Author: buildbot
Date: Fri Apr 13 10:23:57 2018
New Revision: 1028347
Log:
Production update by buildbot for activemq
Modified:
websites/production/activemq/content/cache/main.pageCache
websites/production/activemq/content/virtual-destinations.html
Modified: websites/production/activemq/content/cache/main.pageCache
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+++ websites/production/activemq/content/virtual-destinations.html Fri Apr 13
10:23:57 2018
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<title>
Apache ActiveMQ ™ -- Virtual Destinations
@@ -71,7 +81,11 @@
<tbody>
<tr>
<td valign="top" width="100%">
-<div class="wiki-content maincontent"><p><em>Virtual Destinations</em> allow
us to create logical destinations that clients can use to produce and consume
from but which map onto one or more <em>physical destinations</em>. It allows
us to provide more flexible loosely coupled messaging configurations.</p><h2
id="VirtualDestinations-VirtualTopics">Virtual Topics</h2><p>The idea behind
<em>publish subscribe</em> is a great one. Allow producers to be decoupled from
consumers so that they do not even know how many consumers are interested in
the messages they publish. The JMS specification defines support for durable
topics however they have limitations as we will describe...</p><h3
id="VirtualDestinations-ThelimitationsofJMSdurabletopics">The limitations of
JMS durable topics</h3><p>A JMS durable subscriber MessageConsumer is created
with a unique JMS clientID and durable subscriber name. To be JMS compliant
only one JMS connection can be active at any point in time for one JMS clientI
D, and only one consumer can be active for a clientID and subscriber name.
i.e., only <strong>one</strong> thread can be actively consuming from a given
logical topic subscriber. This means we cannot implement</p><ul><li>load
balancing of messages.</li><li>fast failover of the subscriber if that one
process running that one consumer thread dies.</li></ul><p>Now <em>queue</em>
semantics in JMS offer the ability to load balance work across a number of
consumers in a reliable way - allowing many threads, processes and machines to
be used to process messages. Then we have sophisticated sticky load balancing
techniques like <a shape="rect" href="message-groups.html">Message Groups</a>
to load balance and parallelise work while maintaining ordering.</p><p>Another
added benefit of having physical queues for each logical topic subscriber is we
can then monitor the queue depths via <a shape="rect" href="jmx.html">JMX</a>
to monitor system performance together with being able to browse these
physical queues.</p><h3
id="VirtualDestinations-VirtualTopicstotherescue">Virtual Topics to the
rescue</h3><p>The idea behind virtual topics is that producers send to a topic
in the usual JMS way. Consumers can continue to use the Topic semantics in the
JMS specification. However if the topic is virtual, consumer can consume from a
physical queue for a logical topic subscription, allowing many consumers to be
running on many machines & threads to load balance the load.</p><p>E.g.,
let's say we have a topic called <strong>VirtualTopic.Orders</strong>. (Where
the prefix VirtualTopic. indicates its a virtual topic). And we logically want
to send orders to systems A and B. Now with regular durable topics we'd create
a JMS consumer for clientID_A and "A" along with clientID_B and "B".</p><p>With
virtual topics we can just go right ahead and consume to queue
<strong>Consumer.A.VirtualTopic.Orders</strong> to be a consumer for system A
or consume to <strong>Consumer.B.VirtualTopic.Orde
rs</strong> to be a consumer for system B.</p><p>We can now have a pool of
consumers for each system which then compete for messages for systems A or B
such that all the messages for system A are processed exactly once and
similarly for system B.</p><h3
id="VirtualDestinations-Customizingtheout-of-the-boxdefaults">Customizing the
out-of-the-box defaults</h3><p>The out-of-the-box defaults are described above.
Namely that the only virtual topics available must be within the
<strong>VirtualTopic.></strong> namespace and that the consumer queues are
named <strong>Consumer.*.VirtualTopic.></strong>.</p><p>You can configure
this to use whatever naming convention you wish. The following <a shape="rect"
class="external-link"
href="https://svn.apache.org/repos/asf/incubator/activemq/trunk/activemq-unit-tests/src/test/resources/org/apache/activemq/broker/virtual/global-virtual-topics.xml";>example</a>
shows how to make all topics virtual topics. The example below is using the
name <stron
g>></strong> to indicate 'match all topics'. You could use this wildcard to
apply different virtual topic policies in different hierarchies.</p><pre><span
style="color: rgb(0,0,0);"><destinationInterceptors>
</span></pre><pre><span style="color: rgb(0,0,0);">
<virtualDestinationInterceptor> </span></pre><pre><span style="color:
rgb(0,0,0);"> <virtualDestinations> </span></pre><pre><span style="color:
rgb(0,0,0);"> <virtualTopic name=">" prefix="VirtualTopicConsumers.*."
