I’m guessing that since it is for registration and all, the usage might be write-driven, or at least equally balance between writes and reads. In addition, you

I’m guessing that since it is for registration and all, the usage might be write-driven, or at least equally balance between writes and reads. In addition, you really care about integrity of your data so you probably want serializability, avoid concurrency and potential write/update conflicts, and to only do the writes on a single server. For this reason it sounds to me that partitioning/sharding is the only way to really scale this: have different write servers that care about different primary keys.