How is my GAE app able to run without app.yaml?

Question

Although I started development for Google App Engine using Endpoints a while ago, I hadn't noticed this - Google's ref. page for Project structure says this:

Your development file hierarchy should look like this:

MyDir/
  [pom.xml]
  [build.gradle]
  [index.yaml]
  [cron.yaml]
  [dispatch.yaml]
  src/main/
    appengine/
      app.yaml
    docker/
      [Dockerfile]
    java/
      com.example.mycode/
        MyCode.java
    webapp/
      [index.html]
      [jsp.jsp]
      WEB-INF/
        [web.xml]

You'll need to define an app.yaml file that looks like this:
...

Note that the app.yaml is deemed compulsory as per the docs. In my case, I spawned a backend module(through the Wizard) in Android Studio that builds on Gradle. I have been able to build and deploy this module on GAE successfully but now I needed to switch from automatic scaling to basic/manual scaling, I found this to be done through app.yaml file.

Here is the thing: I don't have an app.yaml in place and it works fine. Where is then the config info. that GAE requires to deploy the App.

Specifically,

app.yaml specifies the environment - Java. But, I found the java plugin in build.gradle for that. Aren't 2 config places for the same thing confusing?
Is it possible to ditch app.yaml entirely for equivalent config. in build.gradle?
Why is Google claiming app.yaml to be compulsory when I am able to do without it?

The Java runtime is special and is known to have XML config instead of YAML. — Tatsuyuki Ishi
@TatsuyukiIshi : Where can I change the scaling config. for the App? — Manish Kumar Sharma

Tatsuyuki Ishi Tatsuyuki Ishi · Accepted Answer · 2017-04-14T10:13:02

The App Engine Java runtime uses its own configuration schema in XML, while others are YAML.

To set the scaling elements, follow the official reference.

How is my GAE app able to run without app.yaml?

1 Answers