Anyone who volunteers for app level sharding is going to find themselves deep in Special Hell. Step one is to rewrite all your queries so they no longer expect really exotic use cases like "SELECT ... WHERE" to actually work until you somehow figure out which nodes to run them on (and if it was supposed to be a join between entities crossing shards, good luck with that). Step two is to find working XA-aware drivers (haha). For step three you need a knife and a goat....

Apparently really high-end clustered databases solve this problem correctly, i.e., the schema is extended to specify where any record can be found, the cluster uses that to work out a minimally-stupid query plan, and record->shard mapping becomes merely a tuning decision. I've never had the opportunity to work with one. But I'm not bitter.