You want to create “fully baked” or “golden” Compute Engine images for your application.

Embed the appropriate database connection string in the image. Create a different image for each environment.

When creating the Compute Engine instance, add a tag with the name of the database to be connected. In your application, query the Compute Engine API to pull the tags for the current instance, and use the tag to construct the appropriate database connection string.

When creating the Compute Engine instance, create a metadata item with a key of “DATABASE” and a value for the appropriate database connection string. In your application, read the “DATABASE” environment variable, and use the value to connect to the appropriate database.

When creating the Compute Engine instance, create a metadata item with a key of “DATABASE” and a value for the appropriate database connection string. In your application, query the metadata server for the “DATABASE” value, and use the value to connect to the appropriate database.