/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.commons.configuration2.sync;

/**
 * <p>
 * An interface controlling synchronization of configuration instances.
 * </p>
 * <p>
 * Each {@code Configuration} object derived from {@link org.apache.commons.configuration2.AbstractConfiguration
 * AbstractConfiguration} has an associated {@code Synchronizer} object. Before an operation on the configuration is
 * performed (e.g. a property read or an update), the {@code Synchronizer} is invoked. Depending on the concrete
 * implementation of the {@code Synchronizer} used, the configuration can be made thread-safe.
 * </p>
 * <p>
 * Whether a configuration has to be thread-safe or not is a matter of a concrete use case. For instance, an application
 * that just reads some configuration settings on startup does need a thread-safe configuration implementation. A
 * configuration in contrast which is shared between multiple components and updated concurrently should better be
 * thread-safe. In order to satisfy both kinds of use cases, the support for thread-safety has been extracted out of the
 * configuration implementation and refactored into this {@code Synchronizer} interface. By assigning different
 * {@code Synchronizer} implementations to a configuration instance, the instance's support for concurrent access can be
 * adapted to the concrete use case.
 * </p>
 * <p>
 * The methods defined by this interface are similar to a <em>read-write lock</em>. The {@code Synchronizer} is notified
 * when read or write operations start and end. A concrete implementation can then apply a specific policy to decide
 * when threads need to block or when access to the configuration for the desired operation is granted.
 * </p>
 *
 * @since 2.0
 */
public interface Synchronizer {
    /**
     * Notifies this {@code Synchronizer} that the current thread is going to start a read operation on the managed
     * configuration. This call can block if a concrete implementation decides that the thread has to wait until a specific
     * condition is fulfilled.
     */
    void beginRead();

    /**
     * Notifies this {@code Synchronizer} that the current thread is going to start a write operation on the managed
     * configuration. This call may block. For instance, a concrete implementation may suspend the thread until all read
     * operations currently active are finished,
     */
    void beginWrite();

    /**
     * Notifies this {@code Synchronizer} that the current thread has finished its read operation. This may cause other
     * waiting threads to be granted access to the managed configuration.
     */
    void endRead();

    /**
     * Notifies this {@code Synchronizer} that the current thread has finished its write operation. This may cause other
     * waiting threads to be granted access to the managed configuration.
     */
    void endWrite();
}