selectorAware="false"/> </span> </pre><pre>
</virtualDestinations></pre><pre><span style="color: rgb(0,0,0);">
</virtualDestinationInterceptor> </span></pre><pre><span style="color:
rgb(0,0,0);"></destinationInterceptors></span></pre><p> </p><p>Note
that making a topic virtual does add a small CPU overhead when sending messages
to the topic but it is fairly small.</p><div class="table-wrap"><table
class="confluenceTable"><tbody><tr><th colspan="1" rowspan="1" c
lass="confluenceTh">Option</th><th colspan="1" rowspan="1"
class="confluenceTh">Default</th><th colspan="1" rowspan="1"
class="confluenceTh">Description</th></tr><tr><td colspan="1" rowspan="1"
class="confluenceTd">selectorAware</td><td colspan="1" rowspan="1"
class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">only messages that match one of the existing subscribers
are actually dispatched. Using this option prevents the build up of unmatched
messages when selectors are used by exclusive consumers</td></tr><tr><td
colspan="1" rowspan="1" class="confluenceTd">local</td><td colspan="1"
rowspan="1" class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">when true, don't fan out messages that were received over
a network</td></tr><tr><td colspan="1" rowspan="1"
class="confluenceTd">concurrentSend</td><td colspan="1" rowspan="1"
class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">when true, use an executor t
o fanout such that sends occur in parallel. This allows the journal to batch
writes which will reduce disk io (5.12)</td></tr><tr><td colspan="1"
rowspan="1" class="confluenceTd">transactedSend</td><td colspan="1" rowspan="1"
class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">when true, use a transaction for fanout sends such that
there is a single disk sync. A local broker transaction will be created if
there is no client transaction (5.13)</td></tr><tr><td colspan="1" rowspan="1"
class="confluenceTd"><span>dropOnResourceLimit</span></td><td colspan="1"
rowspan="1" class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">when true, ignore any ResourceAllocationException thrown
during fanout (see: sendFailIfNoSpace policy entry)
(5.16)</td></tr></tbody></table></div><p> </p><h2
id="VirtualDestinations-CompositeDestinations">Composite
Destinations</h2><p>Composite Destinations allow for one-to-many relationships
on individual d
estinations; the main use case is for <em>composite queues</em>. For example
when a message is sent to queue A you may want to forward it also to queues B
and C and topic D. Composite destinations are then a mapping from a virtual
destination to a collection of other physical destinations. In this case the
mapping is broker side and the client is unaware of the mapping between the
destinations. This is different from client side <a shape="rect"
href="composite-destinations.html">Composite Destinations</a> where the client
uses a URL notation to specify the actual physical destinations that a message
must be sent to.</p><p>The following <a shape="rect" class="external-link"
href="http://svn.apache.org/repos/asf/incubator/activemq/trunk/activemq-unit-tests/src/test/resources/org/apache/activemq/broker/virtual/composite-queue.xml";>example</a>
shows how to set up a <strong><compositeQueue/></strong> element in the
XML configuration so that when a message is sent to <code>MY.QUEUE<
/code> then it is really forwarded to the physical queue <code>FOO</code> and
the topic <code>BAR</code>.</p><pre><span style="color:
rgb(0,0,0);"><destinationInterceptors></span></pre><pre><span
style="color: rgb(0,0,0);"> <virtualDestinationInterceptor>
</span></pre><pre><span style="color: rgb(0,0,0);"> <virtualDestinations>
</span></pre><pre><span style="color: rgb(0,0,0);"> <compositeQueue
name="MY.QUEUE"></span></pre><pre><span style="color: rgb(0,0,0);">
<forwardTo></span></pre><pre><span style="color: rgb(0,0,0);"> <queue
physicalName="FOO" /> </span></pre><pre><span style="color: rgb(0,0,0);">
<topic physicalName="BAR" /></span></pre><pre><span style="color:
rgb(0,0,0);"> </forwardTo></span></pre><pre><span style="color:
rgb(0,0,0);"> </compositeQueue></span></pre><pre><span style="color:
rgb(0,0,0);"> </virtualDestinations></span></pre><pre><span style="color:
rgb(0,0,0);"> </virtualDestinationInterceptor></
span></pre><pre><span style="color:
rgb(0,0,0);"></destinationInterceptors></span></pre><p> </p><p>By
default, subscribers cannot consume messages directly from a composite queue or
topic - it is a logical construct only. Given the configuration above,
