How thin is “thin”? If I build a chat app on top of Redis, is it safe? There’s a tutorial on the Redis website: https://developer.redis.com/howtos/chatapp/ — is it a sign that it’s too simple to be distinct?
Or I can just migrate my app to Valkey and be sure I’m safe from lawyers.
Its a chat app not a data store, so it fails the "primary purpose" test so its safe: its pretty obvious that a chat app serves a different purpose to a key value data store.
Nothing is definitely "safe from lawyers". In some ways BSD is less safe than SSPL - no patent licensing clauses for example.
It will be if your app is a data store service . i.e. your apps is a thin wrapper around Redis.
If your app is more than that, then its clear the value does not primarily derive from Redis.
There are inevitably grey areas (e.g. if your app is a Redis based data store but adds a lot of functionality) but 99% of users do not need to worry.