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For reference see: https://jira.springsource.org/browse/INT-2882 * Verify spacing * Ensure all source code samples are typed: e.g. <programlisting language="xml"> * Ensure source code fits space in PDF format
60 lines
3.5 KiB
XML
60 lines
3.5 KiB
XML
<?xml version="1.0" encoding="UTF-8"?>
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<section xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="bridge"
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xmlns:xlink="http://www.w3.org/1999/xlink">
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<title>Messaging Bridge</title>
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<section id="bridge-introduction">
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<title>Introduction</title>
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<para>
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A Messaging Bridge is a relatively trivial endpoint that simply connects two Message Channels or Channel
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Adapters. For example, you may want to connect a <interfacename>PollableChannel</interfacename> to a
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<interfacename>SubscribableChannel</interfacename> so that the subscribing endpoints do not have to worry
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about any polling configuration. Instead, the Messaging Bridge provides the polling configuration.
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</para>
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<para>
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By providing an intermediary poller between two channels, a Messaging Bridge can be used to throttle inbound
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Messages. The poller's trigger will determine the rate at which messages arrive on the second channel, and the
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poller's "maxMessagesPerPoll" property will enforce a limit on the throughput.
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</para>
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<para>
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Another valid use for a Messaging Bridge is to connect two different systems. In such a scenario, Spring
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Integration's role would be limited to making the connection between these systems and managing a poller
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if necessary. It is probably more common to have at least a <emphasis>Transformer</emphasis> between the
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two systems to translate between their formats, and in that case, the channels would be provided as the
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'input-channel' and 'output-channel' of a Transformer endpoint. If data format translation is not required,
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the Messaging Bridge may indeed be sufficient.
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</para>
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</section>
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<section id="bridge-namespace">
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<title>Configuring Bridge</title>
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<para>
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The <bridge> element is used to create a Messaging Bridge between two Message Channels or Channel Adapters.
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Simply provide the "input-channel" and "output-channel" attributes:
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<programlisting language="xml"><![CDATA[<int:bridge input-channel="input" output-channel="output"/>]]></programlisting>
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As mentioned above, a common use case for the Messaging Bridge is to connect a
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<interfacename>PollableChannel</interfacename> to a <interfacename>SubscribableChannel</interfacename>, and when
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performing this role, the Messaging Bridge may also serve as a throttler:
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<programlisting language="xml"><![CDATA[<int:bridge input-channel="pollable" output-channel="subscribable">
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<int:poller max-messages-per-poll="10" fixed-rate="5000"/>
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</int:bridge>]]></programlisting>
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</para>
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<para>
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Connecting Channel Adapters is just as easy. Here is a simple echo example between the "stdin" and "stdout"
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adapters from Spring Integration's "stream" namespace.
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<programlisting language="xml"><![CDATA[<int-stream:stdin-channel-adapter id="stdin"/>
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<int-stream:stdout-channel-adapter id="stdout"/>
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<int:bridge id="echo" input-channel="stdin" output-channel="stdout"/>]]></programlisting>
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Of course, the configuration would be similar for other (potentially more useful) Channel Adapter bridges, such
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as File to JMS, or Mail to File. The various Channel Adapters will be discussed in upcoming chapters.
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</para>
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<note>
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<para>If no 'output-channel' is defined on a bridge, the reply channel provided by the inbound Message will
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be used, if available. If neither output or reply channel is available, an Exception will be thrown.</para>
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</note>
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</section>
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</section>
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