Spring Integration and Apache Camel

Spring Integration and Apache Camel are open source frameworks providing a simpler solution for the Integration problems in the enterprise, to quote from their respective websites:

Apache Camel -

Apache Camel is a powerful open source integration framework based on known Enterprise Integration Patterns with powerful Bean Integration.

Spring Integration -

It provides an extension of the Spring programming model to support the well-known Enterprise Integration Patterns while building on the Spring Framework's existing support for enterprise integration.

Essentially Spring Integration and Apache Camel enable applications to integrate with other systems.

This article seeks to provide an implementation for an integration problem using both Spring Integration and Apache Camel. The objective is to show how easy it is to use these frameworks for a fairly complicated integration problem and to recommend either of these great products for your next Integration challenge.

Problem:

To illustrate the use of these frameworks consider a simple integration scenario, described using EIP terminology:

 

The application needs to get a "Report" by aggregating "Sections" from a Section XML over http service. Each request for Report consists of a set of request for sections – in this specific example there are requests for three sections, the header, body and footer. The XML over http service returns a Section for the Section Request. The responses need to be aggregated into a single report. A sample test for this scenario is of the following type:

        ReportGenerator reportGenerator = reportGeneratorFactory.createReportGenerator();
        List<SectionRequest> sectionRequests = new ArrayList<SectionRequest>();
        
        String entityId="A Company";
        
        sectionRequests.add(new SectionRequest(entityId,"header"));
        sectionRequests.add(new SectionRequest(entityId,"body"));
        sectionRequests.add(new SectionRequest(entityId,"footer"));        

        ReportRequest reportRequest = new ReportRequest(sectionRequests);

        Report report = reportGenerator.generateReport(reportRequest);
        List<Section> sectionOfReport = report.getSections();
        System.out.println(report);
        assertEquals(3, sectionOfReport.size());

The “ReportGenerator” is the messaging gateway, hiding the details of the underlying messaging infrastructure and in this specific case also the integration API – Apache Camel or Spring Integration. To start with, let us implement a solution to this integration problem using Spring Integration as the Framework, followed by Apache Camel. The complete working code using Spring Integration and Apache Camel is also available with the article.

 

Solution Using Spring Integration:

The Gateway component is easily configured using the following entry in the Spring Configuration. Internally Spring Integration uses AOP to hook up a component which routes the requests from an internal input channel and waits for the response in the response channel.

<si:gateway default-reply-channel="exit" default-request-channel="enter" id="reportGenerator" service-interface="org.bk.report.ReportGenerator">
</si:gateway>

The component to Split the Input Report Request to Section Request is fairly straightforward:

public class SectionRequestSplitter {    
    public List split(ReportRequest reportRequest){
        return reportRequest.getSectionRequests();
    }

}

and to hook this splitter with Spring Integration:

<bean class="org.bk.report.common.SectionRequestSplitter" id="sectionRequestSplitterBean">

<si:splitter id="sectionRequestSplitter" input-channel="enter" method="split" output-channel="sectionRequestToXMLChannel" ref="sectionRequestSplitterBean">

</si:splitter></bean>

Next, to transform the Section Request to an XML format - The component is the following:

public class SectionRequestToXMLTransformer {
    public String transform(SectionRequest sectionRequest){
 //this needs to be optimized...purely for demonstration of the concept
        String sectionRequestAsString = "<section><meta><entityId>" + sectionRequest.getEntityId()
        + "</entityId><sectionName>" + sectionRequest.getSectionId()
        + "</sectionName></meta></section>";
        
        return sectionRequestAsString;
        
    }
}

and is hooked up in the Spring Integration configuration file in the following way:

<bean id="sectionRequestToXMLBean" class="org.bk.report.common.SectionRequestToXMLTransformer"/>
<si:transformer input-channel="sectionRequestToXMLChannel" ref="sectionRequestToXMLBean" method="transform" output-channel="sectionRequestChannel"/>

To send an XML over http request using the Section Request XML to a section Service:

<http:outbound-gateway id="httpHeaderGateway"
  request-channel="sectionRequestChannel" reply-channel="sectionResponseChannel"
  default-url="${sectionBaseURL}/section" extract-request-payload="true"
  charset="UTF-8" request-timeout="1200" />

