spring-integration/src/reference/docbook/gateway.xml

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<section xmlns="http://docbook.org/ns/docbook" version="5.0" xml:id="gateway"
    xmlns:xlink="http://www.w3.org/1999/xlink">
  <title>Messaging Gateways</title>
  <para>
      The primary purpose of a Gateway is to hide the messaging API provided by
      Spring Integration. It allows your application's business logic to be completely
      unaware of the Spring Integration API and using a generic Gateway, your code
      interacts instead with a simple interface, only.
  </para>
  <section id="gateway-proxy">
    <title>Enter the GatewayProxyFactoryBean</title>
    <para>
      As mentioned above, it would be great to have no dependency on the Spring
      Integration API at all - including the gateway class. For that reason, Spring
      Integration provides the <classname>GatewayProxyFactoryBean</classname> that
      generates a proxy for any interface and internally invokes the gateway
      methods shown below. Using dependency injection you can then expose the interface
      to your business methods.
    </para>
    <para>
      Here is an example of an interface that can be used to interact with Spring
      Integration:
    </para>

    <programlisting language="java"><![CDATA[package org.cafeteria;

public interface Cafe {

    void placeOrder(Order order);

}]]></programlisting>

  </section>
  <section id="gateway-namespace">
    <title>Gateway XML Namespace Support</title>
    <para>
      Namespace support is also
      provided which allows you to configure such an interface as a service as demonstrated by the following example.
    </para>
      <programlisting language="xml"><![CDATA[<int:gateway id="cafeService"
         service-interface="org.cafeteria.Cafe"
         default-request-channel="requestChannel"
         default-reply-channel="replyChannel"/>]]></programlisting>
    <para>
      With this configuration defined, the "cafeService" can now be injected into other beans, and the code that invokes the methods on that
      proxied instance of the Cafe interface has no awareness of the Spring Integration API. The general
      approach is similar to that of Spring Remoting (RMI, HttpInvoker, etc.). See the "Samples" Appendix for
      an example that uses this "gateway" element (in the Cafe demo).
    </para>
  </section>
    <section id="gateway-default-reply-channel">
      <title>Setting the Default Reply Channel</title>
      <para>
	      Typically you don't have to specify the <code>default-reply-channel</code>,
	      since a Gateway will auto-create a temporary, anonymous reply channel,
	      where it will listen for the reply. However, there are some cases which
	      may prompt you to define a <code>default-reply-channel</code> (or <code>reply-channel</code>
	      with adapter gateways such as HTTP, JMS, etc.).
	  </para>
	  <para>
		  For some background, we'll quickly discuss some of the inner-workings of the Gateway.
	      A Gateway will create a temporary point-to-point reply channel which is anonymous and is added
	      to the Message Headers with the name <code>replyChannel</code>.
	      When providing an explicit <code>default-reply-channel</code> (<code>reply-channel</code> with remote adapter gateways),
	      you have the option to point to a publish-subscribe channel, which is so named because you can add more than one subscriber to it.
	      Internally Spring Integration will create a Bridge between the temporary <code>replyChannel</code> and the explicitly defined
	      <code>default-reply-channel</code>.
      </para>
      <para>
		  So let's say you want your reply to go not only to the gateway, but also to some other consumer. In this case you
		  would want two things: <emphasis>a) a named channel you can subscribe to and b) that channel is a publish-subscribe-channel.</emphasis>
		  The default strategy used by the gateway will not satisfy those needs, because the reply channel added to the header is anonymous and
		  point-to-point. This means that no other subscriber can get a handle to it and even if it could, the channel
		  has point-to-point behavior such that only one subscriber would get the Message. So by defining a <code>default-reply-channel</code>
		  you can point to a channel of your choosing, which in this case would be a <code>publish-subscribe-channel</code>.
		  The Gateway would create a bridge from it to the temporary, anonymous reply channel that is stored in the header.
      </para>
      <para>
	      Another case where you might want to provide a reply channel explicitly is for monitoring or auditing via an interceptor
	      (e.g., wiretap). You need a named channel in order to configure a Channel Interceptor.
      </para>
    </section>
    <section id="gateway-configuration-annotations">
      <title>Gateway Configuration with Annotations and/or XML</title>
      <para>
        The reason that the attributes on the 'gateway' element are named 'default-request-channel' and
        'default-reply-channel' is that you may also provide per-method channel references by using the
        <classname>@Gateway</classname> annotation.
      </para>

