Author: buildbot
Date: Thu Aug 6 12:20:57 2015
New Revision: 960958
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
==============================================================================
Binary files - no diff available.
Modified: websites/production/activemq/content/virtual-destinations.html
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--- websites/production/activemq/content/virtual-destinations.html (original)
+++ websites/production/activemq/content/virtual-destinations.html Thu Aug 6
12:20:57 2015
@@ -83,40 +83,7 @@
<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 clientID, 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 them 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.Orders</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 <strong>></strong> to indicate 'match all topics'. You could use this
wildcard to apply different virtual topic policies in different hierarchies.</p>
-
-<div class="code panel pdl" style="border-width: 1px;"><div class="codeContent
panelContent pdl">
+<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 them 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><div
class="code panel pdl" style="border-width: 1px;"><div class="codeContent
panelContent pdl">
<script class="brush: xml; gutter: false; theme: Default"
type="syntaxhighlighter"><![CDATA[
<beans
xmlns="http://www.springframework.org/schema/beans"
@@ -140,17 +107,7 @@
</beans>
]]></script>
-</div></div>
-
-<p>Note that making a topic virtual does add a small CPU overhead when sending
messages to the topic but it is fairly small. From version
<strong>5.4</strong>, dispatch from virtual topics to subscription queues can
be <strong>selectorAware</strong> such that 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.</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 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>
-
-<div class="code panel pdl" style="border-width: 1px;"><div class="codeContent
panelContent pdl">
+</div></div>Note that making a topic virtual does add a small CPU overhead
when sending messages to the topic but it is fairly small.<div
class="table-wrap"><table class="confluenceTable"><tbody><tr><th colspan="1"
rowspan="1" class="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 to fanout such that sends occur
in parallel. This allows the journal to batch writes which will reduce disk io
(5.12)</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 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 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><div class="code panel pdl" style="border-width:
1px;"><div class="codeContent panelContent pdl">
<script class="brush: xml; gutter: false; theme: Default"
type="syntaxhighlighter"><![CDATA[
<beans
xmlns="http://www.springframework.org/schema/beans"
@@ -179,33 +136,13 @@
</beans>
]]></script>
-</div></div>
-
-<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>
-
-<div class="code panel pdl" style="border-width: 1px;"><div class="codeContent
panelContent pdl">
+</div></div>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>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><div
class="code panel pdl" style="border-width: 1px;"><div class="codeContent
panelContent pdl">
<pre class="brush: xml; gutter: false; theme: Default"
style="font-size:12px;"><compositeQueue name="IncomingOrders"
forwardOnly="false">
<forwardTo>
<topic physicalName="Notifications" />
</forwardTo>
</compositeQueue></pre>
-</div></div>
-
-<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>
-
-<div class="code panel pdl" style="border-width: 1px;"><div class="codeContent
panelContent pdl">
+</div></div><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><div
class="code panel pdl" style="border-width: 1px;"><div class="codeContent
panelContent pdl">
<script class="brush: xml; gutter: false; theme: Default"
type="syntaxhighlighter"><![CDATA[
<beans
xmlns="http://www.springframework.org/schema/beans"
@@ -234,18 +171,8 @@
</beans>
]]></script>
-</div></div>
-
-
-<h2
id="VirtualDestinations-AvoidingDuplicateMessageinaNetworkofBrokers">Avoiding
Duplicate Message in a Network of Brokers</h2>
-
-<p>You have to make sure that the messages sent to the
<strong>Consumer.*.VirtualTopic.></strong> destination are not forwarded
when you're using both queue-based and non-queue based subscribers to the
virtual topic (that is, if you have normal topic subscribers to the virtual
topic). If you use Virtual Topics in a network of brokers, it is likely you
will get duplicate messages if you use the default network configuration. This
is because a network node will not only forward message sent to the virtual
topic, but also the associated physical queues. To fix this, you should
disable forwarding messages on the associated physical queues. </p>
-
-<p>Here is an example of how to do that:</p>
-
-<div class="code panel pdl" style="border-width: 1px;"><div class="codeContent
panelContent pdl">
-<pre class="brush: xml; gutter: false; theme: Default" style="font-size:12px;">
- <networkConnectors>
+</div></div><h2
id="VirtualDestinations-AvoidingDuplicateMessageinaNetworkofBrokers">Avoiding
Duplicate Message in a Network of Brokers</h2><p>You have to make sure that the
messages sent to the <strong>Consumer.*.VirtualTopic.></strong> destination
are not forwarded when you're using both queue-based and non-queue based
subscribers to the virtual topic (that is, if you have normal topic subscribers
to the virtual topic). If you use Virtual Topics in a network of brokers, it is
likely you will get duplicate messages if you use the default network
configuration. This is because a network node will not only forward message
sent to the virtual topic, but also the associated physical queues. To fix
this, you should disable forwarding messages on the associated physical
queues.</p><p>Here is an example of how to do that:</p><div class="code panel
pdl" style="border-width: 1px;"><div class="codeContent panelContent pdl">
+<pre class="brush: xml; gutter: false; theme: Default"
style="font-size:12px;"> <networkConnectors>
<networkConnector uri="static://(tcp://localhost:61617)">
<excludedDestinations>
<queue physicalName="Consumer.*.VirtualTopic.>"/>
