How are cross-shard joins and transactions handled? Are there query patterns that would require application changes despite using the same drivers and ORM?
cross shard joins break down into single shard selects, and are joined in the routers. There are some query patterns that are currently unsupported, but we are working on adding support for them. The biggest thing to keep in mind when running on a sharded database of any kind is decreasing the number of cross shard queries.