How to automatically delete old Google App Engine version instances?

To stop all instances of all non-default versions of all modules (independently of what languages those modules are in), you could add a small control module, written in Python, using the modules API:

from google.appengine.api.modules import modules

# core logic (inside a cron or other handler)
for m in modules.get_modules():
    dv = modules.get_default_version(m)
    for v in modules.get_versions(m):
        if v != dv: modules.stop_version(m, v)

This doesn't delete the non-default versions (the modules API does not appear to currently support deletion), but does ensure that none of their instances are running (so no charges would be incurred for them).

This core logic is meant for you to wrap it inside a handler, so you can trigger it as required, for example in a cron job, or in a "regular" handler that you trigger from the outside (with appropriate auth) e.g. via wget or curl within your bash scripts.

I don't believe there's a Ruby version of Python's google.appengine.api.modules.modules , but, I could be wrong... I just couldn't find one. However, a simple Python-coded module should let you control modules coded in whatever other App Engine language (since App Engine lets you mix and match modules coded in different languages).

We deploy an App Engine instance from Cloud Build, which is a common deployment pattern. At the end of the cloudbuild.yaml, we specify the following commands as the last build step, which deletes every version except the last 5 versions:

versions=$(gcloud app versions list \
  --service default \
  --sort-by '~VERSION.ID' \
  --format 'value(VERSION.ID)' | sed 1,5d)
for version in $versions; do
  gcloud app versions delete "$version" \
    --service default \
    --quiet
done

The following removes all version except 5 most recent: Uses ghead (on macOS, brew install coreutils), replace with head for linux.

gcloud app versions delete `gcloud app versions list | sed 's/  */:/g' | cut -f 2 -d : | tail -n +2 | ghead -n -5 | tr "\n" " "`