Yarn之状态机分析

前言

hadoop2.x.x版本的底层实现中作了很多优化：用状态机对各种对象生命周期和状态转移进行管理；采用事件机制避免线程同步与阻塞。主要分析下yarn中状态机的实现机制

一、事件

YARN中的很多组件之间进行通信，主要借助于事件。为了可读性、可维护性及可扩展性，YARN中的事件由事件名称和事件类型组成。比如JobImpl处理的事件名称为JobEvent，而事件类型为JobEventType.

二、状态

YARN中的很多组件之间进行通信，主要借助于事件。为了可读性、可维护性及可扩展性，YARN中的事件由事件名称和事件类型组成。比如JobImpl处理的事件名称为JobEvent，而事件类型为JobEventType,如下所示

public enum JobStateInternal {
  NEW,
  SETUP,
  INITED,
  RUNNING,
  COMMITTING,
  SUCCEEDED,
  FAIL_WAIT,
  FAIL_ABORT,
  FAILED,
  KILL_WAIT,
  KILL_ABORT,
  KILLED,
  ERROR,
  REBOOT
}

我们看到JobImpl的内部状态包括新建（NEW）、初始化（INITED）、运行中（RUNNING）、提交中（COMMITTING）、成功（SUCCEEDED）、失败（FAILED）等

三、转化

我们已经了解了事件与状态的基本实现与概念，那么事件与状态有什么关系?一个对象当前处于状态state0,当对象接收到事件Event后,将引发转换动作transition,最终当前对象的状态过渡到state1.

1）Yarn中与状态转化相关的类图如下

image.png

2）SingleArcTransition

@Public
@Evolving
public interface SingleArcTransition {
  /**
   * Transition hook.
   * 
   * @param operand the entity attached to the FSM, whose internal 
   *                state may change.
   * @param event causal event
   */
  public void transition(OPERAND operand, EVENT event);

}

由于SingleArcTransition的具体实现类只负责接收到事件后的具体操作或行为，并没有包含状态相关的信息，所以在状态机执行状态过渡时，并不是直接调用SingleArcTransition具体实现类的transition方法，而是由接口Transition定义（见代码清单3）真正的转态过渡（包括行为和状态改变）。

Transition接口

private interface Transition,
          EVENTTYPE extends Enum, EVENT> {
    STATE doTransition(OPERAND operand, STATE oldState,
                       EVENT event, EVENTTYPE eventType);
  }

SingleInternalArc接口

SingleInternalArc作为Transition接口的实现类，在代理SingleArcTransition的同时，负责状态变换

private class SingleInternalArc
                    implements Transition {

    private STATE postState;
    private SingleArcTransition hook; // transition hook

    SingleInternalArc(STATE postState,
        SingleArcTransition hook) {
      this.postState = postState;
      this.hook = hook;
    }

    @Override
    public STATE doTransition(OPERAND operand, STATE oldState,
                              EVENT event, EVENTTYPE eventType) {
      if (hook != null) {
        hook.transition(operand, event);
      }
      return postState;
    }
  }

3）MultipleArcTransition

@Public
@Evolving
public interface MultipleArcTransition
        > {

  /**
   * Transition hook.
   * @return the postState. Post state must be one of the 
   *                      valid post states registered in StateMachine.
   * @param operand the entity attached to the FSM, whose internal 
   *                state may change.
   * @param event causal event
   */
  public STATE transition(OPERAND operand, EVENT event);

}

由于MultipleArcTransition的具体实现类只负责接收到事件后的具体操作或行为，并没有包含状态相关的信息，所以在状态机执行状态过渡时，并不是直接调用MultipleArcTransition具体实现类的transition方法，而是通过代理类MultipleInternalArc。MultipleInternalArc也实现了Transition接口，并在代理 MultipleArcTransition 的转换行为的同时，负责状态变换.

MultipleInternalArc

private class MultipleInternalArc
              implements Transition{

    // Fields
    private Set validPostStates;
    private MultipleArcTransition hook;  // transition hook

    MultipleInternalArc(Set postStates,
                   MultipleArcTransition hook) {
      this.validPostStates = postStates;
      this.hook = hook;
    }

    @Override
    public STATE doTransition(OPERAND operand, STATE oldState,
                              EVENT event, EVENTTYPE eventType)
        throws InvalidStateTransitonException {
      STATE postState = hook.transition(operand, event);

      if (!validPostStates.contains(postState)) {
        throw new InvalidStateTransitonException(oldState, eventType);
      }
      return postState;
    }
  }

