/**
    * 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.hadoop.hbase.ipc;
  
  import java.util.Deque;
  import java.util.concurrent.BlockingQueue;
  import java.util.concurrent.ConcurrentLinkedDeque;
  import java.util.concurrent.Semaphore;
  import java.util.concurrent.atomic.AtomicInteger;
  
  import org.apache.hadoop.conf.Configuration;
  import org.apache.hadoop.hbase.Abortable;
  import org.apache.hadoop.hbase.classification.InterfaceAudience;
  
  /**
   * Balanced queue executor with a fastpath. Because this is FIFO, it has no respect for
   * ordering so a fast path skipping the queuing of Calls if an Handler is available, is possible.
   * Just pass the Call direct to waiting Handler thread. Try to keep the hot Handlers bubbling
   * rather than let them go cold and lose context. Idea taken from Apace Kudu (incubating). See
   * https://gerrit.cloudera.org/#/c/2938/7/src/kudu/rpc/service_queue.h
   */
  @InterfaceAudience.Private
  public class FastPathBalancedQueueRpcExecutor extends BalancedQueueRpcExecutor {
    // Depends on default behavior of BalancedQueueRpcExecutor being FIFO!
  
    /*
     * Stack of Handlers waiting for work.
     */
    private final Deque<FastPathHandler> fastPathHandlerStack = new ConcurrentLinkedDeque<>();
  
    public FastPathBalancedQueueRpcExecutor(final String name, final int handlerCount,
        final int maxQueueLength, final PriorityFunction priority, final Configuration conf,
        final Abortable abortable) {
      super(name, handlerCount, maxQueueLength, priority, conf, abortable);
  
    }
  
    public FastPathBalancedQueueRpcExecutor(final String name, final int handlerCount,
        final String callQueueType, final int maxQueueLength, final PriorityFunction priority,
        final Configuration conf, final Abortable abortable) {
      super(name, handlerCount, callQueueType, maxQueueLength, priority, conf, abortable);
    }
  
    @Override
    protected Handler getHandler(String name, double handlerFailureThreshhold,
        BlockingQueue<CallRunner> q, AtomicInteger activeHandlerCount) {
      return new FastPathHandler(name, handlerFailureThreshhold, q, activeHandlerCount,
          fastPathHandlerStack);
    }
  
    @Override
    public boolean dispatch(CallRunner callTask) throws InterruptedException {
      FastPathHandler handler = popReadyHandler();
      return handler != null? handler.loadCallRunner(callTask): super.dispatch(callTask);
    }
  
    /**
     * @return Pop a Handler instance if one available ready-to-go or else return null.
     */
    private FastPathHandler popReadyHandler() {
      return this.fastPathHandlerStack.poll();
    }
  
    class FastPathHandler extends Handler {
      // Below are for fast-path support. Push this Handler on to the fastPathHandlerStack Deque
      // if an empty queue of CallRunners so we are available for direct handoff when one comes in.
      final Deque<FastPathHandler> fastPathHandlerStack;
      // Semaphore to coordinate loading of fastpathed loadedTask and our running it.
      private Semaphore semaphore = new Semaphore(0);
      // The task we get when fast-pathing.
      private CallRunner loadedCallRunner;
  
      FastPathHandler(String name, double handlerFailureThreshhold, BlockingQueue<CallRunner> q,
          final AtomicInteger activeHandlerCount,
          final Deque<FastPathHandler> fastPathHandlerStack) {
        super(name, handlerFailureThreshhold, q, activeHandlerCount);
        this.fastPathHandlerStack = fastPathHandlerStack;
      }
  
      protected CallRunner getCallRunner() throws InterruptedException {
        // Get a callrunner if one in the Q.
        CallRunner cr = this.q.poll();
        if (cr == null) {
          // Else, if a fastPathHandlerStack present and no callrunner in Q, register ourselves for
         // the fastpath handoff done via fastPathHandlerStack.
         if (this.fastPathHandlerStack != null) {
           this.fastPathHandlerStack.push(this);
           this.semaphore.acquire();
           cr = this.loadedCallRunner;
           this.loadedCallRunner = null;
         } else {
           // No fastpath available. Block until a task comes available.
           cr = super.getCallRunner();
         }
       }
       return cr;
     }
 
     /**
      * @param task Task gotten via fastpath.
      * @return True if we successfully loaded our task
      */
     boolean loadCallRunner(final CallRunner cr) {
       this.loadedCallRunner = cr;
       this.semaphore.release();
       return true;
     }
   }
 }