Wednesday, November 26, 2008

A Web Site Concentrator to ease AJAX development

Every now and then I do a little AJAX development and there is one thing that I always find frustrating during the development of AJAX apps: The browser's single-origin policy.

Now I agree that for non-development cases this policy is absolutely justified, but during development it's a big PITA.
Let's say you are developing an AJAX app that calls into a back-end app over XML and you want to debug both. Like in my case I have a little GWT app that invokes on a Distributed OSGi Service running on the back end in CXF over port 9000. Ultimately you'd deploy it all over a single server, so there wouldn't really be a problem in production, but during development my GWT app is running on http://localhost:8888/org.coderthoughts.dinner.DinnerClient while my CXF back end runs on http://localhost:9000/org/apache/cxf/dosgi/samples/springdm/DinnerService

If I try to invoke my back-end app in this setup my AJAX app complains:

Since your front-end and back-end are probably written in different languages chances are that they have different development and debugging environments, which makes adhering to the single-origin hard to do. Besides, you don't want to go through the hassle of rebuilding your .WAR files (or whatever format you might use) for every single change. I know MSIE has some options to alleviate this, but they don't always work. Also using JSON as a data format gives you some options to bypass the problem, but that only works if your data format is JSON. In my case it's XML.

Enter ProxyServlet. There's a few of these around, but they're generally a bit raw. I particularly liked the one from Frank Spieleck because it has no dependencies, so is simple to use.
Typically these ProxyServlets forward requests from one context to another location, so to build the multi-site concentrator that you need to make the browser think both of them come from the same source you need at least two of them. In my case I need the following proxy settings:

  • org.coderthoughts.dinner.DinnerClient => http://localhost:8888/org.coderthoughts.dinner.DinnerClient

  • org => http://localhost:9000/org

To ease this task I wrote a little commandline wrapper around Frank's ProxyServlet that simply generates a .WAR file with the right name to set the root context, the correct bits in the web.xml etc... So I generate 2 .WAR files:

java -jar genwebproxy.jar \
  org.coderthoughts.dinner.DinnerClient \
  Created: org.coderthoughts.dinner.DinnerClient.war

java -jar genwebproxy.jar org http://localhost:9000/org
  Created: org.war
and simply deploy them into Tomcat by copying them into the webapps directory. These are standard .WAR files so you can deploy them into any Webapp container. I've also put the PAX-WEB headers in, in case you want to deploy them into that.

Et voilĂ . It works!

Note that I access my AJAX app using the Tomcat port of 8080 instead of the GWT debug port of 8888. I did have to change the port number at with my webapp makes an invocation from 9000 to 8080 as well, but that's easy.

With Frank's permission I put the ProxyServlet plus my wrapping code for the genwebproxy.jar file in SVN here:

You can download the binary from here, the command line is:
java -jar genwebproxy.jar [context_root] [real_location]