      <programlisting language="java"><![CDATA[ public interface Cafe {

     @Gateway(requestChannel="orders")
     void placeOrder(Order order);

 }]]></programlisting>
    <para>
      You may alternatively provide such content in <code>method</code> sub-elements if you prefer XML configuration (see the next paragraph).
    </para>
    <para>
      It is also possible to pass values to be interpreted as Message headers on the Message
      that is created and sent to the request channel by using the @Header annotation:
    </para>
    <programlisting language="java"><![CDATA[ public interface FileWriter {

     @Gateway(requestChannel="filesOut")
     void write(byte[] content, @Header(FileHeaders.FILENAME) String filename);

 }]]></programlisting>

    <para>
      If you prefer the XML approach of configuring Gateway methods, you can provide <emphasis>method</emphasis> sub-elements
      to the gateway configuration.
    </para>

     <programlisting language="xml"><![CDATA[<int:gateway id="myGateway" service-interface="org.foo.bar.TestGateway"
      default-request-channel="inputC">
  <int:method name="echo" request-channel="inputA" reply-timeout="2" request-timeout="200"/>
      <int:method name="echoUpperCase" request-channel="inputB"/>
      <int:method name="echoViaDefault"/>
</int:gateway>]]></programlisting>

    <para>
      You can also provide individual headers per method invocation via XML.
      This could be very useful if the headers you want to set are static in nature and you don't want
      to embed them in the gateway's method signature via <classname>@Header</classname> annotations.
      For example, in the Loan Broker example we want to influence how aggregation of the Loan quotes
      will be done based on what type of request was initiated (single quote or all quotes). Determining the
      type of the request by evaluating what gateway method was invoked, although possible, would
      violate the separation of concerns paradigm (the method is a java artifact),  but expressing your
      intention (meta information) via Message headers is natural in a Messaging architecture.
    </para>

      <programlisting language="xml"><![CDATA[<int:gateway id="loanBrokerGateway"
         service-interface="org.springframework.integration.loanbroker.LoanBrokerGateway">
  <int:method name="getLoanQuote" request-channel="loanBrokerPreProcessingChannel">
    <int:header name="RESPONSE_TYPE" value="BEST"/>
  </int:method>
  <int:method name="getAllLoanQuotes" request-channel="loanBrokerPreProcessingChannel">
    <int:header name="RESPONSE_TYPE" value="ALL"/>
  </int:method>
</int:gateway>]]></programlisting>
    <para>
      In the above case you can clearly see how a different value will be set for the 'RESPONSE_TYPE'
      header based on the gateway's method.
    </para>

    </section>
    <section id="gateway-calling-no-argument-methods">
      <title>Invoking No-Argument Methods</title>
      <para>
          When invoking methods on a Gateway interface that do not have any arguments,
          the default behavior is to <emphasis>receive</emphasis> a <code>Message</code> from a
          <interfacename>PollableChannel</interfacename>.
      </para>
      <para>
          At times however, you may want to trigger no-argument methods so that
          you can in fact interact with other components downstream that do not require
          user-provided parameters, e.g. triggering no-argument SQL calls or Stored
          Procedures.
      </para>
      <para>
          In order to achieve <emphasis>send-and-receive</emphasis> semantics, you must provide a payload.
          In order to generate a payload, method parameters on the interface are
          not necessary. You can either use the <code>@Payload</code> annotation
          or the <code>payload-expression</code> attribute in XML on the <code>method</code>
          sub-element. Below please find a few examples of what the payloads could be:
      </para>

      <itemizedlist>
          <listitem>a literal string</listitem>
          <listitem>#method (for the method name)</listitem>
          <listitem>new java.util.Date()</listitem>
          <listitem>@someBean.someMethod()'s return value</listitem>
      </itemizedlist>

      <para>
          Here is an example using the <code>@Payload</code> annotation:
      </para>
      <programlisting language="xml"><![CDATA[public interface Cafe {

    @Payload("new java.util.Date()")
    List<Order> retrieveOpenOrders();

}]]></programlisting>
      <para>
		If a method has no argument and no return value, but does contain a
		payload expression, it will be treated as a <emphasis>send-only</emphasis>
		operation.
      </para>
    </section>

    <section id="gateway-error-handling">
      <title>Error Handling </title>
      <para>

