SourcePollerRunner.java

/**
 * Copyright (c) Codice Foundation
 * <p/>
 * This is free software: you can redistribute it and/or modify it under the terms of the GNU Lesser
 * General Public License as published by the Free Software Foundation, either version 3 of the
 * License, or any later version.
 * <p/>
 * 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 GNU
 * Lesser General Public License for more details. A copy of the GNU Lesser General Public License
 * is distributed along with this program and can be found at
 * <http://www.gnu.org/licenses/lgpl.html>.
 */
package ddf.catalog.util.impl;

import java.util.List;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.CopyOnWriteArrayList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

import org.slf4j.LoggerFactory;
import org.slf4j.ext.XLogger;

import ddf.catalog.source.Source;

/**
 * The poller to check the availability of all configured sources. This class is instantiated by the
 * CatalogFramework's blueprint and is scheduled by the {@link SourcePoller} to execute at a fixed
 * rate, i.e., the polling interval.
 *
 * This class maintains a list of all of the sources to be polled for their availability. Sources
 * are added to this list when they come online and when they are deleted. A cached map is
 * maintained of all the sources and their last availability states.
 *
 */
public class SourcePollerRunner implements Runnable {

    private static final XLogger LOGGER = new XLogger(
            LoggerFactory.getLogger(SourcePollerRunner.class));

    private List<Source> sources;

    private Map<Source, CachedSource> cachedSources = new ConcurrentHashMap<Source, CachedSource>();

    private ExecutorService pool;

    private Map<Source, Lock> sourceStatusThreadLocks = new ConcurrentHashMap<Source, Lock>();

    /**
     * Creates an empty list of {@link Source} sources to be polled for availability. This
     * constructor is invoked by the CatalogFramework's blueprint.
     */
    public SourcePollerRunner() {

        LOGGER.info("Creating source poller runner.");
        sources = new CopyOnWriteArrayList<Source>();
    }

    /**
     * Checks the availability of each source in the list of sources to be polled.
     */
    @Override
    public void run() {

        LOGGER.trace("RUNNER checking source statuses");

        for (Source source : sources) {

            if (source != null) {

                checkStatus(source);

            }

        }

    }

    /**
     * Checks if the specified source is available, updating the internally maintained map of
     * sources and their status. Lock ensures only one status thread is running per source.
     *
     * @param source
     *            the source to check if it is available
     */
    private void checkStatus(final Source source) {

        if (pool == null) {
            pool = Executors.newCachedThreadPool();

        }
        final Runnable statusRunner = new Runnable() {

            public void run() {

                final CachedSource cachedSource = cachedSources.get(source);
                if (cachedSource != null) {
                    Lock sourceStatusThreadLock = sourceStatusThreadLocks.get(source);

                    if (sourceStatusThreadLock.tryLock()) {
                        LOGGER.debug("Acquired lock for Source [{}] with id [{}]", source,
                                source.getId());

                        try {
                            cachedSource.checkStatus();
                        } finally {
                            // release the lock acquired initially
                            sourceStatusThreadLock.unlock();
                            LOGGER.debug("Released lock for Source [{}] with id [{}]", source,
                                    source.getId());
                        }
                    } else {
                        LOGGER.debug("Unable to get lock for Source [{}] with id [{}]."
                                        + "  A status thread is already running.", source,
                                source.getId());
                    }
                }
            }
        };
        pool.execute(statusRunner);
    }

    /**
     * Adds the {@link Source} instance to the list and sets its current status to UNCHECKED,
     * indicating it will checked at the next polling interval.
     *
     * @param source
     *            the source to add to the list
     */
    public void bind(Source source) {

        LOGGER.info("Binding source: {}", source);
        if (source != null) {
            LOGGER.debug("Marking new source {} as UNCHECKED.", source);
            sources.add(source);
            sourceStatusThreadLocks.put(source, new ReentrantLock());
            cachedSources.put(source, new CachedSource(source));
            checkStatus(source);

        }
    }

    /**
     * Removes the {@link Source} instance from the list of references so that its availability is
     * no longer polled.
     *
     * @param source
     *            the source to remove from the list
     */
    public void unbind(Source source) {
        LOGGER.info("Unbinding source [{}]", source);
        if (source != null) {
            cachedSources.remove(source);
            sources.remove(source);
            sourceStatusThreadLocks.remove(source);
        }
    }

    /**
     * Retrieves a {@link CachedSource} which contains cached values from the 
     * specified {@link Source}. Returns a {@link Source} with values from the 
     * last polling interval. If the {@link Source} is not known, null is returned.
     *
     * @param source
     *            the source to get the {@link CachedSource} for
     *
     * @return a {@link CachedSource} which contains cached values
     */
    public CachedSource getCachedSource(Source source) {
        return cachedSources.get(source);
    }

    /**
     *
     * Calls the @link ExecutorService to shutdown immediately
     */
    public void shutdown() {
        LOGGER.trace("Shutting down status threads");
        if (pool != null) {
            pool.shutdownNow();
        }
        LOGGER.trace("Status threads shut down");
    }
}