4）ApplicableTransition

为了将所有状态机中的状态过渡与状态建立起映射关系，YARN中提供了ApplicableTransition接口用于将SingleInternalArc和 MultipleInternalArc 添加到状态机的拓扑表中，提高在检索状态对应的过渡实现时的性能，ApplicableTransition的实现类为ApplicableSingleOrMultipleTransition类，其apply方法用于代理SingleInternalArc和MultipleInternalArc ，将它们添加到状态拓扑表中。

ApplicableTransition接口定义

private interface ApplicableTransition
             ,
              EVENTTYPE extends Enum, EVENT> {
    void apply(StateMachineFactory subject);
  }

ApplicableSingleOrMultipleTransition

static private class ApplicableSingleOrMultipleTransition
             ,
              EVENTTYPE extends Enum, EVENT>
          implements ApplicableTransition {
    final STATE preState;
    final EVENTTYPE eventType;
    final Transition transition;

    ApplicableSingleOrMultipleTransition
        (STATE preState, EVENTTYPE eventType,
         Transition transition) {
      this.preState = preState;
      this.eventType = eventType;
      this.transition = transition;
    }

    @Override
    public void apply
             (StateMachineFactory subject) {
      Map> transitionMap
        = subject.stateMachineTable.get(preState);
      if (transitionMap == null) {
        // I use HashMap here because I would expect most EVENTTYPE's to not
        //  apply out of a particular state, so FSM sizes would be 
        //  quadratic if I use EnumMap's here as I do at the top level.
        transitionMap = new HashMap>();
        subject.stateMachineTable.put(preState, transitionMap);
      }
      transitionMap.put(eventType, transition);
    }
  }

可以看到ApplicableSingleOrMultipleTransition的apply方法就是为构建状态拓扑表

四、状态机

YARN中状态机的实现类是StateMachineFactory，它主要包含4个属性信息：

transitionsListNode：就是将状态机的一个个过渡的ApplicableTransition实现串联为一个列表，每个节点包含一个ApplicableTransition实现及指向下一个节点的引用

private class TransitionsListNode {
    final ApplicableTransition transition;
    final TransitionsListNode next;

    TransitionsListNode
        (ApplicableTransition transition,
        TransitionsListNode next) {
      this.transition = transition;
      this.next = next;
    }
  }

transitionsListNode形成的过渡列表节点如下：

image.png

stateMachineTable:状态拓扑表,为了提高检索状态对应的过渡map而冗余的数据结构，此结构在optimized为真时，通过对transitionsListNode链表进行处理产生

image.png

defaultInitialState：对象创建时，内部有限状态机的默认初始状态。比如：JobImpl的内部状态机默认初始状态是JobStateInternal.NEW。
optimized：布尔类型，用于标记当前状态机是否需要优化性能，即构建状态拓扑表stateMachineTable。

公共构造器

StateMachineFactory 的公有构造器只有一个。

public StateMachineFactory(STATE defaultInitialState) {
  this.transitionsListNode = null;
  this.defaultInitialState = defaultInitialState;
  this.optimized = false;
  this.stateMachineTable = null;
}

私有构造器

StateMachineFactory 的私有构造器有两个，其中代码清单11中的构造器在addTransition方法中使用。
从其实现看出，此构造器的主要作用是构建transitionsListNode链表。

private StateMachineFactory
    (StateMachineFactory that,
     ApplicableTransition t) {
  this.defaultInitialState = that.defaultInitialState;
  this.transitionsListNode 
      = new TransitionsListNode(t, that.transitionsListNode);
  this.optimized = false;
  this.stateMachineTable = null;
}

下面的构造器则用于installTopology方法

private StateMachineFactory
   (StateMachineFactory that,
    boolean optimized) {
 this.defaultInitialState = that.defaultInitialState;
 this.transitionsListNode = that.transitionsListNode;
 this.optimized = optimized;
 if (optimized) {
   makeStateMachineTable();
 } else {
   stateMachineTable = null;
 }
}

构造器当optimized参数为true时，调用了makeStateMachineTable方法，makeStateMachineTable的实现如下:

private void makeStateMachineTable() {
  Stack> stack =
    new Stack>();

  Map>>
    prototype = new HashMap>>();

  prototype.put(defaultInitialState, null);

  // I use EnumMap here because it'll be faster and denser.  I would
  //  expect most of the states to have at least one transition.
  stateMachineTable
     = new EnumMap>>(prototype);

  for (TransitionsListNode cursor = transitionsListNode;
       cursor != null;
       cursor = cursor.next) {
    stack.push(cursor.transition);
  }

  while (!stack.isEmpty()) {
    stack.pop().apply(this);
  }
}

1.创建堆栈stack，用于将transitionsListNode链表中各个节点持有的ApplicableSingleOrMultipleTransition压入栈中；

