ReadOnlyRoutingDataSource.java

/*
 * Copyright 2013-2014 the original author or authors.
 *
 * 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
 *
 *      http://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 org.springframework.cloud.aws.jdbc.datasource;

import org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource;
import org.springframework.transaction.support.TransactionSynchronizationManager;

import java.util.ArrayList;
import java.util.List;
import java.util.Map;

/**
 * {@link org.springframework.jdbc.datasource.lookup.AbstractRoutingDataSource} implementation that routes to different
 * read only data source in a random fashion if the current transaction is read-only. This is useful for database
 * platforms that support read-replicas (like MySQL) to scale up the access to the database for read-only accesses.
 * <p>
 * <b>Note:</b> In order to use read-only replicas it is necessary to wrap this data source with a {@link
 * org.springframework.jdbc.datasource.LazyConnectionDataSourceProxy} to ensure that the connection is not fetched
 * during transaction creation, but during the first physical access. See the LazyConnectionDataSourceProxy
 * documentation for more details.
 * </p>
 *
 * @author Agim Emruli
 * @see org.springframework.jdbc.datasource.LazyConnectionDataSourceProxy
 */
public class ReadOnlyRoutingDataSource extends AbstractRoutingDataSource {

    private final List<Object> dataSources = new ArrayList<>();
    private List<Object> dataSourceKeys;

    @Override
    public void setTargetDataSources(Map<Object, Object> targetDataSources) {
        super.setTargetDataSources(targetDataSources);
        this.dataSourceKeys = new ArrayList<>(targetDataSources.keySet());
        this.dataSources.addAll(targetDataSources.values());
    }

    @Override
    public void setDefaultTargetDataSource(Object defaultTargetDataSource) {
        super.setDefaultTargetDataSource(defaultTargetDataSource);
        this.dataSources.add(defaultTargetDataSource);
    }

    @Override
    protected Object determineCurrentLookupKey() {
        if (TransactionSynchronizationManager.isCurrentTransactionReadOnly() && !this.dataSourceKeys.isEmpty()) {
            return this.dataSourceKeys.get(getRandom(this.dataSourceKeys.size()));
        }

        return null;
    }

    public List<Object> getDataSources() {
        return this.dataSources;
    }

    private static int getRandom(int high) {
        //noinspection UnsecureRandomNumberGeneration
        return (int) (Math.random() * high);
    }
}