Earlier quoted context omitted.
Heroku is single-AZ within US-East, not even multiple-AZ.
Sorry but that is not even remotely true. We are hosted across all availability zones in the US-EAST region.
Is Heroku resilient against single-AZ failure (so only some subset of customers go down, and then it restarts), or is it exposed so that if any AZ goes down, core stuff also goes down? The sites I care about on Heroku seem to go down whenever any US-East badness happens at all, even if it is "limited to a single AZ" per Amazon.