agent-conf.json file violates twelve-factor methodology of Heroku apps

[hheimbuerger]

I'm running an app on Heroku, which is a PaaS built upon the twelve-factor methodology.

Currently, the agent-conf.json file links my codebase to a specific instance of Librato. This violates a core concept of twelve-factor, namely the strict separation of configuration from code:

Apps sometimes store config as constants in the code. This is a violation of twelve-factor, which requires strict separation of config from code. Config varies substantially across deploys, code does not.

As the twelve-factor page I linked to earlier describes in more detail and much more clearly than I ever could, this is a problem because I'm deploying the same codebase to various instances, in my case production, staging, various demo instances for sales as well as a review app for each feature branch.

Please provide and document a way to support a maintainable Heroku setup, following the twelve-factor methodology.

[hheimbuerger]

I have multiple Heroku apps, representing different stages in the apps lifecycle: production, staging, reviews, etc.
My main interest was to selectively enable Librato for some Heroku apps (production) and have it completely disabled (i.e. the instrumention being a noop) on all other apps (staging, review, etc.).

I really wish this was just built into librato-python-web, but for now I've built my own wrapper script as a workaround:

bin/librato-wrapper in my Heroku codebase:

#!/usr/bin/env bash
set -o errexit
set -o pipefail
set -o nounset

if [ "${LIBRATO_ENABLED:-}" = "true" ]
then
    echo "Found LIBRATO_ENABLED, enabling Librato instrumentation..."
    exec librato-launch "$@"
else
    echo "Skipping Librato instrumentation as LIBRATO_ENABLED was not set."
    exec "$@"
fi

(I'm not a bash expert, so suggestions are very welcome. I'm specifically hoping that this doesn't break Heroku's process watching and restarting. I've made the mistake of using && in my Procfile before and it took me months to track that down.)

My Procfile looks like this:

web: bin/librato-wrapper gunicorn myapp.wsgi --bind 0.0.0.0:$PORT ...

Now the librato-launch instrumentation is only run when the environment variable LIBRATO_ENABLED is set to true (in my case: only in production) and is skipped in all other cases.