exec/java-exec/src/main/java/org/apache/drill/exec/rpc/DynamicSemaphore.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.drill.exec.rpc;

import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.locks.ReentrantLock;

/**
 * Dynamic credit based flow control:
 * The sender initially sends batch to the sender by the initial static credit (3).
 * The receiver will calculate a runtime credit value while sampling some received batches. The runtime
 * generated credit value will be sent to the receiver as an ack value. The sender will change to the
 * runtime generated credit value while received the ack credit value.
 *
 * The ack credit value has three type numeric value with different meaning:
 *  0 : meaning no explicit credit value to the sender, just keep the initial static credit based fc.
 *  -1: meaning the receiver has gone to wrong to solve the query.
 *  value greater than 0:  meaning a runtime generated credit value for the sender to change to.
 */
public class DynamicSemaphore {

    private static final int INITIAL_STATIC_CREDIT = 3;
    private final Semaphore semaphore = new Semaphore(INITIAL_STATIC_CREDIT);
    private final AtomicBoolean changed = new AtomicBoolean(false);
    private final ReentrantLock lock = new ReentrantLock();
    private int firstReceivedCredit = -1;

    public DynamicSemaphore() {

    }

    public void acquire() throws InterruptedException {
        semaphore.acquire();
    }

    public void release() {
        semaphore.release();
    }

    /**
     * received an advice credit to transfer from the
     * initial static value
     *
     * @param suggestedSemaphoreVal
     */
    public void tryToIncreaseCredit(int suggestedSemaphoreVal) {
        if (suggestedSemaphoreVal < INITIAL_STATIC_CREDIT) {
            return;
        }
        if (changed.get()) {
            return;
        }
        try {
            lock.lock();
            if (suggestedSemaphoreVal > firstReceivedCredit) {
                firstReceivedCredit = suggestedSemaphoreVal;
            } else {
                return;
            }
            int increasedSemaphoreNumber = suggestedSemaphoreVal - INITIAL_STATIC_CREDIT;
            //meaning increase the semaphore
            semaphore.release(increasedSemaphoreNumber);
            changed.compareAndSet(false, true);
        } finally {
            lock.unlock();
        }
    }
}