Github user michaelandrepearce commented on a diff in the pull request:
https://github.com/apache/activemq-artemis/pull/2490#discussion_r246518313
--- Diff:
artemis-server/src/main/java/org/apache/activemq/artemis/core/server/impl/QueueConsumersImpl.java
---
@@ -0,0 +1,124 @@
+/*
+ * 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.activemq.artemis.core.server.impl;
+
+import org.apache.activemq.artemis.core.PriorityAware;
+import org.apache.activemq.artemis.utils.collections.PriorityCollection;
+import org.apache.activemq.artemis.utils.collections.ResettableIterator;
+
+import java.util.Collection;
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.Set;
+import java.util.Spliterator;
+import java.util.concurrent.CopyOnWriteArraySet;
+import java.util.concurrent.atomic.AtomicReferenceFieldUpdater;
+import java.util.function.Consumer;
+
+/**
+ * This class's purpose is to hold the consumers.
+ *
+ * CopyOnWriteArraySet is used as the underlying collection to the
PriorityCollection, as it is concurrent safe,
+ * but also lock less for a read path, which is our HOT path.
+ * Also it was the underlying collection previously used in QueueImpl,
before we abstracted it out to support priority consumers.
+ *
+ * There can only be one resettable iterable view,
+ * A new iterable view is created on modification, this is to keep the
read HOT path performent, BUT
+ * the iterable view changes only after reset so changes in the underlying
collection are only seen after a reset,
+ *
+ * All other iterators created by iterators() method are not reset-able
and are created on delegating iterator().
+ *
+ * @param <T> The type this class may hold, this is generic as can be
anything that extends PriorityAware,
+ * but intent is this is the QueueImpl:ConsumerHolder.
+ */
+public class QueueConsumersImpl<T extends PriorityAware> implements
QueueConsumers<T> {
+
+ private final PriorityCollection<T> consumers = new
PriorityCollection<>(CopyOnWriteArraySet::new);
+ private final Collection<T> unmodifiableConsumers =
Collections.unmodifiableCollection(consumers);
+ private final AtomicReferenceFieldUpdater<QueueConsumersImpl,
ResettableIterator> changedIteratorFieldUpdater =
AtomicReferenceFieldUpdater.newUpdater(QueueConsumersImpl.class,
ResettableIterator.class, "changedIterator");
+ private volatile ResettableIterator<T> changedIterator;
+ private ResettableIterator<T> currentIterator =
consumers.resettableIterator();
+
+ @Override
+ public Set<Integer> getPriorites() {
+ return consumers.getPriorites();
+ }
+
+ @Override
+ public boolean hasNext() {
+ return currentIterator.hasNext();
+ }
+
+ @Override
+ public T next() {
+ return currentIterator.next();
+ }
+
+ @Override
+ public QueueConsumers<T> reset() {
+ if (changedIterator != null) {
+ currentIterator = changedIterator;
+ changedIteratorFieldUpdater.compareAndSet(this, changedIterator,
null);
--- End diff --
If it fails it means whilst we were reseting and changing reference to the
new iterator. a consumer was added or removed and another new iterator was
created (as this can occur whilsy we are iterating or resetting.) And means
simply whilst this reset updated its iterator, on next reset it needs to switch
over its again. We wouldnt want to lazy set to null
---