subscribers can only consume messages from <code>FOO</code> and
<code>BAR</code>; but not <code>MY.QUEUE</code>.</p><p>This behaviour can be
altered to implement use cases such as watching a queue by sending the same
messages to a notification topic (wire tapping), by setting the optionally set
<code>forwardOnly</code> attribute to false.</p><pre><compositeQueue
name="IncomingOrders" forwardOnly="false"> </pre><pre>
<forwardTo></pre><pre> <topic physicalName="Notifications"
/></pre><pre> </forwardTo></pre><pre>
</compositeQueue></pre><p> </p><p>Messages sent to
<code>IncomingOrders</code> will all be copied and forwarded to
<code>Notifications</code>, before being placed on the phys
ical <code>IncomingOrders</code> queue for consumption by
subscribers.</p><p>Where the <code>forwardOnly</code> attribute is not defined
or is set to <code>true</code>, there is no logical difference between a
<code>compositeQueue</code> and a <code>compositeTopic</code> - they can be
used interchangeably. It is only when a composite destination is made physical
through the use of <code>forwardOnly</code> that the choice of
<code>compositeTopic</code>/<code>compositeQueue</code> has an impact on
behavior.</p><h3 id="VirtualDestinations-Usingfiltereddestinations">Using
filtered destinations</h3><p>From Apache ActiveMQ <strong>4.2</strong> onwards
you can now use selectors to define virtual destinations.</p><p>You may wish to
create a virtual destination which forwards messages to multiple destinations
but applying a selector first to decide if the message really does have to go
to a particular destination.</p><p>The following example shows how a message
sent to the virtual destinatio
n <strong>MY.QUEUE</strong> will be forwarded to <strong>FOO</strong> and
<strong>BAR</strong> if the selectors
match</p><pre><destinationInterceptors>
<virtualDestinationInterceptor> <virtualDestinations> </pre><pre>
<compositeQueue name="MY.QUEUE"></pre><pre>
<forwardTo></pre><pre> <filteredDestination selector="odd = 'yes'"
queue="FOO"/></pre><pre> <filteredDestination selector="i = 5"
topic="BAR"/></pre><pre> </forwardTo></pre><pre>
</compositeQueue></pre><pre></virtualDestinations>
</virtualDestinationInterceptor>
</destinationInterceptors></pre><p> </p><h2
id="VirtualDestinations-AvoidingDuplicateMessageinaNetworkofBrokers">Avoiding
Duplicate Message in a Network of Brokers</h2><p><strong>TLDR</strong>: bridge
consumer queues or virtual topics, not both.</p><p>Typically you would network
consumer queues. In this case it is important to not bridge any normal topic
consumer on the virtu
al topic because any forwarded message would again get fanned out to consumer
queues on the networked broker, leading to duplicates.</p><p>It is also
possible to bridge the virtual topic in which case it is necessary exclude the
consumer queues from any network connector configuration. </p><p>Here is
an example of how to exclude virtual topic consumer queues:</p><pre><span
style="color: rgb(0,0,0);"><networkConnectors> <networkConnector
uri="static://(<a shape="rect" href="tcp://localhost:61617"
rel="nofollow">tcp://localhost:61617</a>)"></span></pre><pre><span
style="color: rgb(0,0,0);"> <excludedDestinations>
</span></pre><pre><span style="color: rgb(0,0,0);"> <queue
physicalName="Consumer.*.VirtualTopic.>"/> </span></pre><pre><span
style="color: rgb(0,0,0);"> </excludedDestinations>
</span></pre><pre><span style="color: rgb(0,0,0);"></networkConnector>
</networkConnectors></span></pre><p> </p></div>
+<div class="wiki-content maincontent"><p><em>Virtual Destinations</em> allow
us to create logical destinations that clients can use to produce and consume
from but which map onto one or more <em>physical destinations</em>. It allows
us to provide more flexible loosely coupled messaging configurations.</p><h2
id="VirtualDestinations-VirtualTopics">Virtual Topics</h2><p>The idea behind
<em>publish subscribe</em> is a great one. Allow producers to be decoupled from
consumers so that they do not even know how many consumers are interested in
the messages they publish. The JMS specification defines support for durable
topics however they have limitations as we will describe...</p><h3
id="VirtualDestinations-ThelimitationsofJMSdurabletopics">The limitations of
JMS durable topics</h3><p>A JMS durable subscriber MessageConsumer is created
with a unique JMS clientID and durable subscriber name. To be JMS compliant
only one JMS connection can be active at any point in time for one JMS clientI
D, and only one consumer can be active for a clientID and subscriber name.