    Of course, the Gateway invocation might result in errors.
    By default any error that has occurred downstream will be re-thrown as a
    <classname>MessagingException</classname> (RuntimeException)
    upon the Gateway's method invocation. However there are times when you may want
    to simply log the error rather than propagating it, or you may want to treat an
    Exception as a valid reply, by mapping it to a Message that will conform to some
    "error message" contract that the caller understands. To accomplish this, our
    Gateway provides support for a Message Channel dedicated to the errors via the
    <emphasis>error-channel</emphasis> attribute. In the example below, you can see
    that a 'transformer' is used to create a reply Message from the Exception.
    </para>

<programlisting language="xml"><![CDATA[<int:gateway id="sampleGateway"
    default-request-channel="gatewayChannel"
    service-interface="foo.bar.SimpleGateway"
    error-channel="exceptionTransformationChannel"/>

<int:transformer input-channel="exceptionTransformationChannel"
        ref="exceptionTransformer" method="createErrorResponse"/>
]]></programlisting>
    <para>
  The <emphasis>exceptionTransformer</emphasis> could be a simple POJO that
  knows how to create the expected error response objects. That would then be
  the payload that is sent back to the caller. Obviously, you could do many
  more elaborate things in such an "error flow" if necessary. It might involve
  routers (including Spring Integration's ErrorMessageExceptionTypeRouter),
  filters, and so on. Most of the time, a simple 'transformer' should be sufficient,
  however.
     </para>
     <para>
       Alternatively, you might want to only log the Exception (or send it somewhere
       asynchronously). If you provide a one-way flow, then nothing would be sent
       back to the caller. In the case that you want to completely suppress Exceptions,
       you can provide a reference to the global "nullChannel" (essentially a /dev/null
       approach). Finally, as mentioned above, if no "error-channel" is defined at all,
       then the Exceptions will propagate as usual.
     </para>
     <para>
       <important>
       Exposing the messaging system via simple POJI Gateways obviously provides benefits, but "hiding" the reality
       of the underlying messaging system does come at a price so there are certain things you should consider.

            We want our Java method to return as quickly as possible and not hang for an indefinite amount of time while
            the caller is waiting on it to return (void, return value, or a thrown Exception). When regular methods are
            used as a proxies in front of the Messaging system, we have to take into account the potentially asynchronous
            nature of the underlying messaging. This means that there might
            be a chance that a Message that was initiated by a Gateway could be dropped by a Filter, thus never reaching a
            component that is responsible for producing a reply. Some Service Activator method might result in an Exception,
            thus providing no reply (as we don't generate Null messages). So as you can see there are multiple scenarios
            where a reply message might not be coming. That is perfectly natural in messaging systems. However think about the
            implication on the gateway method. The Gateway's method input arguments  were incorporated into a Message and
            sent downstream. The reply Message would be converted to a return value of the Gateway's method. So you might want
            to ensure that for each Gateway call there will always be a reply Message.
            Otherwise, your Gateway method might never return and will hang indefinitely.
One of the ways of handling this situation is via an Asynchronous Gateway (explained later in this section). Another way of handling it is to explicitly set the reply-timeout attribute. That way, the gateway will not hang any longer than the time specified by the reply-timeout and will return 'null' if that timeout does elapse. Finally, you might want to consider setting downstream flags such as 'requires-reply' on
 a service-activator or 'throw-exceptions-on-rejection' on a filter. These options will be discussed in more detail in the final section
of this chapter.
       </important>
     </para>
  </section>
  <section id="async-gateway">
    <title>Asynchronous Gateway</title>
    <para>
      As a pattern the Messaging Gateway is a very nice way to hide messaging-specific code while still exposing the full capabilities of the
      messaging system. As you've seen, the <classname>GatewayProxyFactoryBean</classname> provides a convenient way to expose a Proxy over a service-interface
      thus giving you POJO-based access to a messaging system (based on objects in your own domain, or primitives/Strings, etc).  But when a
      gateway is exposed via simple POJO methods which return values it does imply that for each Request message (generated when the method is invoked)
      there must be a Reply message (generated when the method has returned). Since Messaging systems naturally are asynchronous you may not always be
      able to guarantee the contract where <emphasis>"for each request there will always be be a reply"</emphasis>. 
      With Spring Integration 2.0 we are introducing support for an <emphasis>Asynchronous Gateway</emphasis> which is a convenient way to initiate
      flows where you may not know if a reply is expected or how long will it take for replies to arrive.
      </para>
      <para>
      A natural way to handle these types of scenarios in Java would be relying upon <emphasis>java.util.concurrent.Future</emphasis> instances, and
      that is exactly what Spring Integration uses to support an <emphasis>Asynchronous Gateway</emphasis>.
      </para>
      <para>
         From the XML configuration, there is nothing different and you still define <emphasis>Asynchronous Gateway</emphasis> the same way as a regular Gateway.
      </para>
      <programlisting language="xml"><![CDATA[<int:gateway id="mathService" 
     service-interface="org.springframework.integration.sample.gateway.futures.MathServiceGateway"
     default-request-channel="requestChannel"/>]]></programlisting>
     <para>
      However the Gateway Interface (service-interface) is a bit different.
     </para>
      <programlisting language="java">public interface MathServiceGateway {
  Future&lt;Integer&gt; multiplyByTwo(int i);
}</programlisting>

