LearnerSyncThrottler.java

/*
 * 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.zookeeper.server.quorum;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Utility class to limit the number of concurrent syncs from a leader to
 * observers and followers or from a follower to observers.  {@link LearnerHandler}
 * objects should call {@link #beginSync(boolean)} before sending a sync and
 * {@link #endSync()} after finishing, successfully or not.
 *
 */
public class LearnerSyncThrottler {

    private static final Logger LOG = LoggerFactory.getLogger(LearnerSyncThrottler.class);

    private final Object countSyncObject = new Object();
    private int syncInProgress;

    private volatile int maxConcurrentSyncs;

    public enum SyncType {
        DIFF,
        SNAP
    }

    private final SyncType syncType;

    /**
     * Constructs a new instance limiting the concurrent number of syncs to
     * <code>maxConcurrentSyncs</code>.
     * @param maxConcurrentSyncs maximum concurrent number of syncs
     * @param syncType either a snapshot sync or a txn-based diff sync
     * @throws java.lang.IllegalArgumentException when <code>maxConcurrentSyncs</code>
     *                                            is less than 1
     */
    public LearnerSyncThrottler(int maxConcurrentSyncs, SyncType syncType) throws IllegalArgumentException {
        if (maxConcurrentSyncs <= 0) {
            String errorMsg = "maxConcurrentSyncs must be positive, was " + maxConcurrentSyncs;
            throw new IllegalArgumentException(errorMsg);
        }

        this.maxConcurrentSyncs = maxConcurrentSyncs;
        this.syncType = syncType;

        synchronized (countSyncObject) {
            syncInProgress = 0;
        }
    }

    /**
     * Indicates that a new sync is about to be sent.
     *
     * @param essential if <code>true</code>, do not throw an exception even
     *                  if throttling limit is reached
     * @throws SyncThrottleException if throttling limit has been exceeded
     *                                   and <code>essential == false</code>,
     *                                   even after waiting for the timeout
     *                                   period, if any
     * @throws InterruptedException if thread is interrupted while trying
     *                              to start a sync; cannot happen if
     *                              timeout is zero
     */
    protected void beginSync(boolean essential) throws SyncThrottleException, InterruptedException {

        synchronized (countSyncObject) {
            if (essential || syncInProgress < maxConcurrentSyncs) {
                syncInProgress++;
            } else {
                throw new SyncThrottleException(syncInProgress + 1, maxConcurrentSyncs, syncType);
            }
        }
    }

    /**
     * Indicates that a sync has been completed.
     */
    public void endSync() {
        int newCount;
        synchronized (countSyncObject) {
            syncInProgress--;
            newCount = syncInProgress;
            countSyncObject.notify();
        }

        if (newCount < 0) {
            String errorMsg = "endSync() called incorrectly; current sync count is " + newCount;
            LOG.error(errorMsg);
        }
    }

    public void setMaxConcurrentSyncs(int maxConcurrentSyncs) {
        this.maxConcurrentSyncs = maxConcurrentSyncs;
    }

    public int getSyncInProgress() {
        return syncInProgress;
    }

}