To transform the Section Response XML to a Section Object - The component is the following:

public class SectionResponseXMLToSectionTransformer {
 public Section transform(String sectionXML) {
  SAXReader saxReader = new SAXReader();
  Document document;
  String sectionName = "";
  String entityId = "";
  try {
   document = saxReader.read(new StringReader(sectionXML));

   sectionName = document
     .selectSingleNode("/section/meta/sectionName").getText();
   entityId = document.selectSingleNode("/section/meta/entityId")
     .getText();
  } catch (DocumentException e) {
   e.printStackTrace();
  }
  return new Section(entityId, sectionName, sectionXML);
 }
}

and is hooked up in the Spring Integration configuration file in the following way:

<bean id="sectionResponseXMLToSectionBean" class="org.bk.report.common.SectionResponseXMLToSectionTransformer" />
<si:transformer input-channel="sectionXMLResponseChannel"
  ref="sectionResponseXMLToSectionBean" method="transform" output-channel="sectionResponseChannel" />

To aggregate the Sections together into a report, the component is the following::

public class SectionResponseAggregator {

    public Report aggregate(List<Section> sections) {
        return new Report(sections);
    }

}

and is hooked up in the Spring Integration configuration file in the following way:

<bean id="sectionResponseAggregator" class="org.bk.report.common.SectionResponseAggregator"/>
<si:aggregator input-channel="sectionResponseChannel" output-channel="exit" ref="sectionResponseAggregator" method="aggregate"/>

This completes the Spring Integration implementation for this Integration Problem. The following is the complete Spring Integration configuration file:

<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:context="http://www.springframework.org/schema/context" xmlns:si="http://www.springframework.org/schema/integration"
	xmlns:http="http://www.springframework.org/schema/integration/http"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
	http://www.springframework.org/schema/integration http://www.springframework.org/schema/integration/spring-integration-1.0.xsd	
	http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd
	http://www.springframework.org/schema/integration/http http://www.springframework.org/schema/integration/http/spring-integration-http-1.0.xsd
	">

	<context:property-placeholder />
	<si:channel id="enter" />

	<si:channel id="exit" />

	<si:gateway id="reportGenerator" default-request-channel="enter"
		default-reply-channel="exit" service-interface="org.bk.report.ReportGenerator" />

	<si:channel id="sectionRequestToXMLChannel" />
	<si:splitter id="sectionRequestSplitter" input-channel="enter"
		ref="sectionRequestSplitterBean" method="split" output-channel="sectionRequestToXMLChannel" />

	<si:channel id="sectionRequestChannel" />
	<si:transformer input-channel="sectionRequestToXMLChannel"
		ref="sectionRequestToXMLBean" method="transform" output-channel="sectionRequestChannel" />

	<si:channel id="sectionXMLResponseChannel" />
	<http:outbound-gateway id="httpHeaderGateway"
		request-channel="sectionRequestChannel" reply-channel="sectionXMLResponseChannel"
		default-url="${sectionBaseURL}/section" extract-request-payload="true"
		charset="UTF-8" request-timeout="1200" />

	<si:channel id="sectionResponseChannel" />
	<si:transformer input-channel="sectionXMLResponseChannel"
		ref="sectionResponseXMLToSectionBean" method="transform" output-channel="sectionResponseChannel" />

	<si:aggregator input-channel="sectionResponseChannel"
		output-channel="exit" ref="sectionResponseAggregator" method="aggregate" />

	<bean id="sectionRequestSplitterBean" class="org.bk.report.common.SectionRequestSplitter" />
	<bean id="sectionRequestToXMLBean" class="org.bk.report.common.SectionRequestToXMLTransformer" />
	<bean id="sectionResponseXMLToSectionBean" class="org.bk.report.common.SectionResponseXMLToSectionTransformer" />
	<bean id="sectionResponseAggregator" class="org.bk.report.common.SectionResponseAggregator" />
		
</beans>

 

A working sample is provided with the article(Download, extract and run "mvn test")

 

Solution using Apache Camel:

Apache Camel allows the route to be defined using multiple DSL implementations – Java DSL, Scala DSL and an XML based DSL. The recommended approach is to use Spring CamelContext as a runtime and the Java DSL for route development. The following is to build the Spring Camel Context:

<camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">
  <template id="camelTemplate" />
  <routeBuilder ref="routeBuilder"/>
 </camelContext>

The route is configured by the Java based DSL:

public class CamelRouteBuilder extends RouteBuilder {
 private String serviceURL;

 @Override
 public void configure() throws Exception {
  
  from("direct:start")
  .split().method("sectionRequestSplitterBean", "split")
  .aggregationStrategy(new ReportAggregationStrategy())
  .transform().method("sectionRequestToXMLBean", "transform")
  .to(serviceURL)
  .transform().method("sectionResponseXMLToSectionBean", "transform");
 }

 public void setServiceURL(String serviceURL) {
  this.serviceURL = serviceURL;
 }
}

Apache Camel does not provide an out of the box Message Gateway feature, however it is fairly easy to create a wrapper component that can hide the underlying details in the following way:

Reader davsclaus has provided references to two mechanisms with Apache Camel to provide an out of the box Messaging Gateway - Messaging Gateway EIP and Camel Proxy which allows a POJO to be used as a Mesaging Gateway. 

