€1.3mil is the monthly cost for the whole company, not just any single app. It’s a very big company with thousands of developers and dozens of products. The team I’m working with spends around €5k/month on AWS for all their apps, databases and other associated infra (like EMR clusters and such).
Keeping stuff in memory is fine, but there’s little perceivable difference to a user between a Redis Cache and an ETS cache. It’s another one of those “10x faster” situation, but it’s 20ms vs 2ms. I don’t even think the difference is that big, though.
I wouldn’t use mnesia for something where losing data isn’t an option… It’s just not meant for that.
