source: josm/trunk/src/org/openstreetmap/josm/data/cache/HostLimitQueue.java@ 8510

// License: GPL. For details, see LICENSE file.
package org.openstreetmap.josm.data.cache;

import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;

import org.openstreetmap.josm.Main;

/**
 * @author Wiktor Niesiobędzki
 *
 * Queue for ThreadPoolExecutor that implements per-host limit. It will acquire a semaphore for each task
 * and it will set a runnable task with semaphore release, when job has finished.
 *
 * This implementation doesn't guarantee to have at most hostLimit connections per host[1], and it doesn't
 * guarantee that all threads will be busy, when there is work for them[2].
 *
 * [1] More connection per host may happen, when ThreadPoolExecutor is growing its pool, and thus
 *     tasks do not go through the Queue
 * [2] If we have a queue, and for all hosts in queue we will fail to acquire semaphore, the thread
 *     take the first available job and wait for semaphore. It might be the case, that semaphore was released
 *     for some task further in queue, but this implementation doesn't try to detect such situation
 *
 *
 */

public class HostLimitQueue extends LinkedBlockingDeque<Runnable> {
    private static final long serialVersionUID = 1L;

    private final Map<String, Semaphore> hostSemaphores = new ConcurrentHashMap<>();
    private final int hostLimit;

    /**
     * Creates an unbounded queue
     * @param hostLimit how many parallel calls to host to allow
     */
    public HostLimitQueue(int hostLimit) {
        super(); // create unbounded queue
        this.hostLimit = hostLimit;
    }

    private JCSCachedTileLoaderJob<?, ?> findJob() {
        for (Iterator<Runnable> it = iterator(); it.hasNext();) {
            Runnable r = it.next();
            if (r instanceof JCSCachedTileLoaderJob) {
                JCSCachedTileLoaderJob<?, ?> job = (JCSCachedTileLoaderJob<?, ?>) r;
                if (tryAcquireSemaphore(job)) {
                    if (remove(job)) {
                        return job;
                    } else {
                        // we have acquired the semaphore, but we didn't manage to remove it, as someone else did
                        // release the semaphore and look for another candidate
                        releaseSemaphore(job);
                    }
                } else {
                    Main.info("TMS - Skipping job {0} because host limit reached", job.getUrl());
                }
            }
        }
        return null;
    }

    @Override
    public Runnable poll(long timeout, TimeUnit unit) throws InterruptedException {
        Runnable job = findJob();
        if (job != null) {
            return job;
        }
        job = pollFirst(timeout, unit);
        if (job != null) {
            acquireSemaphore(job);
        }
        return job;
    }

    @Override
    public Runnable take() throws InterruptedException {
        Runnable job = findJob();
        if (job != null) {
            return job;
        }
        job = takeFirst();
        if (job != null) {
            acquireSemaphore(job);
        }
        return job;
    }

    private  Semaphore getSemaphore(JCSCachedTileLoaderJob<?, ?> job) {
        String host = job.getUrl().getHost();
        Semaphore limit = hostSemaphores.get(host);
        if (limit == null) {
            synchronized (hostSemaphores) {
                limit = hostSemaphores.get(host);
                if (limit == null) {
                    limit = new Semaphore(hostLimit);
                    hostSemaphores.put(host, limit);
                }
            }
        }
        return limit;
    }

    private void acquireSemaphore(Runnable job) throws InterruptedException {
        if (job instanceof JCSCachedTileLoaderJob) {
            final JCSCachedTileLoaderJob<?, ?> jcsJob = (JCSCachedTileLoaderJob<?, ?>) job;
            Semaphore limit = getSemaphore(jcsJob);
            if (limit != null) {
                limit.acquire();
                jcsJob.setFinishedTask(new Runnable() {
                    @Override
                    public void run() {
                        releaseSemaphore(jcsJob);
                    }
                });
            }
        }
    }

    private boolean tryAcquireSemaphore(final JCSCachedTileLoaderJob<?, ?> job) {
        boolean ret = true;
        Semaphore limit = getSemaphore(job);
        if (limit != null) {
            ret = limit.tryAcquire();
            if (ret) {
                job.setFinishedTask(new Runnable() {
                    @Override
                    public void run() {
                        releaseSemaphore(job);
                    }
                });
            }
        }
        return ret;
    }

    private void releaseSemaphore(JCSCachedTileLoaderJob<?, ?> job) {
        Semaphore limit = getSemaphore(job);
        if (limit != null) {
            limit.release();
            if (limit.availablePermits() > hostLimit) {
                Main.warn("More permits than it should be");
            }
        }
    }
}