Okay so yesterday's test was easy. Set four browsers running with the Reload plug-in set for every 10 seconds (with one browser set to 5 seconds). This meant that there would be 18,780 hits a minute. Now there are a bunch of quotas on Google App Engine and as I've noticed before its pretty much only the raw time one
that gets culled on an individual request.
So it scales for the cloud in terms of breaking down the problem but now we are running up against another quota. The 5,000,000 page views a month. This sounds like a lot, and it would be if each page was 1 request, but in this AJAX and Web 2.0 each page can be made of lots of small requests (625 images + the main page for starters). Now Google say that they throttle before your limit rather than just going up to it and stopping... and indeed they do.
That shows the requests coming in. Notice the two big troughs? That could be the test machines bandwidth dropping out for a second, or an issue on the App Engine side. More investigation required. That profile of usage soon hit the throttle
This looks like you can take a slashdotting for about 2 hours before the throttle limit kicks in. The throttle is also quickly released when the demand goes down. The issue however here is that it isn't clear how close I am to a quota and how much I have left, there isn't a monthly page count view and, as noted before, the bandwidth and cycles quotas don't appear to work at the moment
It still says I've used 0% of my CPU and bandwidth which is a little bit odd given this really does cane the CPU. Bug fixing required there I think!
So basically so far it appears that App Engine is running on two real quotas, one is the real time that a request takes and the other is related to the number of page views. If you are looking to scale on the cloud it is important to understand the metrics you need to really measure and those which are more informational. As Google become tighter on bandwidth and CPU then those will become real metrics but for now its all about the number of requests and the time those requests take.