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Scheduler of java processes that seeks to optimize the execution of processing tasks by the JVM with minimal threads in order to reduce the overhead of context switching, optimize the utilization of CPU cores, and provide controls to the application over the scheduling of processing tasks.
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/*
 *   Copyright 2015 AML Innovation & Consulting LLC
 *
 *   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 com.amlinv.javasched.impl;

import com.amlinv.javasched.BlockingSchedulerEngine;
import com.amlinv.javasched.NonBlockingSchedulerEngine;
import com.amlinv.javasched.Scheduler;
import com.amlinv.javasched.SchedulerProcess;
import com.amlinv.javasched.SchedulerProcessExecutionSlip;
import com.amlinv.javasched.Step;
import com.amlinv.javasched.engine.ProcessStepExecutionListener;
import com.amlinv.javasched.engine.ProcessStepExecutionSlipFactory;
import com.amlinv.javasched.engine.StandardProcessExecutionSlip;
import com.amlinv.javasched.engine.StandardProcessExecutionSlipFactory;

import java.util.HashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;

/**
 * Scheduler of processes, each of which is a serialized source of processing steps in which each
 * step can be blocking or non-blocking.  Steps are fed into the blocking and non-blocking engines
 * based on the type of step, and processes iterate over steps until no more steps are available, or
 * one of the steps has a fatal error.
 *
 * Created by art on 12/10/14.
 */
public class RoundRobinScheduler implements Scheduler {

  //  private final Set processSet;
  private final Map processes;
  private final Object schedulerSync = new Object();

  private NonBlockingSchedulerEngine nonBlockingEngine = new StandardNonBlockingSchedulerEngine();
  private BlockingSchedulerEngine blockingEngine = new StandardBlockingSchedulerEngine();

  private boolean started;

  private ProcessStepExecutionSlipFactory stepExecutionSlipFactory;
  private StandardProcessExecutionSlipFactory processExecutionSlipFactory;

  public RoundRobinScheduler() {
    this.processes = new HashMap<>();
    this.started = false;
    this.stepExecutionSlipFactory = new ProcessStepExecutionSlipFactory();
    this.processExecutionSlipFactory = new StandardProcessExecutionSlipFactory();
  }

  public NonBlockingSchedulerEngine getNonBlockingEngine() {
    return this.nonBlockingEngine;
  }

  public void setNonBlockingEngine(NonBlockingSchedulerEngine newNonBlockingEngine) {
    this.nonBlockingEngine = newNonBlockingEngine;
  }

  public BlockingSchedulerEngine getBlockingEngine() {
    return blockingEngine;
  }

  public void setBlockingEngine(BlockingSchedulerEngine newEngine) {
    this.blockingEngine = newEngine;
  }

  public ProcessStepExecutionSlipFactory getStepExecutionSlipFactory() {
    return stepExecutionSlipFactory;
  }

  public void setStepExecutionSlipFactory(
      ProcessStepExecutionSlipFactory stepExecutionSlipFactory) {
    this.stepExecutionSlipFactory = stepExecutionSlipFactory;
  }

  @Override
  public void start() {
    synchronized (this.schedulerSync) {
      if (this.started) {
        throw new IllegalStateException("already started");
      }
      this.started = true;
    }

    this.nonBlockingEngine.start();
    this.blockingEngine.start();
  }

  @Override
  public SchedulerProcessExecutionSlip startProcess(SchedulerProcess newProcess) {
    synchronized (schedulerSync) {
      if (!this.started) {
        throw new IllegalStateException("not yet started");
      }
    }

    StandardProcessExecutionSlip executionSlip = this.prepareProcessExecutionSlip(newProcess);

    boolean previouslySubmitted;
    synchronized (this.processes) {
      previouslySubmitted = this.processes.containsKey(newProcess);
      if (!previouslySubmitted) {
        this.processes.put(newProcess, executionSlip);
      }
    }

    if (previouslySubmitted) {
      throw new IllegalStateException("process is already active");
    }

    executionSlip.processStarted();
    this.advanceProcessExecution(newProcess);

    return executionSlip;
  }

  @Override
  public List getProcessList() {
    LinkedList result;
    synchronized (this.processes) {
      result = new LinkedList(this.processes.keySet());
    }

    return result;
  }

  protected void onProcessCompletion(SchedulerProcess process) {
    StandardProcessExecutionSlip executionSlip;
    synchronized (this.processes) {
      executionSlip = this.processes.remove(process);
    }

    if (executionSlip != null) {
      executionSlip.processStopped();
    }
  }

  protected void advanceProcessExecution(SchedulerProcess process) {
    Step nextStep = process.getNextStep();

    if (nextStep == null) {
      //
      // No more steps for this process; cleanup.
      //
      this.onProcessCompletion(process);
    } else {
      //
      // Prepare an execution slip that will notify back the state of the step's execution.
      //
      Step executionSlip = this.prepareStepExecutionSlip(process, nextStep);

      //
      // Hand-off to the blocking or non-blocking engine based on whether the step is blocking or
      //  non-blocking.
      //
      if (nextStep.isBlocking()) {
        this.blockingEngine.submit(executionSlip);
      } else {
        this.nonBlockingEngine.submit(executionSlip);
      }
    }
  }

  protected void onProcessStepException(SchedulerProcess process, Throwable exc) {
    this.onProcessCompletion(process);
  }

  protected StandardProcessExecutionSlip prepareProcessExecutionSlip(
      final SchedulerProcess theProcess) {

    return this.processExecutionSlipFactory.createSlip(theProcess);
  }

  protected Step prepareStepExecutionSlip(SchedulerProcess process, Step step) {
    ProcessStepExecutionListener listener = this.prepareStepExecutionListener(process);

    Step result = this.stepExecutionSlipFactory.createProcessStepExecutionSlip(step, listener);

    return result;
  }

  /**
   * Prepare a process step execution listener that will transition the process to the next state in
   * its execution.
   *
   * @param process the process for which a step will be executed.
   * @return the new listener that will advance the state of the given process on completion of the
   * current step.
   */
  protected ProcessStepExecutionListener prepareStepExecutionListener(
      final SchedulerProcess process) {

    return new ProcessStepExecutionListener() {
      private RoundRobinScheduler parent = RoundRobinScheduler.this;

      @Override
      public void onStepStarted() {
      }

      @Override
      public void onStepStopped() {
        //
        // Last step finished; advance the process execution to the next step.
        //
        parent.advanceProcessExecution(process);
      }

      @Override
      public void onStepException(Throwable exc) {
        //
        // Fatal exception on processing the step; cleanup.
        //
        parent.onProcessStepException(process, exc);
      }
    };
  }
}