Sunday, April 19, 2009

Deployment on GAE/J and the minimal Google App Engine/J application

Continuing the exploration of the Google App Engine/J I decided to take a look at how the deployment process works.

I started by creating a tiny GAE/J application consisting only of one JSP file. In addition to the JSP file the GAE/J environment requires two additional files: the standard Java deployment descriptor web.xml and the GAE/J specific appengine-web.xml. The files required for our application are hence: index.jsp, WEB-INF/appengine-web.xml and WEB-INF/web.xml.

The smallest possible valid appengine-web.xml contains only the required elements appengine-web-app/application and appengine-web-app/version. If you want to use HTTP-sessions you're required to add an element appengine-web-app/sessions-enabled set to true. The appconfig-web.xml is documented here.

This is the smallest possible valid appengine-web.xml:

The smallest possible valid web.xml file that is accepted by GAE/J contains only the root node web-app. For some reason <web-app /> is not accepted whereas the equivalent <web-app></web-app> is. The GAE/J documentation covering web.xml is found here.

The smallest possible web.xml that is accepted by GAE/J is:

Now that we have the JSP-file and our minimal web.xml and appengine-web.xml we're ready to deploy.

To make the working of the deployment process as clear as possible I choose to deploy by issuing the deploy command directly rather than relying on the Ant scripts provided by the GAE/J SDK. In order to deploy we issue the command:
java -cp /some/path/to/appengine-java-sdk-1.2.0/lib/appengine-tools-api.jar update .

The application should now be deployed.

If we examine our GAE/J "home dir" (see this previous post about "GAE/J home dirs") after deployment we'll find the following files:

  • /base/data/home/apps/[app-id]/[version-id]/index.jsp

  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/classes/org/apache/jsp/

  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/classes/org/apache/jsp/index_jsp.class

  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/web.xml
  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/appengine-web.xml
  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/lib/jasper-compiler-5.0.28.jar
  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/lib/jakarta-jstl-1.1.2.jar
  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/lib/commons-el-1.0.jar
  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/lib/jakarta-standard-1.1.2.jar
  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/lib/jasper-runtime-5.0.28.jar
  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/lib/ant-launcher-1.6.5.jar
  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/lib/commons-logging-1.1.1.jar
  • /base/data/home/apps/[app-id]/[version-id]/WEB-INF/lib/ant-1.6.5.jar

Note that a couple of JAR:s have been to our application. Apache Jasper runtime (+ dependencies) is used by the servlet (org.apache.jsp.index_jsp) that is generated from our JSP-file (index.jsp). But what about the other JAR:s (say ant, ant-launcher and jasper-compiler) - should those really be required in the live environment? Is the uploading of those files a bug?

The web.xml that is uploaded to the live environment is altered during the deployment process. Servlet-mappings are added to the original web.xml to match JSP filenames against the servlets that are generated from said JSP files -- in this case the URL-pattern /index.jsp is mapped to the servlet org.apache.jsp.index_jsp.

Note also that both the original JSP file (index.jsp) and the corresponding generated servlet Java file ( are uploaded although those files should not be needed in the live environment. This can be seen either as a feature (in the sense you have a backup of your source code in the live environment) or as a bug (in the sense that only what's required in the live environment should be in the live environment).

This concludes the exploration of the GAE/J deployment process this time.

I'd like to hear from you what you think about the three (very minor) issues discussed -- <web-app /> not accepted as a valid web.xml + JSP:s and Java-files uploaded to live environment + additional JAR:s uploaded of which some redundant (?). Are any of these bugs that should be reported? What do you think? Please leave a comment.

I should probably add that I'm very impressed with how smooth the deployment process is handled when using GAE/J. In particular I have not yet seen a case where deploying a new version means downtime visible for the end-user. That's very nice!


  1. Hi!

    Thanks for all your testing! I believe most of your thoughts are correct, and they should be filed as bug. It's not good if we have to upload useless files at each deployment, because bandwidth is usually limited.

    I think the general jars which are needed for (almost) every application should be stored in a global space, so everybody can read from it (classpath of the container, maybe?)

  2. Thanks for your comments Bálint!

    I've now filed three bug reports - see this blog post.

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