i.e., only <strong>one</strong> thread can be actively consuming from a given
logical topic subscriber. This means we cannot implement</p><ul><li>load
balancing of messages.</li><li>fast failover of the subscriber if that one
process running that one consumer thread dies.</li></ul><p>Now <em>queue</em>
semantics in JMS offer the ability to load balance work across a number of
consumers in a reliable way - allowing many threads, processes and machines to
be used to process messages. Then we have sophisticated sticky load balancing
techniques like <a shape="rect" href="message-groups.html">Message Groups</a>
to load balance and parallelise work while maintaining ordering.</p><p>Another
added benefit of having physical queues for each logical topic subscriber is we
can then monitor the queue depths via <a shape="rect" href="jmx.html">JMX</a>
to monitor system performance together with being able to browse these
physical queues.</p><h3
id="VirtualDestinations-VirtualTopicstotherescue">Virtual Topics to the
rescue</h3><p>The idea behind virtual topics is that producers send to a topic
in the usual JMS way. Consumers can continue to use the Topic semantics in the
JMS specification. However if the topic is virtual, consumer can consume from a
physical queue for a logical topic subscription, allowing many consumers to be
running on many machines & threads to load balance the load.</p><p>E.g.,
let's say we have a topic called <strong>VirtualTopic.Orders</strong>. (Where
the prefix VirtualTopic. indicates its a virtual topic). And we logically want
to send orders to systems A and B. Now with regular durable topics we'd create
a JMS consumer for clientID_A and "A" along with clientID_B and "B".</p><p>With
virtual topics we can just go right ahead and consume to queue
<strong>Consumer.A.VirtualTopic.Orders</strong> to be a consumer for system A
or consume to <strong>Consumer.B.VirtualTopic.Orde
rs</strong> to be a consumer for system B.</p><p>We can now have a pool of
consumers for each system which then compete for messages for systems A or B
such that all the messages for system A are processed exactly once and
similarly for system B.</p><h3
id="VirtualDestinations-Customizingtheout-of-the-boxdefaults">Customizing the
out-of-the-box defaults</h3><p>The out-of-the-box defaults are described above.
Namely that the only virtual topics available must be within the
<strong>VirtualTopic.></strong> namespace and that the consumer queues are
named <strong>Consumer.*.VirtualTopic.></strong>.</p><p>You can configure
this to use whatever naming convention you wish. The following <a shape="rect"
class="external-link"
href="https://svn.apache.org/repos/asf/incubator/activemq/trunk/activemq-unit-tests/src/test/resources/org/apache/activemq/broker/virtual/global-virtual-topics.xml";>example</a>
shows how to make all topics virtual topics. The example below is using the
name <stron
g>></strong> to indicate 'match all topics'. You could use this wildcard to
apply different virtual topic policies in different hierarchies.</p><pre><span
style="color: rgb(0,0,0);"><destinationInterceptors>
</span></pre><pre><span style="color: rgb(0,0,0);">
<virtualDestinationInterceptor> </span></pre><pre><span style="color:
rgb(0,0,0);"> <virtualDestinations> </span></pre><pre><span style="color:
rgb(0,0,0);"> <virtualTopic name=">" prefix="VirtualTopicConsumers.*."
