My reply was mainly aimed at the example posted here, really how often will a Dog Walker's set of Dogs change? Keep in mind that transactions can have retries too, so it's not enough for 2 requests to collide, it needs to be entity groups with > 1 write per second for that to be an issue.
But beyond that, there are other app engine tools like task queues, which can make dealing with contention pretty easy.
An unrelated query: does anyone know of companies (like Streak.com) using app engine datastore in production? We are newbie programmers, and developing our app on app engine (because it was easy to learn) - but it feels a bit lonely sometimes :)
But beyond that, there are other app engine tools like task queues, which can make dealing with contention pretty easy.