How can I pass a variable from github actions workflow to a GAE app.yaml file?

Question

I have a django project I want to put into maintenance mode before I update (migrate) the database.

So, my github workflow

1. deploys my project with a variable MAINTENANCE_MODE set to true. This new deploy I understand will reboot any running instances, ensuring all instances only show my 'Site down for maintenance' 503.html page and won't be interacting with the database.
2. I launch a django VM in github actions, run my migrate, run collectstatic.
3. I set MAINTENANCE_MODE to false, I deploy a second time. This will re-enable production server with new code that now accesses a migrated database.

My question is, I am trying to use a single app.yaml file for both deploys. To pass the MAINTENANCE_MODE variable from github actions workflow to the app.yaml file, how can I do this?

I know you can import secrets like so:

runtime: python38
instance_class: F2
env_variables:
  DB_URL: $ {{ secrets.DB_URL }}

But I don't know how to modify a secret in the workflow. Perhaps its not a secret but some other type of variable one can set in the workflow and access in the app.yaml?

Answer 1

So it appears that Google App Engine's yaml files do not support dynamic environment variable substitution. Static substitution (like using github's secrets) works, because github compiles the file with the github environment variable before the workflow runs, but there's no clear way to modify a file with a variable that is going to change during a workflow.

A method however that does work is to compile a new GAE yaml file during the workflow. Here's what I came up with in the end...

      - name: Put in Maintenance mode
        run: |
          MAINTENANCE_MODE=1 envsubst < app_eng_staging.yml.template  > app.yaml
          cat app.yaml
          gcloud app deploy --project staging-project --quiet
      - name: Collectstatic and migrate
        env:
          RUNNING_ENVIRONMENT: 'Staging_Server'
          DJANGO_DEBUG: 'False'
        run: |          
          pipenv run python manage.py collectstatic --noinput
          pipenv run python manage.py migrate
      - name: Turn off Maintenance and Deploy
        run: |
          MAINTENANCE_MODE=0 envsubst < app_eng_staging.yml.template  > app.yaml
          gcloud app deploy --project staging-project --quiet

The trick is to use the linux envsubst command. We start with a app_eng_staging.yml.template file, which looks like so:

runtime: python38
instance_class: F2
env_variables:
  RUNNING_ENVIRONMENT: 'Staging_Server'
  StagingServerDB: $ {{ secrets.STAGINGSERVER_DB }}
  MAINTENANCE_MODE: ${MAINTENANCE_MODE}
  FRONTEND_URL: $ {{ secrets.STAGING_FRONTEND_URL }}

envsubst then populates ${MAINTENANCE_MODE} with the value 1 and the result is saved to a new file app.yaml.

After we finish working with out database migration, we can use envsubst to create a new app.yaml with MAINTENANCE_MODE set to zero (off), and re-deploy.

Neat huh?

Answer 2

There is a feature in GitHub Action for this called "GAE environment variable compiler"

Please read here