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/*
 * Copyright 2015-2016 ksyun.com, Inc. or its affiliates. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License").
 * You may not use this file except in compliance with the License.
 * A copy of the License is located at
 *
 *  http://ksyun.com/apache2.0
 *
 * or in the "license" file accompanying this file. This file 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 com.ksc.http;

import com.ksc.annotation.SdkTestInternalApi;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.http.conn.HttpClientConnectionManager;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.concurrent.TimeUnit;

/**
 * Daemon thread to periodically check connection pools for idle connections.
 * 
 * Connections sitting around idle in the HTTP connection pool for too long will
 * eventually be terminated by the AWS end of the connection, and will go into
 * CLOSE_WAIT. If this happens, sockets will sit around in CLOSE_WAIT, still
 * using resources on the client side to manage that socket. Many sockets stuck
 * in CLOSE_WAIT can prevent the OS from creating new connections.
 * 

 * This class closes idle connections before they can move into the CLOSE_WAIT
 * state.
 * 

 * This thread is important because by default, we disable Apache HttpClient's
 * stale connection checking, so without this thread running in the background,
 * cleaning up old/inactive HTTP connections, we'd see more IO exceptions when
 * stale connections (i.e. closed on the AWS side) are left in the connection
 * pool, and requests grab one of them to begin executing a request.
 */
public final class IdleConnectionReaper extends Thread {

    /**
     * Shared log for any errors during connection reaping.
     */
    static final Log log = LogFactory.getLog(IdleConnectionReaper.class);
    /**
     * The period between invocations of the idle connection reaper.
     */
    private static final int PERIOD_MILLISECONDS = 1000 * 60 * 1;
    /**
     * The list of registered connection managers, whose connections
     * will be periodically checked and idle connections closed.
     */
    private static final ArrayList
            connectionManagers = new ArrayList();
    /**
     * Singleton instance of the connection reaper.
     */
    private static IdleConnectionReaper instance;
    /**
     * Set to true when shutting down the reaper;  Once set to true, this
     * flag is never set back to false.
     */
    private volatile boolean shuttingDown;

    /**
     * Private constructor - singleton pattern.
     */
    private IdleConnectionReaper() {
        super("java-sdk-http-connection-reaper");
        setDaemon(true);
    }

    /**
     * Registers the given connection manager with this reaper;
     *
     * @return true if the connection manager has been successfully registered;
     * false otherwise.
     */
    public static synchronized boolean registerConnectionManager(HttpClientConnectionManager connectionManager) {
        if (instance == null) {
            instance = new IdleConnectionReaper();
            instance.start();
        }
        return connectionManagers.add(connectionManager);
    }

    /**
     * Removes the given connection manager from this reaper,
     * and shutting down the reaper if there is zero connection manager left.
     *
     * @return true if the connection manager has been successfully removed;
     * false otherwise.
     */
    public static synchronized boolean removeConnectionManager(HttpClientConnectionManager connectionManager) {
        boolean b = connectionManagers.remove(connectionManager);
        if (connectionManagers.isEmpty())
            shutdown();
        return b;
    }

    @SdkTestInternalApi
    public static synchronized List getRegisteredConnectionManagers() {
        return Collections.unmodifiableList(connectionManagers);
    }

    /**
     * Shuts down the thread, allowing the class and instance to be collected.
     * 
     * Since this is a daemon thread, its running will not prevent JVM shutdown.
     * It will, however, prevent this class from being unloaded or garbage
     * collected, in the context of a long-running application, until it is
     * interrupted. This method will stop the thread's execution and clear its
     * state. Any use of a service client will cause the thread to be restarted.
     *
     * @return true if an actual shutdown has been made; false otherwise.
     */
    public static synchronized boolean shutdown() {
        if (instance != null) {
            instance.markShuttingDown();
            instance.interrupt();
            connectionManagers.clear();
            instance = null;
            return true;
        }
        return false;
    }

    /**
     * For testing purposes.
     * Returns the number of connection managers currently monitored by this
     * reaper.
     */
    static synchronized int size() {
        return connectionManagers.size();
    }

    private void markShuttingDown() {
        shuttingDown = true;
    }

    @SuppressWarnings("unchecked")
    @Override
    public void run() {
        while (true) {
            if (shuttingDown) {
                log.debug("Shutting down reaper thread.");
                return;
            }
            try {
                Thread.sleep(PERIOD_MILLISECONDS);

                // Copy the list of managed ConnectionManagers to avoid possible
                // ConcurrentModificationExceptions if registerConnectionManager or
                // removeConnectionManager are called while we're iterating (rather
                // than block/lock while this loop executes).
                List connectionManagers = null;
                synchronized (IdleConnectionReaper.class) {
                    connectionManagers = (List) IdleConnectionReaper.connectionManagers.clone();
                }
                for (HttpClientConnectionManager connectionManager : connectionManagers) {
                    // When we release connections, the connection manager leaves them
                    // open so they can be reused.  We want to close out any idle
                    // connections so that they don't sit around in CLOSE_WAIT.
                    try {
                        connectionManager.closeIdleConnections(60, TimeUnit.SECONDS);
                    } catch (Exception t) {
                        log.warn("Unable to close idle connections", t);
                    }
                }
            } catch (Throwable t) {
                log.debug("Reaper thread: ", t);
            }
        }
    }
}