It's trivial to define your entity groups in Google Cloud Datastore such that you have strongly consistent carts. It's how you would naturally build an app, and

It's trivial to define your entity groups in Google Cloud Datastore such that you have strongly consistent carts. It's how you would naturally build an app, and it doesn't require http sessions (which I've never used on GAE and never expect to in the future). I've built multiple ecommerce sites this way and they work great. What's hard is to do strongly consistent queries across carts. But that isn't something anyone cares about.