Tuesday, November 19, 2013

Camel on Google App Engine Tutorial


Goal of this tutorial is to get a non-trivial Camel application running on Google App Engine (GAE). For developing that application the Camel Components for Google App Engine are used. The example web application generates a weather report for a city that is entered by the user into a form and sends the weather report via email to either the currently logged-in user or a user-defined receiver. The following figure gives an overview. Users of this application need to login with their Google account.
POSTed form data are dispatched to the Camel application via the ghttp component. The application transforms the report request and enqueues it with the gtask component for further background processing. It then generates an immediate HTML response containing information about the city and the receiver of the report. The response also contains a link logging out from the application. In the background, the application retrieves weather data from the Google Weather Service, transforms the data to generate a simple weather report and sends the report by email via the gmail component.
Java clients
Clients other than web browsers may also interact with this application by POSTing the form data and an authentication cookie. At the moment the example application doesn't include a standalone Java client that demonstrates how to do that. This is work in progress. In the meantime, refer to the glogin component documentation for instructions how to access security-enabled GAE services.
Tutorial online
The next two sections describe how to checkout, build and deploy the example application. If you want to skip these steps, go directly to the Usage section and use the pre-deployed example application at http://camelcloud.appspot.com.


  • Sign up for a Google App Engine account if you don't have one.
  • Create a new application via the admin console or reuse an existing one for uploading the example.
  • Install the Google App Engine SDK for Java. This tutorial has been tested with version 1.3.6.


First, checkout the sources from the Camel code repository.
svn co http://svn.apache.org/repos/asf/camel/trunk camel 
Navigate to the camel-example-gae and install from here.
Open the camel-example-gae/src/main/webapp/WEB-INF/application-web.xml file and replace the template application name replaceme with the name of the application that you created in the previous section. Optionally, adjust the version number if needed.
xml version="1.0" encoding="utf-8"?>
<appengine-web-app xmlns="http://appengine.google.com/ns/1.0">
        <exclude path="/index.html" />
        <property name="java.util.logging.config.file" value="WEB-INF/logging.properties"/>
Then go to the camel-example-gae directory and enter
mvn install
This will create the application war file in the target directory. Finally use the appcfg command-line tool of the App Engine SDK to deploy the application.
appcfg update target/camel-example-gae-
where version needs to be replaced with the version of Camel you're using. You will be prompted for the email address and password of your Google App Engine account. After deployment the example application is ready to use.


In the following, the application name camelcloud will be used as an example. If you deployed the example application somewhere else, use your application name instead. Go to http://camelcloud.appspot.com. The application will redirect you to a login page (see also Security for Camel GAE Applications).
After login, the application displays the tutorial's main page where you can enter the name of a city and optionally enter an email address where to send the waether report. If you check Send report to me the report will be send to the email address that you used for login.
After pressing Submit the response is
The home link brings you back to the main page, the logout link is to logout from the application (which brings you back to the login page). In the background, the current weather conditions for the user-entered city will be retrieved from the Google weather service and a formatted weather report will be send by email. Submitting the form the first time initializes the application on Google App Engine which can take several seconds. Subsequent submissions (within a certain time frame) are served much faster. Check your emails and you should now see a new email with subject Wheather report and content similar to this one:
Weather report for:  London, England
Current condition:   Klar
Current temperature: 12 (Celsius) 
The report is partly internationalized, depending on the language settings of your browser.

Code walkthrough

The TutorialRouteBuilder implements the message processing routes shown in the Overview section. Input form data are received via theghttp component. After receiving the request a RequestProcessor adds the form data and information about the current user to a ReportDataPOJO. The ReportData object is then serialized and queued for background processing. Queueing messages on GAE is done with the gtaskcomponent. After adding the ReportData object to the queue an HTML response is generated with the ResponseProcessor.
package org.apache.camel.example.gae;
import org.w3c.dom.Document;
import org.apache.camel.Exchange;
import org.apache.camel.builder.RouteBuilder;
import org.apache.camel.component.gae.mail.GMailBinding;
import org.apache.camel.processor.aggregate.AggregationStrategy;
public class TutorialRouteBuilder extends RouteBuilder {
    public void configure() throws Exception {
            .process(new RequestProcessor())
            .process(new ResponseProcessor());
            .setHeader(Exchange.HTTP_QUERY, constant("weather=").append(ReportData.city()))
            .enrich("ghttp://www.google.com/ig/api", reportDataAggregator())
            .setHeader(GMailBinding.GMAIL_SUBJECT, constant("Weather report"))
            .setHeader(GMailBinding.GMAIL_SENDER, ReportData.requestor())
            .setHeader(GMailBinding.GMAIL_TO, ReportData.recipient())
            .process(new ReportGenerator())       
    private static AggregationStrategy reportDataAggregator() {
        return new AggregationStrategy() {
            public Exchange aggregate(Exchange reportExchange, Exchange weatherExchange) {
                ReportData reportData = reportExchange.getIn().getBody(ReportData.class);
                return reportExchange;
Background processing of the queued messages starts from("gtask://default"). The first step is to enrich the previously generatedReportData object with data from the Google weather service. The service URL is dynamically constructed by setting the Exchange.HTTP_QUERYheader to weather=. For example, if the user entered London in the city field of the form the resulting URL isghttp://www.google.com/ig/api?weather=London. The weather service is accessed with the ghttp component and the weather service response is aggregated into the ReportData object using a custom aggregator returned by the reportDataAggregator() method. Generating a simple weather report from ReportData is done by the ReportGenerator. The report is then sent by email with the gmail component.


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