I used AWS Cloudwatch and some simple server side checks (ianheggie/health_check for Rails is great) for a very long time. It's not perfect, but it's (1) cheap

I used AWS Cloudwatch and some simple server side checks (ianheggie/health_check for Rails is great) for a very long time. It's not perfect, but it's (1) cheap (2) easy (3) quick (the mythical trifecta). It misses some of issues due to high loads (but still technically available) but works perfectly when things actually crash (like queue workers deciding to turn off).