Camel Components for Google App Engine
Tutorial
A good starting point for using Camel on GAE is the Camel on Google App Engine Tutorial
The Camel components for Google App Engine (GAE) are part of the camel-gae
project and provide connectivity to GAE's cloud computing services. They make the GAE cloud computing environment accessible to applications via Camel interfaces. Following this pattern for other cloud computing environments could make it easier to port Camel applications from one cloud computing provider to another. The following table lists the cloud computing services provided by Google App Engine and the supporting Camel components. The documentation of each component can be found by following the link in the Camel Component column.
GAE service |
Camel component |
Component description |
---|---|---|
Provides connectivity to the GAE URL fetch service but can also be used to receive messages from servlets. |
||
Supports asynchronous message processing on GAE by using the task queueing service as message queue. |
||
Supports sending of emails via the GAE mail service. Receiving mails is not supported yet but will be added later. |
||
|
Not supported yet. |
|
|
Not supported yet. |
|
|
Not supported yet. |
|
|
Not supported yet. |
|
|
Not supported yet. |
Camel context
Usage of the CamelContext
implementations org.apache.camel.impl.DefaultCamelContext
and org.apache.camel.spring.SpringCamelContext
on GAE is not possible out of the box. JMX must be disabled because the javax.management
package is not part of the GAE JRE. Usage of the Camel-specific Spring configuration XML schema from the http://camel.apache.org/schema/spring
namespace doesn't work either. Camel uses a JAXB-dependent bean definition parser but JAXB is (currently) not supported by GAE. To work around these restrictions camel-gae
provides the following CamelContext
implementations.
org.apache.camel.component.gae.context.GaeDefaultCamelContext
(extendsorg.apache.camel.impl.DefaultCamelContext
)org.apache.camel.component.gae.context.GaeSpringCamelContext
(extendsorg.apache.camel.spring.SpringCamelContext
)
Both disable JMX before startup. The GaeSpringCamelContext
additionally provides setter methods adding route builders as shown in the next example.
<beans xmlns="http://www.springframework.org/schema/beans" 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-2.5.xsd"> <bean id="camelContext" class="org.apache.camel.component.gae.context.GaeSpringCamelContext"> <property name="routeBuilder" ref="myRouteBuilder" /> </bean> <bean id="myRouteBuilder" class="org.example.MyRouteBuilder"> </bean> </beans>
Alternatively, use the routeBuilders
property of the GaeSpringCamelContext
for setting a list of route builders.
The web.xml
Running Camel on GAE requires usage of the CamelHttpTransportServlet
from camel-servlet
. The following example shows how to configure this servlet together with a Spring application context XML file.
<web-app xmlns="http://java.sun.com/xml/ns/javaee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation=" http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" version="2.5"> <servlet> <servlet-name>CamelServlet</servlet-name> <servlet-class>org.apache.camel.component.servlet.CamelHttpTransportServlet</servlet-class> <init-param> <param-name>contextConfigLocation</param-name> <param-value>appctx.xml</param-value> </init-param> </servlet> <!-- Mapping used for external requests --> <servlet-mapping> <servlet-name>CamelServlet</servlet-name> <url-pattern>/camel/*</url-pattern> </servlet-mapping> <!-- Mapping used for web hooks accessed by task queueing service. --> <servlet-mapping> <servlet-name>CamelServlet</servlet-name> <url-pattern>/worker/*</url-pattern> </servlet-mapping> </web-app>
The location of the Spring application context XML file is given by the contextConfigLocation
init parameter. The appctx.xml
file must be on the classpath. The servlet mapping makes the Camel application accessible under http://<appname>.appspot.com/camel/...
when deployed to Google App Engine where <appname>
must be replaced by a real GAE application name. The second servlet mapping is used internally by the task queueing service for background processing via web hooks. This mapping is relevant for the gtask component and is explained there in more detail.