    <para>
    As you can see from the example above the return type for the gateway method is a <classname>Future</classname>. When
    <classname>GatewayProxyFactoryBean</classname> sees that the
    return type of the gateway method is a <classname>Future</classname>, it immediately switches to the async mode by utilizing
    an <classname>AsyncTaskExecutor</classname>. That is all. The call to such a method always returns immediately with a <classname>Future</classname> instance.
    Then, you can interact with the <classname>Future</classname> at your own pace to get the result, cancel, etc. And, as with
    any other use of Future instances, calling get() may reveal a timeout, an execution exception, and so on.
    <programlisting language="java">MathServiceGateway mathService = ac.getBean("mathService", MathServiceGateway.class);
Future&lt;Integer&gt; result = mathService.multiplyByTwo(number);
// do something else here since the reply might take a moment
int finalResult =  result.get(1000, TimeUnit.SECONDS);</programlisting>
For a more detailed example, please refer to the <emphasis>async-gateway</emphasis> sample distributed within the Spring Integration samples.
    </para>

    <para><emphasis>Asynchronous Gateway and AsyncTaskExecutor</emphasis></para>
    <para>
     By default <classname>GatewayProxyFactoryBean</classname> uses <classname>org.springframework.core.task.SimpleAsyncTaskExecutor</classname>
     when submitting internal <classname>AsyncInvocationTask</classname> instances for any gateway method whose
     return type is <classname>Future.class</classname>. However the <literal>async-executor</literal> attribute in the
     <literal>&lt;gateway/&gt;</literal> element's configuration allows you to provide a reference to any implementation of
     <classname>java.util.concurrent.Executor</classname> available within the Spring application context.
    </para>
  </section>
  <section>
    <title>Gateway behavior when no response arrives</title>
    <para>
    As it was explained earlier, the Gateway provides a convenient way of interacting with a Messaging system via POJO method
    invocations, but realizing that a typical method invocation, which is generally expected to always return (even with an Exception),
    might not always map one-to-one to message exchanges (e.g., a reply message might not arrive - which is equivalent to a
    method not returning). It is important to go over several scenarios especially in the Sync Gateway case and understand
    the default behavior of the Gateway and how to deal with these scenarios to make the Sync Gateway behavior more
    predictable regardless of the outcome of the message flow that was initialed from such Gateway.
    </para>
    <para>
    There are certain attributes that could be configured to make Sync Gateway behavior more predictable,
    but some of them might not always work as you might have expected. One of them is <emphasis>reply-timeout</emphasis>.
    So, lets look at the <emphasis>reply-timeout</emphasis> attribute and see how it can/can't influence the behavior
    of the Sync Gateway in various scenarios. We will look at single-threaded scenario
    (all components downstream are connected via Direct Channel) and multi-threaded scenarios
    (e.g., somewhere downstream you may have Pollable or Executor Channel which breaks single-thread boundary)
    </para>
    <para>
    <emphasis>Long running process downstream</emphasis>
    </para>
    <para>
    <emphasis>Sync Gateway - single-threaded</emphasis>.
  If a component downstream is still running (e.g., infinite loop or a very slow service), then setting a <emphasis>reply-timeout</emphasis>
  has no effect and the Gateway method call will not return until such downstream service exits (via return or exception).
    <emphasis>Sync Gateway - multi-threaded</emphasis>.
  If a component downstream is still running (e.g., infinite loop or a very slow service), in a multi-threaded message
  flow setting the <emphasis>reply-timeout</emphasis> will have an effect by allowing gateway method invocation to
  return once the timeout has been reached, since the <classname>GatewayProxyFactoryBean</classname>  will simply
  poll on the reply channel waiting for a message until the timeout expires. However it could result in a 'null' return
  from the Gateway method if the timeout has been reached before the actual reply was produced. It is also important to understand that
