The most basic and important principle of all runtime environments, including that of Python, is that you can talk to the outside world only by going through Google's own services. It is like a completely sealed and contained sandbox where you are not allowed to write to the disk or to connect to the network. However, no program will be very useful in that kind of isolation. Therefore, you can definitely talk to the outside world but only through the services provided by the App Engine. You can also ship your own code and libraries but they must all be in pure Python code and no C extensions are allowed. This is actually a limitation and tradeoff to ensure that the containers are always identical. Since no external libraries are allowed, it can be ensured that the minimal set of native required libraries is always present on the instance.

At the very beginning, App Engine started with the Python runtime environment, and version 2.5 was the one that was available for you. It had a few external libraries too, and it provided a CGI environment for your web app to talk to the world. That is, when a web request comes in, the environment variables are set from the request, the body goes to stdin and the Python interpreter invoked with given program. It is up to your program to then handle and respond to the request. This runtime environment is now deprecated.

Later, the Python 2.7 runtime environment came along, with new language features and updated shipped libraries. A major departure from the Python 2.5 runtime environment was not only the language version, but also a switch from CGI to WSGI. Because of this switch, it became possible for web apps to process requests concurrently. This boosted the overall throughput per instance. We will examine CGI and WSGI in detail in the next chapter.

Java runtime environment presents a standard Servlet version 2.5 environment, and there are two language versions available—Java 5 and Java 6. The Java 6 runtime environment is deprecated and will be soon removed. The Java 6 runtime environment will be replaced and new applications users can only be able to use Java 7. The app.xml is a file that defines your application, and you have various standard Java APIs available to talk to Google services, such as JPA for persistence, Java Mail for mail, and so on.

This runtime environment is also capable of handling concurrent requests.

This runtime environment uses the new Go programming language from Google. It is a CGI environment too, and it's not possible to handle concurrent requests, the applications are written in Go version 1.4.

This is a preview platform, and the PHP interpreter is modified to fit in the scalable environment with the libraries patched, removed, or the individual functions disabled. You get to develop applications just as you would do for any normal PHP web application, but there are many limitations. Many of the standard library modules are either not available, or are partially functional, the applications are written in PHP version 5.5.