Can we clear up one issue? Sharding on userid -- bad or fine? I say fine. If your algorithm is "if userid between A and M then server0, else (N-Z) server1",

Can we clear up one issue? Sharding on userid -- bad or fine? I say fine. If your algorithm is "if userid between A and M then server0, else (N-Z) server1", then you could get unbalanced. But if your algorithm is "if random 50/50 hash of userid = 0 then server0, else (random 50/50 hash = 1) server1", then you are going to stay almost exactly balanced (law of large numbers). You would never, ever, get anywhere near a 60/40 imbalance. So the choice to use userid, if implemented correctly, is completely fine.