[patio11]

I use MySql and Redis for persistence, depending on the type of data. Both get written to on a purchase: MySql upgrades the account info, Redis holds my A/B testing stats which just had several tests score points. If the MySQL write fails, I have a CS emergency because my customer can't get what she paid for and I probably just ruined a lesson plan for tomorrow. If of the Redis writes fails, my A/B test results that I won't look at for a week anyhow shift in a way that almost certainly doesn't alter my final decision.

It is absolutely OK to lose analytics data occasionally, and indeed with the variety of ways to bork that (js is off, user agent prefetches undisplayed page, bot action, etc) if your stats aren't robust against it you are screwed anyhow.