It turns out that most SQL databases, when used as a key-value store, shard better than most NoSQL databasing. You run into issues with things like JOINs acros

It turns out that most SQL databases, when used as a key-value store, shard better than most NoSQL databasing. You run into issues with things like JOINs across shards, but there's literally no upside even there to NoSQL versus using a SQL databased in a disciplined way. I've designed systems with the same type of infinitely horizontally scalable KVS-type storage for all the changing state in SQL. SQL also means you can do things like: - Do local joins (if e.g. all the data for a user is in the same shard) - Keeping a small set of relational data (not the stuff which needs to scale) and have one technology - Use read replicas to scale some of the stuff which doesn't fit in the "infinitely horizontally scalable KVS" model .... and so on. You can't not think about it, but if you do think about it, it's all upside and no downside.