2.创建状态拓扑表stateMachineTable，并在此拓扑表中插入一个额外的默认初始状态defaultInitialState与null的映射；

3.迭代访问transitionsListNode链表，并将各个节点持有的ApplicableSingleOrMultipleTransition压入栈中；

4.依次弹出栈顶的ApplicableSingleOrMultipleTransition，并应用其apply方法（已在前面小节介绍），持续不断的构建状态拓扑表stateMachineTable。

五、状态机的构建

为了简化叙述，本节以JobImpl中状态机的构建为例。由于JobImpl的状态机预设的（调用addTransition方法）加入的ApplicableSingleOrMultipleTransition非常多，我们节选其中的2个作为典型进行分析。最后还会分析installTopology方法的实现。

protected static final
    StateMachineFactory 
       stateMachineFactory
     = new StateMachineFactory
              (JobStateInternal.NEW)

          // Transitions from NEW state
          .addTransition(JobStateInternal.NEW, JobStateInternal.NEW,
              JobEventType.JOB_DIAGNOSTIC_UPDATE,
              DIAGNOSTIC_UPDATE_TRANSITION)
          .addTransition(JobStateInternal.NEW, JobStateInternal.NEW,
              JobEventType.JOB_COUNTER_UPDATE, COUNTER_UPDATE_TRANSITION)
          .addTransition
              (JobStateInternal.NEW,
              EnumSet.of(JobStateInternal.INITED, JobStateInternal.NEW),
              JobEventType.JOB_INIT,
              new InitTransition())
          // 省略其它addTransition调用
          // create the topology tables
          .installTopology();

构建JobImpl的状态机的步骤如下：

1.调用 StateMachineFactory 构造器创建一个初始的状态机

2.调用addTransition(STATE preState, STATE postState, EVENTTYPE eventType, SingleArcTransition hook)方法添加单弧过渡。从其实现（见代码清单15）可以知道addTransition方法将SingleArcTransition封装为SingleInternalArc，然后将SingleInternalArc封装为ApplicableSingleOrMultipleTransition，最后调用之前说的第一个私有构造器构建transitionsListNode链表

3.调用addTransition(STATE preState, Set postStates, EVENTTYPE eventType, MultipleArcTransition hook)方法添加多弧过渡。从其实现（见代码清单16）可以知道addTransition方法将MultipleArcTransition封装为MultipleInternalArc，然后将MultipleInternalArc封装为ApplicableSingleOrMultipleTransition，最后调用之前说的第一个私有构造器构建transitionsListNode链表；

4.最后调用installTopology方法，其实现见代码清单17。installTopology正是在使用之前说的第二个私有构造器构建状态拓扑表stateMachineTable

public StateMachineFactory
           
        addTransition(STATE preState, STATE postState,
                      EVENTTYPE eventType,
                      SingleArcTransition hook){
  return new StateMachineFactory
      (this, new ApplicableSingleOrMultipleTransition
         (preState, eventType, new SingleInternalArc(postState, hook)));
}

public StateMachineFactory
           
        addTransition(STATE preState, Set postStates,
                      EVENTTYPE eventType,
                      MultipleArcTransition hook){
  return new StateMachineFactory
      (this,
       new ApplicableSingleOrMultipleTransition
         (preState, eventType, new MultipleInternalArc(postStates, hook)));
}

public StateMachineFactory
           
        installTopology() {
  return new StateMachineFactory(this, true);
}

Stack> stack =
    new Stack>();

  Map>>
    prototype = new HashMap>>();

  prototype.put(defaultInitialState, null);

  // I use EnumMap here because it'll be faster and denser.  I would
  //  expect most of the states to have at least one transition.
  stateMachineTable
     = new EnumMap>>(prototype);

  for (TransitionsListNode cursor = transitionsListNode;
       cursor != null;
       cursor = cursor.next) {
    stack.push(cursor.transition);
  }

  //最终在这里,状态路由表都存放到一个StateMachineFactory中了
  while (!stack.isEmpty()) {
    stack.pop().apply(this);
  }

Yarn之状态机分析

Yarn之状态机分析

前言

一、事件

二、状态

三、转化

1）Yarn中与状态转化相关的类图如下

2）SingleArcTransition

3）MultipleArcTransition

4）ApplicableTransition

四、状态机

五、状态机的构建

六、例子

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