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
queues.
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
messages.
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
Andreas
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 <andr...@wayofquality.de>
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]
https://github.com/woq-blended/blended/blob/master/blended-testing/blended-testing-activemq/src/test/scala/de/woq/blended/testing/activemq/DurableSubscriberSpec.scala
<
https://github.com/woq-blended/blended/blob/master/blended-testing/blended-testing-activemq/src/test/scala/de/woq/blended/testing/activemq/DurableSubscriberSpec.scala
Thanks and best regards
Andreas
On 20 Oct 2014, at 17:40, Tim Bain <tb...@alumni.duke.edu> 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 http://activemq.apache.org/networks-of-brokers.html
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 <noel.ocon...@gmail.com>
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" <andr...@wayofquality.de>
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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