聊聊artemis message的priority

序

本文主要研究一下artemis message的priority

priority

activemq-artemis-2.11.0/artemis-core-client/src/main/java/org/apache/activemq/artemis/core/message/impl/CoreMessage.java

public class CoreMessage extends RefCountMessage implements ICoreMessage {

   //......
   protected byte priority;

   public byte getPriority() {
      return priority;
   }

   public CoreMessage setPriority(byte priority) {
      this.priority = priority;
      messageChanged();
      return this;
   }

   //......
}

• CoreMessage定义了priority属性(Values range from 0 (less priority) to 9 (more priority) inclusive)，并提供了getPriority、setPriority方法

messageReferences.add

activemq-artemis-2.11.0/artemis-server/src/main/java/org/apache/activemq/artemis/core/server/impl/QueueImpl.java

public class QueueImpl extends CriticalComponentImpl implements Queue {

   //......

   private final PriorityLinkedList<MessageReference> messageReferences = new PriorityLinkedListImpl<>(QueueImpl.NUM_PRIORITIES, MessageReferenceImpl.getIDComparator());

   //......   

   private synchronized void internalAddTail(final MessageReference ref) {
      refAdded(ref);
      messageReferences.addTail(ref, getPriority(ref));
      pendingMetrics.incrementMetrics(ref);
      enforceRing(false);
   }

   private void internalAddHead(final MessageReference ref) {
      queueMemorySize.addAndGet(ref.getMessageMemoryEstimate());
      pendingMetrics.incrementMetrics(ref);
      refAdded(ref);

      int priority = getPriority(ref);

      messageReferences.addHead(ref, priority);

      ref.setInDelivery(false);
   }

   private void internalAddSorted(final MessageReference ref) {
      queueMemorySize.addAndGet(ref.getMessageMemoryEstimate());
      pendingMetrics.incrementMetrics(ref);
      refAdded(ref);

      int priority = getPriority(ref);

      messageReferences.addSorted(ref, priority);
   }

   private int getPriority(MessageReference ref) {
      try {
         return ref.getMessage().getPriority();
      } catch (Throwable e) {
         ActiveMQServerLogger.LOGGER.unableToGetMessagePriority(e);
         return 4; // the default one in case of failure
      }
   }

   //......
}

• QueueImpl定义了messageReferences，其类型为PriorityLinkedList<MessageReference>；其internalAddTail、internalAddHead、internalAddSorted方法都会调用getPriority方法获取priority，出现异常返回4，之后通过messageReferences的addTail、addHead、addSorted方法添加到队列

PriorityLinkedList

activemq-artemis-2.11.0/artemis-commons/src/main/java/org/apache/activemq/artemis/utils/collections/PriorityLinkedList.java

public interface PriorityLinkedList<T> {

   void addHead(T t, int priority);

   void addTail(T t, int priority);

   void addSorted(T t, int priority);

   T poll();

   void clear();

   /**
    * Returns the size of this list.<br>
    * It is safe to be called concurrently.
    */
   int size();

   LinkedListIterator<T> iterator();

   /**
    * Returns {@code true} if empty, {@code false} otherwise.<br>
    * It is safe to be called concurrently.
    */
   boolean isEmpty();
}

• PriorityLinkedList接口定义了根据priority的addHead、addTail、addSorted方法，其size以及isEmpty要求是线程安全的

