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     Apache ActiveMQ &#8482; -- Discovery Transport Reference
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 <div class="wiki-content maincontent"><h3 id="DiscoveryTransportReference-TheDiscoveryTransport">The Discovery Transport</h3>

 <p>The Discovery transport works just like the <a shape="rect" href="failover-transport-reference.html">Failover</a> transport, except that it uses a discovery agent to locate the list of uri to connect to.  The Discovery transport is also used by the <a shape="rect" href="fanout-transport-reference.html">Fanout</a> transport for discovering brokers to send a fanout message to.</p>

 <h4 id="DiscoveryTransportReference-ConfigurationSyntax">Configuration Syntax</h4>

 <p><strong>discovery:(discoveryAgentURI)?transportOptions</strong><br clear="none">
 or<br clear="none">
 <strong>discovery:discoveryAgentURI</strong></p>

 <p>Note that to be able to use <a shape="rect" href="discovery.html">Discovery</a> to find brokers, the brokers need to have the multicast discovery agent enabled on the broker. </p>

 <p>To configure discovery in a Broker you should use the <a shape="rect" href="xml-configuration.html">Xml Configuration</a>. Here is an <a shape="rect" class="external-link" href="http://svn.apache.org/viewvc/activemq/trunk/activemq-core/src/test/resources/org/apache/activemq/usecases/receiver-discovery.xml?view=co">example</a> of using discovery. Its basically something like the following (see the <strong>discoveryUri</strong>)</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;">
 &lt;broker name="foo"&gt;
   &lt;transportConnectors&gt;
     &lt;transportConnector uri="tcp://localhost:0" discoveryUri="multicast://default"/&gt;
   &lt;/transportConnectors&gt;

   ...
 &lt;/broker&gt;
 </pre>
 </div></div>


 <h5 id="DiscoveryTransportReference-TransportOptions">Transport Options</h5>

 <div class="table-wrap"><table class="confluenceTable"><tbody><tr><th colspan="1" rowspan="1" class="confluenceTh"><p>Option Name</p></th><th colspan="1" rowspan="1" class="confluenceTh"><p>Default Value</p></th><th colspan="1" rowspan="1" class="confluenceTh"><p>Description</p></th></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>reconnectDelay</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>10</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>How long to wait for discovery</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>initialReconnectDelay</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>10</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>How long to wait before the first reconnect attempt to a discovered url</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>maxReconnectDelay</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>30000</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>The maximum amount of time we ever wait between reconnect attempts</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>useExponentialBackOff</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>true</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>Should an exponential backoff be used btween reconnect attempts</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>backOffMultiplier</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>2</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>The exponent used in the exponential backoff attempts</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>maxReconnectAttempts</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>0</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>If not 0, then this is the maximum number of reconnect attempts before an error is sent back to the client</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>group</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>default</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p> an identifier for the group to partition multi cast traffic among collaborating peers; the group forms part of the shared identity of a discovery datagram (since 5.2) </p></td></tr></tbody></table></div>


 <h5 id="DiscoveryTransportReference-ExampleURI">Example URI</h5>

 <div class="preformatted panel" style="border-width: 1px;"><div class="preformattedContent panelContent">
 <pre>discovery:(multicast://default)?initialReconnectDelay=100
 </pre>
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 <div class="confluence-information-macro confluence-information-macro-information"><p class="title">Applying parameters to discovered transports</p><span class="aui-icon aui-icon-small aui-iconfont-info confluence-information-macro-icon"></span><div class="confluence-information-macro-body">
 <p>From 5.4, transport parameters in the URI will also be applied to discovered transports if they are prefixed with <code>discovered.</code>; For example, adding the <code>discovered.connectionTimeout</code> parameter to the URI will apply the parameter to every discovered <a shape="rect" href="tcp-transport-reference.html">TCP</a> transport, even though this parameter is not a Discovery transport option.</p></div></div></div>
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	<div class="wiki-content maincontent"><h3 id="DiscoveryTransportReference-TheDiscoveryTransport">The Discovery Transport</h3>

