Wednesday, June 10, 2009

Changing Quotas To Keep Most Apps Serving Free

Since App Engine launched, our goal has been to offer substantial free hosting resources to as many developers as possible. As previously announced, we are changing our free resource quota levels, effective on June 22nd. Our target level of free support has been 5 million page views per month for a reasonably efficient web application.

When we launched App Engine, we were intentionally generous in our free quotas, both because we didn't know resources usage of a typical request, and because we didn't offer a billing feature to allow developers to buy more resources for a higher-traffic app. Since our billing feature launched in February, developers with high-traffic applications can purchase additional resources far beyond our original fixed free quotas. Having been live for more than a year, we now have good empirical data on the average resource consumption per request, so we're able to set our quotas to more accurately support our 5 million page views target.

This change in the free quotas offered to every application is intended to allow us to continue to offer substantial free application hosting to any interested developer. We have grown a lot in the last year, with over 80,000 applications created, and with these changes to our free quotas, more than 90% of these applications will continue to serve completely free. To empirically determine reasonable levels for our quotas, we measured resource usage for all applications running on App Engine over a recent 7-day period. For each of the quotas, we took the highest daily average resource usage per HTTP request out of the 7-day period:

CPU: 0.14 CPU-seconds/request
Outbound data transfer: 6149 bytes out
Inbound data transfer: 803 bytes in

Multiplied by 5 million requests spread over a 30 day month, these per-request resource statistics translate to daily resource usage of 6.4 CPU-hours and 1.02 gigabytes of outbound data transfer. We top off the numbers by offering 6.5 CPU-hours and 1.07 gigabytes of outbound transfer. Though typically inbound data transfer is a small fraction of outbound data transfer, we made inbound and outbound data transfer symmetric to ease initial data uploads.

Finally — what do we mean by reasonably efficient applications? Simply put, efficient applications avoid unnecessary computation or data transfer, and two techniques common to efficient App Engine applications are the use of caching headers and memcache. Caching headers in an HTTP response prevent a user's browser from needlessly re-downloading information that hasn't changed, both speeding up the user experience and saving bandwidth. Similarly, memcache keeps frequently accessed data in a memory cache on App Engine servers, rather than always reading from disk in the Datastore, therefore saving CPU usage and Datastore load.

Again, these changes ensure we can keep our continuing promise to make it free to get started with App Engine.

1 comment:

Jeremy said...

I just tried to use an app that is being hosted on app engine. It had exceeded its quota. It seems like a relatively small donation on my part of $1.00 to $5.00 would pay for myself to use it for the couple hours I want to and probably a lot of other people as well.

This is a feature request. Make it so any user of a developer's app can buy hosting if the developer exceeds his or her quota.