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/*
 * Copyright (C) 2020 Graylog, Inc.
 *
 * This program is free software: you can redistribute it and/or modify
 * it under the terms of the Server Side Public License, version 1,
 * as published by MongoDB, Inc.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
 * Server Side Public License for more details.
 *
 * You should have received a copy of the Server Side Public License
 * along with this program. If not, see
 * .
 */
package org.graylog2.system.shutdown;

/**
 * Services can implement this to participate in a graceful shutdown of the server.
 * 
 * A service can register itself with the {@link GracefulShutdownService} like this:
 *
 * 
 {@code
 * class MyService implements GracefulShutdownHook {
 *     private final GracefulShutdownService shutdownService;
 *
 *     @Inject
 *     public MyService(GracefulShutdownService shutdownService) {
 *         this.shutdownService = shutdownService;
 *     }
 *
 *     // This will be executed by the GracefulShutdownService on server shutdown
 *     @Override
 *     void doGracefulShutdown() throws Exception {
 *         runShutdownTasks();
 *     }
 *
 *     public void start() {
 *         // Let the GracefulShutdownService know about this service
 *         this.shutdownService.register(this);
 *     }
 *
 *     // This will be executed by some service manager when this service is stopped
 *     public void stop() {
 *         // Remove this service from the GracefulShutdownService because it's stopped before server shutdown and
 *         // we don't need any graceful shutdown for it anymore
 *         this.shutdownService.unregister(this);
 *         runShutdownTasks();
 *     }
 *
 *     private void runShutdownTasks() {
 *         // Run the actual shutdown tasks for the service here
 *     }
 * }
 * }
 */
public interface GracefulShutdownHook {
    /**
     * Execute shutdown tasks for the service that implements this interface.
     * Warning:
     * 
     *     This method is called from another thread so the class that implements {@link GracefulShutdownHook} must be thread-safe.
     *     The server shutdown is waiting for this method call to complete. Blocking this method will block the server
     * shutdown!
     * 
     *
     * @throws Exception
     */
    void doGracefulShutdown() throws Exception;
}