Commands with same priority are not handled in the correct order

[MGathier]

Basic information

• Axon Framework version: 4.5.8
• JDK version: 11
• Complete executable reproducer if available (e.g. GitHub Repo):

Steps to reproduce

AxonServerCommandBus is using a PriorityBlockingQueue to queue commands to execute. When the priority of 2 items is the same,
this queue does not respect the order in which they are queued. A simple testcase for this:

@Test
    public void blockingQueueTest() throws InterruptedException {
        PriorityBlockingQueue<SimpleMessage> priorityBlockingQueue = new PriorityBlockingQueue<SimpleMessage>();
        priorityBlockingQueue.put(new SimpleMessage(0, "1"));
        priorityBlockingQueue.put(new SimpleMessage(0, "2"));
        priorityBlockingQueue.put(new SimpleMessage(0, "3"));
        priorityBlockingQueue.put(new SimpleMessage(0, "4"));
        priorityBlockingQueue.put(new SimpleMessage(0, "5"));
        priorityBlockingQueue.put(new SimpleMessage(0, "6"));

        assertEquals("1", priorityBlockingQueue.take().data);
        assertEquals("2", priorityBlockingQueue.take().data);
        assertEquals("3", priorityBlockingQueue.take().data);
        assertEquals("4", priorityBlockingQueue.take().data);
        assertEquals("5", priorityBlockingQueue.take().data);
        assertEquals("6", priorityBlockingQueue.take().data);
    }
    
    private class SimpleMessage implements Comparable<SimpleMessage>{
        private final long priority;
        private final String data;

        private SimpleMessage(long priority, String data) {
            this.priority = priority;
            this.data = data;
        }

        @Override
        public int compareTo(@NotNull SimpleMessage o) {
            return Long.compare(priority, o.priority);
        }
    }

With high volumes of commands, this could lead to starvation. An older command remains in the queue for a long time as the newer commands are picked up first.

Expected behaviour

When commands with the same priority are queued, I expect that the first command will be taken first.

Actual behaviour

Order in which they are taken is random.

[smcvb]

Closing this issue as it's resolved in pull request #2263.