  the reply message (if produced) will be sent to a reply channel after the Gateway method invocation might have returned, so you must be aware of that and design your flow with this in mind.
    </para>
    <para>
    <emphasis>Downstream component returns 'null'</emphasis>
    </para>
    <para>
    <emphasis>Sync Gateway - single-threaded</emphasis>.
  If a component downstream returns 'null' and no <emphasis>reply-timeout</emphasis> has been configured, the Gateway
  method call will hang indefinitely unless: a) a <emphasis>reply-timeout</emphasis> has been configured or b) the
  <emphasis>requires-reply</emphasis> attribute has been set on the downstream component (e.g., service-activator)
  that might return 'null'. In this case, an Exception would be thrown and propagated to the Gateway.
  <emphasis>Sync Gateway - multi-threaded</emphasis>. Behavior is the same as above.
    </para>
    <para>
    <emphasis>Downstream component return signature is 'void' while Gateway method signature is non-void</emphasis>
    </para>
    <para>
    <emphasis>Sync Gateway - single-threaded</emphasis>.
  If a component downstream returns 'void' and no <emphasis>reply-timeout</emphasis> has been configured,
  the Gateway method call will hang indefinitely unless a <emphasis>reply-timeout</emphasis> has been configured 
  <emphasis>Sync Gateway - multi-threaded</emphasis> Behavior is the same as above.
    </para>
    <para>
    <emphasis>Downstream component results in Runtime Exception (regardless of the method signature)</emphasis>
    </para>
    <para>
    <emphasis>Sync Gateway - single-threaded</emphasis>.
  If a component downstream throws a Runtime Exception, such exception will be propagated via an Error Message back to
  the gateway and re-thrown.
  <emphasis>Sync Gateway - multi-threaded</emphasis> Behavior is the same as above.
    </para>
    <para>
      <important>
    It is also important to understand that by default <emphasis>reply-timeout</emphasis> is unbounded* which means that
    if not explicitly set there are several scenarios (described above) where your Gateway method invocation might
    hang indefinitely. So, make sure you analyze your flow and if there is even a remote possibility of one of these
    scenarios to occur, set the <emphasis>reply-timeout</emphasis> attribute to a 'safe' value or, even better,
    set the <emphasis>requires-reply</emphasis> attribute of the downstream component to 'true' to ensure a timely response
    as produced by the throwing of an Exception as soon as that downstream component does return null internally.
    But also, realize that there are some scenarios (see the very first one)
    where <emphasis>reply-timeout</emphasis> will not help. That means it is also important to analyze your message
    flow and decide when to use a Sync Gateway vs an Async Gateway. As you've seen the latter case is simply a matter of
    defining Gateway methods that return Future instances. Then, you are guaranteed to receive that return value, and
    you will have more granular control over the results of the invocation.
    <para>
    Also, when dealing with a Router you should remember that setting the <emphasis>resolution-required</emphasis> attribute to 'true'
    will result in an Exception thrown by the router if it can not resolve a particular channel. Likewise, when dealing with a Filter,
    you can set the <emphasis>throw-exception-on-rejection</emphasis> attribute. In both of these cases, the resulting flow will
    behave like that containing a service-activator with the 'requires-reply' attribute. In other words, it will help to ensure
    a timely response from the Gateway method invocation.
    </para>
    <para>
        <note>
            <para>
                * <emphasis>reply-timeout</emphasis> is unbounded for <emphasis>&lt;gateway/&gt;</emphasis>
                elements (created by the GatewayProxyFactoryBean). Inbound gateways for external integration
                (ws, http, etc.) share many characteristics and attributes with these gateways. However,
                for those inbound gateways, the default <emphasis>reply-timeout</emphasis> is 1000
                milliseconds (1 second). If a downstream async handoff is made to another thread, you may need to
                increase this attribute to allow enough time for the flow to complete before the
                gateway times out.
            </para>
        </note>
    </para>
      </important>
    </para>
  </section>

</section>