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From Andreas Gies <>
Subject Re: Duplicate messages durable subscriber
Date Tue, 04 Nov 2014 15:16:11 GMT
Hi Tim,

I will make some time tomorrow to try your suggestions.

A Virtual destination would simply replace a durable subscriber with a 
Queue receiver.
Messages posted to a mapped topic would end up in the the underlying 

Now, a consumer that wasn't connected to the peer broker would 
immediately receive
the messages on the broker - that would address the problem of 
temporarily unvisible

As for the duplicated delivery of messages - effectively we would have 
concurrent consumers
(even though most likely onlz one of the would be connected at any one 
point in time).
A message consumed on either side of the NWOB would effectively vanish 
from the underlying
Queue and not be delivered again.

Due to that I was contemplating a distributed broker plugin that would 
kind of synchronize the
consumption of messages of a DS within a NWOB.

However, before going that path I wanted to make sure that I am not 
missing something obvious.

Thanks for the answers so far and best regards

On 03/11/14 20:23, Tim Bain wrote:
> Andreas,
> Your spec added two configuration elements your previous post didn't
> mention, and I'd like to eliminate each of them in turn to see if it's
> causing/contributing to the problem.
>     1. Your networkConnectors are apparently multicast.  Please see what
>     happens if you configure them as
>     static:(tcp://host:port?tcpOptions)?staticOptions, to take the multicast
>     (and the broker discovery that it's presumably doing) out of the equation.
>     I recently experimented with what happens when the failover is allowed to
>     perform a reconnect in a broker-to-broker networkConnector, and the result
>     is duplicate and/or stale subscriptions between the brokers.  That behavior
>     could explain what you're seeing, if multicast is similarly performing
>     reconnects without notifying the static wrapper so it can recreate the
>     network bridge, so let's take it out of the equation to see if the behavior
>     changes.  (I've never used multicast, so this might not make sense; if
>     someone knows that this can't be the issue, please say so.)
>     2. I don't know how gracefully conduitSubscriptions reacts to consumers
>     moving around the network of brokers; I don't believe this should be the
>     problem, but if #1 doesn't produce any change in behavior, can you set
>     conduitSubscriptions=false and see if anything changes?
> I'm not clear on how Virtual Topics will solve the problem; can you
> explain?  To me this feels like a problem with broker-to-broker management
> of subscriptions made on behalf of clients (most likely duplicate
> subscriptions for a client, one from each broker, after a failover), and
> I'm not sure how a Virtual Topic would make it any better if that's the
> case.  But if you know of a way that it would, that might help me to
> understand what's going on.
> Tim
> On Sat, Nov 1, 2014 at 9:50 AM, Andreas Gies <>
> wrote:
>> Hello Tim,
>> thanks for your answer. It took me a bit to digest it - so my apologies for
>> the delay in my answer.
>> I have come up with a test case that shows - and unfortunately confirms
>> my observations.The test case is located at [1].
>> Here is the excerpt of my problem descriptions & observations:
>> /**
>>   * This specification shall help to investigate the duplicate delivery of
>> messages for durable subscribers
>>   * within a network of brokers. The problem has been posted on the
>> ActiveMQ mailing list on Oct. 18th 2014
>>   * and was described as follows:
>>   *
>>   * Suppose you have a network of brokers consisting of two members
>> discovering each other via multicast.
>>   * The network bridge is set up using conduit subscriptions. Now assume
>> that we have a durable subscriber
>>   * named "S" that connects to the network of brokers using a failover uri
>> pointing to both brokers.
>>   *
>>   * First, the subscriber connects to Broker A. It will consume all
>> messages published to either Broker A or B.
>>   * Now the subscriber disconnects and stays offline for a bit, then it
>> reconnects to Broker B. Now it will pick
>>   * up all messages that have been published while it was offline.
>>   *
>>   * Let's say then 10 messages are published. All is well as the subscriber
>> consumes those messages.
>>   * If the subscriber then disconnects and reconnects to Broker A, these 10
>> messages will be consumed
>>   * again by the reconnected subscriber.
>>   *
>>   * According to Tim Bain on Oct., 20th 2014 this indicates a bug rather