PriorityLinkedListImpl

activemq-artemis-2.11.0/artemis-commons/src/main/java/org/apache/activemq/artemis/utils/collections/PriorityLinkedListImpl.java

public class PriorityLinkedListImpl<T> implements PriorityLinkedList<T> {

   private static final AtomicIntegerFieldUpdater<PriorityLinkedListImpl> SIZE_UPDATER = AtomicIntegerFieldUpdater.newUpdater(PriorityLinkedListImpl.class, "size");

   protected LinkedListImpl<T>[] levels;

   private volatile int size;

   private int lastReset;

   private int highestPriority = -1;

   private int lastPriority = -1;

   public PriorityLinkedListImpl(final int priorities) {
      this(priorities, null);
   }


   public PriorityLinkedListImpl(final int priorities, Comparator<T> comparator) {
      levels = (LinkedListImpl<T>[]) Array.newInstance(LinkedListImpl.class, priorities);

      for (int i = 0; i < priorities; i++) {
         levels[i] = new LinkedListImpl<>(comparator);
      }
   }

   private void checkHighest(final int priority) {
      if (lastPriority != priority || priority > highestPriority) {
         lastPriority = priority;
         if (lastReset == Integer.MAX_VALUE) {
            lastReset = 0;
         } else {
            lastReset++;
         }
      }

      if (priority > highestPriority) {
         highestPriority = priority;
      }
   }

   @Override
   public void addHead(final T t, final int priority) {
      checkHighest(priority);

      levels[priority].addHead(t);

      exclusiveIncrementSize(1);
   }

   @Override
   public void addTail(final T t, final int priority) {
      checkHighest(priority);

      levels[priority].addTail(t);

      exclusiveIncrementSize(1);
   }

   @Override
   public void addSorted(T t, int priority) {
      checkHighest(priority);

      levels[priority].addSorted(t);

      exclusiveIncrementSize(1);
   }

   @Override
   public T poll() {
      T t = null;

      // We are just using a simple prioritization algorithm:
      // Highest priority refs always get returned first.
      // This could cause starvation of lower priority refs.

      // TODO - A better prioritization algorithm

      for (int i = highestPriority; i >= 0; i--) {
         LinkedListImpl<T> ll = levels[i];

         if (ll.size() != 0) {
            t = ll.poll();

            if (t != null) {
               exclusiveIncrementSize(-1);

               if (ll.size() == 0) {
                  if (highestPriority == i) {
                     highestPriority--;
                  }
               }
            }

            break;
         }
      }

      return t;
   }

   @Override
   public void clear() {
      for (LinkedListImpl<T> list : levels) {
         list.clear();
      }

      exclusiveSetSize(0);
   }

   private void exclusiveIncrementSize(int amount) {
      SIZE_UPDATER.lazySet(this, this.size + amount);
   }

   private void exclusiveSetSize(int value) {
      SIZE_UPDATER.lazySet(this, value);
   }

   @Override
   public int size() {
      return size;
   }

   @Override
   public boolean isEmpty() {
      return size == 0;
   }

   @Override
   public LinkedListIterator<T> iterator() {
      return new PriorityLinkedListIterator();
   }

   //......
}

• PriorityLinkedListImpl实现了PriorityLinkedList接口，其构造器需要priorities参数，它使用Array.newInstance(LinkedListImpl.class, priorities)来创建并初始化levels数组，其数组元素类型为LinkedListImpl；其addHead、addTail、addSorted先执行checkHighest(priority)维护highestPriority，之后调用对应priority的LinkedListImpl的addHead、addTail、addSorted方法，最后调用exclusiveIncrementSize方法递增size；其poll方法会从highestPriority的LinkedListImpl开始poll

小结

CoreMessage定义了priority属性(Values range from 0 (less priority) to 9 (more priority) inclusive)，并提供了getPriority、setPriority方法；QueueImpl定义了messageReferences，其类型为PriorityLinkedList<MessageReference>；其internalAddTail、internalAddHead、internalAddSorted方法都会调用getPriority方法获取priority，出现异常返回4，之后通过messageReferences的addTail、addHead、addSorted方法添加到队列；PriorityLinkedListImpl实现了PriorityLinkedList接口，其构造器需要priorities参数，它使用Array.newInstance(LinkedListImpl.class, priorities)来创建并初始化levels数组，其数组元素类型为LinkedListImpl；PriorityLinkedListImpl的addHead、addTail、addSorted均委托给LinkedListImpl类

doc

• QueueImpl