A client querying any heroic node in a federation will cause it to fan out to all known shards and merge the result.
Federations tries to be as transparent as possible in the face of problems. Each request that fans out to a shard has the potential to fail, preventing that data to become unavailable.
In the face of errors, successful shards will be returned as normal. The failing shards will be specifically reported as such, and it is left to the client to decide what to do next.