>> than a missing feature in ActiveMQ
>>   * and this Spec shall pinpoint the behavior.
>>   * *
>>   * The test is based on ActiveMQ 5.10
>>   *
>>   * Observations:
>>   * -------------
>>   * Depending on when the durable subscriber is known to the members of the
>> NWOB, messages can be either left pending
>>   * or delivered repeatedly (see the last 2 test cases). Message gaps can
>> occur, if the DS has only connected
>>   * to one broker so far. If the DS then disconnects and after a while
>> reconnects to the other broker it wasn't
>>   * connected to so far, it will not see the messages that have been
>> produced while it was offline (it will see
>>   * those messages after reconnecting to broker 1).
>>   *
>>   * Dupilcate delivery will happen if the DS was already connected to both
>> brokers. From the broker's perspective
>>   * it seems that those DS are handled as two distinct subscribers, so
>> effectively all messages that are published
>>   * will eventually be delivered to both subscribers.
>>   */
>> I know that Virtual Topics could solve the problem - however we in the
>> middleware team are not in control of that
>> particular client application and therefor we cannot change the consumer
>> from a DS to a queue consumer.
>> Can you confirm that we are indeed looking at a missing feature or a bug
>> in ActiveMQ 5.10 ? - Otherwise i would
>> need to get my thinking cap back on and see how I could solve the problem
>> without changing the client code.
>> [1]
>> <
>> Thanks and best regards
>> Andreas
>>> On 20 Oct 2014, at 17:40, Tim Bain <> wrote:
>>> If you have a network of brokers, messages on topics will be forwarded to
>>> whichever broker the consumer connects to, without duplicate delivery of
>>> any messages so long as no messages were processed by the consumer
>> without
>>> being ack'ed.  If you were using queues, there's the potential for
>> messages
>>> to get stranded on a broker if no consumers are left, but this isn't
>>> possible for topics.  (I'm not clear on the reason that topics can't get
>>> messages stranded even when consumers bounce between brokers, and
>>> unfortunately
>> doesn't
>>> describe why that is.)
>>> So I think that ActiveMQ's base capabilities will do exactly what you
>> want,
>>> and if you're seeing redelivery of messages that were successfully acked
>>> when the consumer bounces to another broker, I think that would indicate
>> a
>>> bug in ActiveMQ rather than a missing feature.
>>> On Sun, Oct 19, 2014 at 6:05 PM, Noel OConnor <>
>>> wrote:
>>>> Take a look at idempotent consumers in camel. This may help you out as a
>>>> basis for your plugin if you decide to go with it.
>>>> On Oct 18, 2014 5:47 PM, "Andreas Gies" <>
>> wrote:
>>>>> Hi
>>>>> I am using ActiveMQ 5.10 in an application. So far the requirement for
>>>> the
>>>>> remote locations has been for pure store and forward capabilities,
>>>>> so that a single AMQ broker was sufficient. This has changed in a way
>>>> that
>>>>> now 2 nodes should be present in the remote location for
>>>>> resilience and load balancing. I had considered a master/configuration
>> as
>>>>> the requirement for resilience is stronger than that for load
>> balancing.
>>>>> However, the situation in those locations is that I don’t have a shared
>>>> db
>>>>> nor a shared filesystem. As far as I have understood the replicated
>>>> level db
>>>>> would require at least 3 nodes ?
>>>>> This is why I have chosen a network of brokers in the end, which works
>>>>> well for any Queue based communication.
>>>>> Now my problem is that there is one client application that is provided
>>>> by
>>>>> a 3rd party and uses durable subscriptions. It would be quite an effort
>>>>> to change that application towards using queues, so that I could
>> consider
>>>>> virtual destinations.
>>>>> The problem occurs two-fold:
>>>>> Assume a  Subscriber connects to BrokerA, then disconnects and
>> reconnects
>>>>> to Broker B. It consumes messages for a while, than disconnects
>>>>> and reconnects to Broker A. All messages that have already been
>> consumed
>>>>> while it was connected to Broker B will be delivered again.
>>>>> My question is now whether this could be avoided by means of ActiveMQ
>>>>> alone ? - I was contemplating a broker plugin to track messages that
>>>>> have been consumed on other nodes so that I could avoid redelivering
>> them
>>>>> again.
>>>>> Sorry if thats a bit vague - I am fishing for ideas ….
>>>>> Thanks and best regards
>>>>> Andreas


    Andreas Gies

WoQ – Way of Quality GmbH

Geschäftsführer & CTO

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