> So my conclusion is that for now, the best way to scale read-only queries
for a sharded master is to
> implement map-reduce at the application level.
That's the conclusion I would expect. It's the price you pay for sharding,
it's part of the deal.
But it's also the benefit you get from sharding. Once your read traffic
grows to the point that it's too much for a single host, you're going to
have to re-shard it all again *anyway*. The whole point of sharding is
that it allows you to grow outside the capacities of a single host.