I created a simple web application and put it up on Amazon Web Services for about a week. The cost for a single instance and a single load balancer was around $21, including a few cents for storage and traffic. It would be about $82 a month. So I ported it to Google App Engine, where a low-traffic web application could be hosted for free. It took about a day. I replaced the storage layer that used Amazon S3 with one that used JDO. I replaced the memcached layer using the net.spy.memcached client with one using the JCache (JSR107) API. Those were straightforward and most of the rest of the code didn't need any changes. I also added the configuration files appengine-web.xml and jdoconfig.xml.

I ran into a few snags with the appengine servlet container, some of which might have been due to strange interactions with Google Guice, but that's just speculation without any real investigation.

The first one is probably a bug in the appengine (or jetty) implementation of HttpServletResponse.encodeRedirectURL(). It erroneously added ;jsessionid=sessionid to external URLs. I worked around that by not calling encodeRedirectURL() for external URLs, even though the javadoc says "All URLs sent to the HttpServletResponse.sendRedirect method should be run through this method."

The next one seemed like some weird difference in implementations. I used guice-servlet to configure servlet filters on /index.jsp, but they weren't being invoked for /, while the filters were being invoked under Tomcat and Glassfish. I worked around that by changing filter("/index.jsp").through(MyFilter.class) to filter("/","/index.jsp").through(MyFilter.class).

The last one was the weirdest, and has to be some kind of bug in either appengine, jetty, or guice. Once I got the filters passing the request for / through, instead of serving up /index.jsp, the service returned a redirect to //, which turned into a redirect to ///, etc, until the browser stopped due to too many redirects. I worked around that by kludging in a special servlet for /:

    @Singleton
    public static class RedirectToIndexJSP extends javax.servlet.http.HttpServlet {
        private static final long serialVersionUID = 0L;
        @Override
        protected void doGet(javax.servlet.http.HttpServletRequest request, javax.servlet.http.HttpServletResponse response) throws javax.servlet.ServletException, java.io.IOException {
            request.getRequestDispatcher("/index.jsp").forward(request, response);
        }
    }

After that, it all worked. Compared to Amazon, the service on Google App Engine is a lot slower for the first request, as this is a very low-traffic application, so it's pretty much never running, and a new virtual machine starts up when a request does come in, which seems to take around 10 seconds. Subsequent requests are reasonable, though still slower than Amazon.