When we released version 1.2.4 of the SDK earlier this month, a couple of new features were released that didn't quite make it into the release notes. We think they're really cool features, so we wanted to take the time to highlight them.
The first feature is that when logged into your app as an administrator, App Engine will include a couple of extra headers in all the HTTP responses it sends you. As an example, here's what I see in Firefox's Live HTTP headers plugin when I load my blog:
X-AppEngine-Resource-Usage: ms=293 cpu_ms=500 api_cpu_ms=236
The first header tells me that it took 293 milliseconds to generate the page, and 500 CPU milliseconds were consumed, of which 236 milliseconds were spent doing API calls - such as accessing memcache or the datastore. The second header tells me that App Engine estimates that serving 1000 requests like this one would cost about $0.01 if I was above my free quota - not bad!
You can view these headers using plugins such as Firefox's Live HTTP Headers or Firebug. Note that only logged in administrators see these figures - ordinary users, and users who aren't logged in, won't see them at all.
The second new feature is that we've enabled 'wildcard' domains for App Engine apps serving off appspot.com. What this means is that you can now create multiple subdomains for your App Engine app, and have them all served by the same application. Thus, your users can access 'myapp.appspot.com', or 'developer.myapp.appspot.com', or 'news.myapp.appspot.com', with your app deciding how to handle each request. No setup is required to use the wildcard domains - simply configure your app to serve up requests to these subdomains however you wish. You can detect which domain a user requested by looking at the 'Host' header in the incoming request - for example, in Python's webapp framework you can access this with self.request.headers['Host'].