heroku h12 error code example

Example: heroku node js h21 backend connection refused

2010-10-06T21:51:07-07:00 heroku[router]: at=error code=H21 desc="Backend connection refused" method=GET path="/" host=myapp.herokuapp.com fwd=17.17.17.17 dyno=web.1 connect=1ms service= status=503 bytes=