
 * @author James Leigh
 */
class WritableConnection extends AbstractEchoWriteConnection {

	private int addIndex;

	public WritableConnection(Federation federation, List members) throws SailException {
		super(federation, members);
		int size = members.size();
		int rnd = (new Random().nextInt() % size + size) % size;
		// use round-robin to distribute the load
		for (int i = rnd, n = rnd + size; i < n; i++) {
			try {
				if (members.get(i % size).getRepository().isWritable()) {
					addIndex = i % size;
				}
			} catch (RepositoryException e) {
				throw new SailException(e);
			}
		}
	}

	@Override
	public void addStatementInternal(Resource subj, IRI pred, Value obj, Resource... contexts) throws SailException {
		add(members.get(addIndex), subj, pred, obj, contexts);
	}

	private void add(RepositoryConnection member, Resource subj, IRI pred, Value obj, Resource... contexts)
			throws SailException {
		try {
			member.add(subj, pred, obj, contexts);
		} catch (RepositoryException e) {
			throw new SailException(e);
		}
	}

	@Override
	protected void clearInternal(Resource... contexts) throws SailException {
		removeStatementsInternal(null, null, null, contexts);
	}

	@Override
	public boolean pendingRemovals() {
		return false;
	}
}