selectorAware="false"/> </span> </pre><pre>
</virtualDestinations></pre><pre><span style="color: rgb(0,0,0);">
</virtualDestinationInterceptor> </span></pre><pre><span style="color:
rgb(0,0,0);"></destinationInterceptors></span></pre><p> </p><p>Note
that making a topic virtual does add a small CPU overhead when sending messages
to the topic but it is fairly small.</p><div class="table-wrap"><table
class="confluenceTable"><tbody><tr><th colspan="1" rowspan="1" c
lass="confluenceTh">Option</th><th colspan="1" rowspan="1"
class="confluenceTh">Default</th><th colspan="1" rowspan="1"
class="confluenceTh">Description</th></tr><tr><td colspan="1" rowspan="1"
class="confluenceTd">selectorAware</td><td colspan="1" rowspan="1"
class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">only messages that match one of the existing subscribers
are actually dispatched. Using this option prevents the build up of unmatched
messages when selectors are used by exclusive consumers</td></tr><tr><td
colspan="1" rowspan="1" class="confluenceTd">local</td><td colspan="1"
rowspan="1" class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">when true, don't fan out messages that were received over
a network</td></tr><tr><td colspan="1" rowspan="1"
class="confluenceTd">concurrentSend</td><td colspan="1" rowspan="1"
class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">when true, use an executor t
o fanout such that sends occur in parallel. This allows the journal to batch
writes which will reduce disk io (5.12)</td></tr><tr><td colspan="1"
rowspan="1" class="confluenceTd">transactedSend</td><td colspan="1" rowspan="1"
class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">when true, use a transaction for fanout sends such that
there is a single disk sync. A local broker transaction will be created if
there is no client transaction (5.13)</td></tr><tr><td colspan="1" rowspan="1"
class="confluenceTd"><span>dropOnResourceLimit</span></td><td colspan="1"
rowspan="1" class="confluenceTd">false</td><td colspan="1" rowspan="1"
class="confluenceTd">when true, ignore any ResourceAllocationException thrown
during fanout (see: sendFailIfNoSpace policy entry)
(5.16)</td></tr></tbody></table></div><p> </p><h2
id="VirtualDestinations-VirtualSelectorCacheBrokerPlugin"><span style="color:
rgb(0,0,0);">VirtualSelectorCacheBrokerPlugin</span></h2><p><span style="
color: rgb(0,0,0);">When selectorAware=true, only active consumers are
condidered for selector matching. If consumers disconnect and reconnect they
will miss messages. The intent of selectorAware=true is to not have
messages build up. </span><span style="color:
rgb(0,0,0);">The virtualSelectorCacheBrokerPlugin provides a cache that
tracks the selectors associated with a destination by a consumers such that
they can apply in the absense of that consumer. In this way the just the
selected messages build up. The existing set of selectors can be persisted such
that it can be recovered on restart. the plugin is applied in the normal way to
the plugins section.</span></p><div class="code panel pdl" style="border-width:
1px;"><div class="codeContent panelContent pdl">
+<pre class="brush: java; gutter: false; theme: Default"
style="font-size:12px;"><plugins>
+ <virtualSelectorCacheBrokerPlugin persistFile="<some
path>/selectorcache.data" />
+</plugins></pre>
+</div></div><p>Note: the persistFile option uses java serialisation that
should be locked down <span style="color: rgb(0,0,0);">with an
appropriate </span><a shape="rect" class="external-link"
href="http://openjdk.java.net/jeps/290";
rel="nofollow">jdk.serialFilter</a> that allows
ConcurrentHashMap</p><p><span style="color: rgb(106,106,106);"><span
style="color: rgb(106,106,106);"> </span></span></p><h2
id="VirtualDestinations-CompositeDestinations">Composite
Destinations</h2><p>Composite Destinations allow for one-to-many relationships
on individual destinations; the main use case is for <em>composite queues</em>.
For example when a message is sent to queue A you may want to forward it also
to queues B and C and topic D. Composite destinations are then a mapping from a
virtual destination to a collection of other physical destinations. In this
case the mapping is broker side and the client is unaware of the mapping
between the destinations. This is different from clie
nt side <a shape="rect" href="composite-destinations.html">Composite
Destinations</a> where the client uses a URL notation to specify the actual
physical destinations that a message must be sent to.</p><p>The following <a
shape="rect" class="external-link"
href="http://svn.apache.org/repos/asf/incubator/activemq/trunk/activemq-unit-tests/src/test/resources/org/apache/activemq/broker/virtual/composite-queue.xml";>example</a>
shows how to set up a <strong><compositeQueue/></strong> element in the