	<p>The Discovery transport works just like the <a shape="rect" href="failover-transport-reference.html">Failover</a> transport, except that it uses a discovery agent to locate the list of uri to connect to. The Discovery transport is also used by the <a shape="rect" href="fanout-transport-reference.html">Fanout</a> transport for discovering brokers to send a fanout message to.</p>

	<h4 id="DiscoveryTransportReference-ConfigurationSyntax">Configuration Syntax</h4>

	<p><strong>discovery:(discoveryAgentURI)?transportOptions</strong><br clear="none">
	or<br clear="none">
	<strong>discovery:discoveryAgentURI</strong></p>

	<p>Note that to be able to use <a shape="rect" href="discovery.html">Discovery</a> to find brokers, the brokers need to have the multicast discovery agent enabled on the broker. </p>

	<p>To configure discovery in a Broker you should use the <a shape="rect" href="xml-configuration.html">Xml Configuration</a>. Here is an <a shape="rect" class="external-link" href="http://svn.apache.org/viewvc/activemq/trunk/activemq-core/src/test/resources/org/apache/activemq/usecases/receiver-discovery.xml?view=co">example</a> of using discovery. Its basically something like the following (see the <strong>discoveryUri</strong>)</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;">
	<broker name="foo">
	<transportConnectors>
	<transportConnector uri="tcp://localhost:0" discoveryUri="multicast://default"/>
	</transportConnectors>

	...
	</broker>
	</pre>
	</div></div>


	<h5 id="DiscoveryTransportReference-TransportOptions">Transport Options</h5>

	<div class="table-wrap"><table class="confluenceTable"><tbody><tr><th colspan="1" rowspan="1" class="confluenceTh"><p>Option Name</p></th><th colspan="1" rowspan="1" class="confluenceTh"><p>Default Value</p></th><th colspan="1" rowspan="1" class="confluenceTh"><p>Description</p></th></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>reconnectDelay</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>10</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>How long to wait for discovery</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>initialReconnectDelay</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>10</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>How long to wait before the first reconnect attempt to a discovered url</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>maxReconnectDelay</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>30000</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>The maximum amount of time we ever wait between reconnect attempts</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>useExponentialBackOff</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>true</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>Should an exponential backoff be used btween reconnect attempts</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>backOffMultiplier</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>2</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>The exponent used in the exponential backoff attempts</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>maxReconnectAttempts</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>0</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>If not 0, then this is the maximum number of reconnect attempts before an error is sent back to the client</p></td></tr><tr><td colspan="1" rowspan="1" class="confluenceTd"><p>group</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p>default</p></td><td colspan="1" rowspan="1" class="confluenceTd"><p> an identifier for the group to partition multi cast traffic among collaborating peers; the group forms part of the shared identity of a discovery datagram (since 5.2) </p></td></tr></tbody></table></div>


	<h5 id="DiscoveryTransportReference-ExampleURI">Example URI</h5>

	<div class="preformatted panel" style="border-width: 1px;"><div class="preformattedContent panelContent">
	<pre>discovery:(multicast://default)?initialReconnectDelay=100
	</pre>
	</div></div>

	<div class="confluence-information-macro confluence-information-macro-information"><p class="title">Applying parameters to discovered transports</p><span class="aui-icon aui-icon-small aui-iconfont-info confluence-information-macro-icon"></span><div class="confluence-information-macro-body">
	<p>From 5.4, transport parameters in the URI will also be applied to discovered transports if they are prefixed with <code>discovered.</code>; For example, adding the <code>discovered.connectionTimeout</code> parameter to the URI will apply the parameter to every discovered <a shape="rect" href="tcp-transport-reference.html">TCP</a> transport, even though this parameter is not a Discovery transport option.</p></div></div></div>
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