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Updating Reference Documentation for M4.

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Mark Fisher
2008-05-22 20:16:15 +00:00
parent 1575a2e9fe
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spring-integration-reference/src/overview.xml
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framework while handling that object. It consists of a payload and header and has a unique identifier. The
payload can be of any type and the header holds commonly required information such as timestamp, expiration,
and return address. Developers can also store any arbitrary key-value properties or attributes in the header.
<mediaobject>
<imageobject>
<imagedata align="center" fileref="images/message.png" format="PNG"/>
</imageobject>
</mediaobject>
</para>
</section>
<section id="overview-components-source">
<title>Message Source</title>
<para>
Since a Spring Integration Message is a generic wrapper for any Object, there is no limit to the number of
potential sources for such messages. In fact, a Source implementation can act as an adapter that converts
Objects from any other system into Spring Integration Messages.
<mediaobject>
<imageobject>
<imagedata align="center" fileref="images/source.png" format="PNG"/>
</imageobject>
</mediaobject>
To facilitate the conversion of Objects to Messages, Spring Integration also defines a strategy interface
for creating Messages called <interfacename>MessageCreator</interfacename>. While it is relatively easy to
implement Source directly, an adapter is also available for invoking arbitrary methods on plain Objects. Also,
several Source implementations are already available within the Spring Integration Adapters module. For a
detailed discussion of the various adapters, see <xref linkend="adapters"/>.
</para>
</section>
<section>
<title>Message Target</title>
<para>
Just as a Source enables Message reception, a Target handles the responsibility of sending Messages. As with
a Source, a Target can act as an adapter that converts Messages into the Objects expected by some other system.
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<imagedata align="center" fileref="images/target.png" format="PNG"/>
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</mediaobject>
Spring Integration provides a strategy interface for mapping Messages to Objects called
<interfacename>MessaegMapper</interfacename>. The Target interface may be implemented directly, but an adapter
is also available for invoking arbitrary methods on plain Objects (delegating to the Message-mapping strategy
in the process). As with Sources, several Target implementations are already available within the Spring
Integration Adapters module as discussed in <xref linkend="adapters"/>.
</para>
</section>
<section id="overview-components-handler">
<title>Message Handler</title>
<para>
As described above, the Source and Target components support conversion between Objects and Messages so that
application code and/or external systems can be connected to a Spring Integration application rather easily.
However, both Source and Target are unidirectional while the application code or external system to be invoked
may provide a return value. The Message Handler interface supports these request-reply scenarios.
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<imageobject>
<imagedata align="center" fileref="images/handler.png" format="PNG"/>
</imageobject>
</mediaobject>
As with the Source and Target, Spring Integration also provides an adapter that itself implements the Message
Handler interface while supporting the invocation of arbitrary methods on plain Objects. The adapter relies
upon the message-creating and message-mapping strategies to handle the bidirectional Object/Message conversion.
For more information about the Message Handler, see <xref linkend="api-messagehandler"/>.
</para>
</section>
<section id="overview-components-channel">
<title>Message Channel</title>
<para>
A Message Channel represents the "pipe" of a pipes-and-filters architecture. Producers send Messages to
a MessageChannel, and consumers receive Messages from a MessageChannel. The send and receive methods both come
in two forms: one that blocks indefinitely and one that accepts a timeout (for an immediate return, specify a
timeout value of 0). There are two main types of channels: <emphasis>Point-to-Point</emphasis> channels where
typically a single consumer will receive the Message and <emphasis>Publish-Subscribe</emphasis> channels where
all subscribers should receive the Message.
a channel, and consumers receive Messages from a channel. By providing both send and receive operations, a
Message Channel basically combines the roles of Source and Target.
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<imagedata align="center" fileref="images/channel.png" format="PNG"/>
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Spring Integration provides a number of different channel implementations: QueueChannel, PriorityChannel,
RendezvousChannel, DirectChannel, and ThreadLocalChannel. These are described in detail in
<xref linkend="api-messagechannel"/>.