XML configuration so that when a message is sent to <code>MY.QUEUE</code> then
it is really forwarded to the physical queue <code>FOO</code> and the topic
<code>BAR</code>.</p><pre><span style="color:
rgb(0,0,0);"><destinationInterceptors></span></pre><pre><span
style="color: rgb(0,0,0);"> <virtualDestinationInterceptor>
</span></pre><pre><span style="color: rgb(0,0,0);"> <virtualDestinations>
</span></pre><pre><span style="color: rgb(0,0,0);"> <compositeQueue name="
MY.QUEUE"></span></pre><pre><span style="color: rgb(0,0,0);">
<forwardTo></span></pre><pre><span style="color: rgb(0,0,0);"> <queue
physicalName="FOO" /> </span></pre><pre><span style="color: rgb(0,0,0);">
<topic physicalName="BAR" /></span></pre><pre><span style="color:
rgb(0,0,0);"> </forwardTo></span></pre><pre><span style="color:
rgb(0,0,0);"> </compositeQueue></span></pre><pre><span style="color:
rgb(0,0,0);"> </virtualDestinations></span></pre><pre><span style="color:
rgb(0,0,0);"> </virtualDestinationInterceptor></span></pre><pre><span
style="color:
rgb(0,0,0);"></destinationInterceptors></span></pre><p> </p><p>By
default, subscribers cannot consume messages directly from a composite queue or
topic - it is a logical construct only. Given the configuration above,
subscribers can only consume messages from <code>FOO</code> and
<code>BAR</code>; but not <code>MY.QUEUE</code>.</p><p>This behaviour can be
altered to implement
use cases such as watching a queue by sending the same messages to a
notification topic (wire tapping), by setting the optionally set
<code>forwardOnly</code> attribute to false.</p><pre><compositeQueue
name="IncomingOrders" forwardOnly="false"> </pre><pre>
<forwardTo></pre><pre> <topic physicalName="Notifications"
/></pre><pre> </forwardTo></pre><pre>
</compositeQueue></pre><p> </p><p>Messages sent to
<code>IncomingOrders</code> will all be copied and forwarded to
<code>Notifications</code>, before being placed on the physical
<code>IncomingOrders</code> queue for consumption by subscribers.</p><p>Where
the <code>forwardOnly</code> attribute is not defined or is set to
<code>true</code>, there is no logical difference between a
<code>compositeQueue</code> and a <code>compositeTopic</code> - they can be
used interchangeably. It is only when a composite destination is made physical
through the use of <code>forwardOnly</code> that the choice
of <code>compositeTopic</code>/<code>compositeQueue</code> has an impact on
behavior.</p><h3 id="VirtualDestinations-Usingfiltereddestinations">Using
filtered destinations</h3><p>From Apache ActiveMQ <strong>4.2</strong> onwards
you can now use selectors to define virtual destinations.</p><p>You may wish to
create a virtual destination which forwards messages to multiple destinations
but applying a selector first to decide if the message really does have to go
to a particular destination.</p><p>The following example shows how a message
sent to the virtual destination <strong>MY.QUEUE</strong> will be forwarded to
<strong>FOO</strong> and <strong>BAR</strong> if the selectors
match</p><pre><destinationInterceptors>
<virtualDestinationInterceptor> <virtualDestinations> </pre><pre>
<compositeQueue name="MY.QUEUE"></pre><pre>
<forwardTo></pre><pre> <filteredDestination selector="odd = 'yes'"
queue="FOO"/></pre><pre> <filteredDestinatio
n selector="i = 5" topic="BAR"/></pre><pre>
</forwardTo></pre><pre>
</compositeQueue></pre><pre></virtualDestinations>
</virtualDestinationInterceptor>
</destinationInterceptors></pre><p> </p><h2
id="VirtualDestinations-AvoidingDuplicateMessageinaNetworkofBrokers">Avoiding
Duplicate Message in a Network of Brokers</h2><p><strong>TLDR</strong>: bridge
consumer queues or virtual topics, not both.</p><p>Typically you would network
consumer queues. In this case it is important to not bridge any normal topic
consumer on the virtual topic because any forwarded message would again get
fanned out to consumer queues on the networked broker, leading to
duplicates.</p><p>It is also possible to bridge the virtual topic in which case
it is necessary exclude the consumer queues from any network connector
configuration. </p><p>Here is an example of how to exclude virtual topic
consumer queues:</p><pre><span style="color: rgb(0,0,0);"><networkConnec
tors> <networkConnector uri="static://(<a shape="rect"
href="tcp://localhost:61617"
rel="nofollow">tcp://localhost:61617</a>)"></span></pre><pre><span
style="color: rgb(0,0,0);"> <excludedDestinations>
</span></pre><pre><span style="color: rgb(0,0,0);"> <queue
physicalName="Consumer.*.VirtualTopic.>"/> </span></pre><pre><span
style="color: rgb(0,0,0);"> </excludedDestinations>
</span></pre><pre><span style="color: rgb(0,0,0);"></networkConnector>
</networkConnectors></span></pre><p> </p></div>
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