Camel Proxy will be used with the article, and can be configured in the Camel Configuration files in the following way:

	<camelContext id="camel" xmlns="http://camel.apache.org/schema/spring">

		<proxy id="consumerMessageGateway" serviceInterface="org.bk.report.ReportGenerator"
			serviceUrl="direct:start" />

		<template id="camelTemplate" />
		<routeBuilder ref="routeBuilder" />
	</camelContext>

Per davsclaus, there is a bug in Apache Camel(2.1 or older) when invoking a bean later in the route(the splitter bean), which is to be fixed in Apache Camel 2.2. To work around this bug, a convertBody step will be introduced in the route:

        from("direct:start")
        .convertBodyTo(ReportRequest.class)
        .split(bean("sectionRequestSplitterBean", "split"), new ReportAggregationStrategy())
        .transform().method("sectionRequestToXMLBean", "transform")
    	.to(serviceURL)
        .transform().method("sectionResponseXMLToSectionBean", "transform");

 

The component to Split the Input Report Request to Section Request is exactly same as Spring Integration component:

public class SectionRequestSplitter {
    
    public List<SectionRequest> split(ReportRequest reportRequest){
        return reportRequest.getSectionRequests();
    }

}

To hook the component with Apache Camel:

<bean id="sectionRequestSplitterBean" class="org.bk.report.si.SectionRequestSplitter" />

from("direct:start")
.split().method("sectionRequestSplitterBean", "split")
....

Next to transform the Section Request to an XML format, again this is exactly same as the implementation for Spring Integration, with hook being provided in the following manner:

......
.transform().method("sectionRequestToXMLBean", "transform")
......

To send an XML over http request using the Section Request XML to a section Service:

......
.transform().method("sectionRequestToXMLBean", "transform")
.to(serviceURL)
.........

To transform the Section Response XML to a Section object, the component is exactly same as the one used with Spring Integration, with the following highlighted hook in the Camel route:

......
.transform().method("sectionResponseXMLToSectionBean", "transform");

To aggregate the Section responses together into a report, the component is a bit more complicated than Spring Integration. Apache Camel supports a Scatter/Gather pattern using a route of the following type:

......
.split().method("sectionRequestSplitterBean", "split")
.aggregationStrategy(new ReportAggregationStrategy())

with an aggregation strategy being passed on to the Splitter, the aggregation strategy implementation is the following:

public class ReportAggregationStrategy implements AggregationStrategy {
 @Override
 public Exchange aggregate(Exchange oldExchange, Exchange newExchange) {
  if (oldExchange == null) {
   Section section = newExchange.getIn().getBody(Section.class);
   Report report = new Report();
   report.addSection(section);
   newExchange.getIn().setBody(report);
   return newExchange;
  }

  Report report = oldExchange.getIn().getBody(Report.class);
  Section section = newExchange.getIn().getBody(Section.class);
  report.addSection(section);
  oldExchange.getIn().setBody(report);
  return oldExchange;

 }
}

This completes the Apache Camel based implementation. A working sample for Camel is provided with the article - just download, extract and run "mvn test".

Conclusion:

Spring Integration and Apache Camel provide a simple and clean approach for the Integration problems in a typical enterprise. They are lightweight frameworks – Spring Integration builds on top of Spring portfolio and extends the familiar programming model for the Integration domain and is easy to pick up, Apache camel provides a good Java based DSL and integrates well with Spring Core, with a fairly gentle learning curve. The article does not recommend one product over the other but encourages the reader to evaluate and learn from both these frameworks.

References:

Spring Integration Website: http://www.springsource.org/spring-integration
Apache Camel Website: http://camel.apache.org/
Spring Integration Reference: http://static.springsource.org/spring-integration/reference/htmlsingle/spring-integration-reference.html
Apache Camel User Guide: http://camel.apache.org/user-guide.html
Plug for my blog: http://biju-allandsundry.blogspot.com/