</para>
</section>
<section id="overview-components-endpoint">
<title>Message Endpoint</title>
<para>
A Message Endpoint represents the "filter" of a pipes-and-filters architecture. The endpoint's primary role is
to connect application code to the messaging framework and to do so in a non-invasive manner. In other words,
the application code should have no awareness of the messaging framework. This is similar to the role of a
Controller in the MVC paradigm. Just as a Controller handles HTTP requests, the endpoint handles Messages. Just
as Controllers are mapped to URL patterns, endpoints are mapped to Message Channels. The goal is the same in
both cases: isolate application code from the infrastructure. In Spring Integration, the Message Endpoint
"hosts" and delegates to a <interfacename>MessageHandler</interfacename> strategy interface as described in
<xref linkend="api-messagehandler"/>.
Thus far, the component diagrams show Consumers, Producers, and Requesters invoking the Source, Target, and
Message Handlers respectively. However, one of the primary goals of Spring Integration is to simplify the
development of enterprise integration solutions through <emphasis>inversion of control</emphasis>. This means
that you should not have to implement such Producers, Consumers, and Requesters directly. Instead, you should
be able to focus on your domain logic with an implementation based on plain Objects. Then, by providing
declarative configuration, you can "connect" your application code to the messaging infrastructure provided by
Spring Integration. The components responsible for these connections are Message Endpoints.
</para>
<para>
A Message Endpoint represents the "filter" of a pipes-and-filters architecture. As mentioned above, the
endpoint's primary role is to connect application code to the messaging framework and to do so in a
non-invasive manner. In other words, the application code should have no awareness of the Message objects or
the Message Channels. This is similar to the role of a Controller in the MVC paradigm. Just as a Controller
handles HTTP requests, the Message Endpoint handles Messages. Just as Controllers are mapped to URL patterns,
Message Endpoints are mapped to Message Channels. The goal is the same in both cases: isolate application code
from the infrastructure. Spring Integration provides three types of endpoints - one for each of the component
types described above: Source Endpoint, Target Endpoint, and Handler Endpoint.
</para>
<section>
<title>Source Endpoint</title>
<para>
A Source Endpoint connects any Source implementation to a Message Channel. The invocation of the Source's
receive operation is controlled by scheduling information provided within the Source Endpoint's
configuration. Any time the receive operation returns a non-null Message, it is sent to the channel.
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<imagedata align="center" fileref="images/source-endpoint.png" format="PNG"/>
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</para>
</section>
<section>
<title>Target Endpoint</title>
<para>
A Target Endpoint connects a Message Channel to any Target implementation. The invocation of the Message
Channel's receive operation is controlled by scheduling information provided within the Target Endpoint's
configuration. Any time a non-null Message is received from the channel, it is sent to the Target.
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</para>
</section>
<section>
<title>Handler Endpoint</title>
<para>
Since Message Handler's are capable of returning reply Messages, the Handler Endpoint has some additional
responsibilities. The general behavior is the same as the Target Endpoint, but the Handler Endpoint must
make a distinction between "input-channel" and "output-channel". Whenever the Message Handler does return
a reply Message, that Message is sent to the output channel. If no output channel has been configured, then
the reply will be sent to the channel specified as the Message header's "return address" if available.
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</para>
</section>
</section>
<section id="overview-component-router">
<title>Message Router</title>
@@ -133,18 +246,11 @@
receiving a Message and then deciding what channel or channels should receive the Message next. Typically the
decision is based upon the Message's content and/or metadata. A Message Router is often used as a dynamic
alternative to configuring the input and output channels for an endpoint.
</para>
</section>
<section id="overview-component-channeladapter">
<title>Channel Adapter</title>
<para>
A Channel Adapter is used to connect components to a Message Channel when those components are not themselves
Message Endpoints. These adapters provide a mechanism for connecting to external systems, such as JMS queues
or a File system. Channel Adapters may be configured for input and/or output. An input (source) adapter will
receive (or poll for) data, convert that data to a Message, and then send that Message to its Message Channel.
An output (target) adapter is simply another type of <interfacename>MessageHandler</interfacename>, but when it
receives a Message, it will convert it to the target's expected type and then "send" it (publish to a JMS
queue, write to a File, etc.).
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</para>
</section>
<section id="overview-component-bus">